kernel.ex

# Use elixir_bootstrap module to be able to bootstrap Kernel.
# The bootstrap module provides simpler implementations of the
# functions removed, simple enough to bootstrap.
import Kernel,
  except: [@: 1, defmodule: 2, def: 1, def: 2, defp: 2, defmacro: 1, defmacro: 2, defmacrop: 2]

import :elixir_bootstrap

defmodule Kernel do
  @moduledoc """
  `Kernel` is Elixir's default environment.

  It mainly consists of:

    * basic language primitives, such as arithmetic operators, spawning of processes,
      data type handling, and others
    * macros for control-flow and defining new functionality (modules, functions, and the like)
    * guard checks for augmenting pattern matching

  You can invoke `Kernel` functions and macros anywhere in Elixir code
  without the use of the `Kernel.` prefix since they have all been
  automatically imported. For example, in IEx, you can call:

      iex> is_number(13)
      true

  If you don't want to import a function or macro from `Kernel`, use the `:except`
  option and then list the function/macro by arity:

      import Kernel, except: [if: 2, unless: 2]

  See `Kernel.SpecialForms.import/2` for more information on importing.

  Elixir also has special forms that are always imported and
  cannot be skipped. These are described in `Kernel.SpecialForms`.

  ## The standard library

  `Kernel` provides the basic capabilities the Elixir standard library
  is built on top of. It is recommended to explore the standard library
  for advanced functionality. Here are the main groups of modules in the
  standard library (this list is not a complete reference, see the
  documentation sidebar for all entries).

  ### Built-in types

  The following modules handle Elixir built-in data types:

    * `Atom` - literal constants with a name (`true`, `false`, and `nil` are atoms)
    * `Float` - numbers with floating point precision
    * `Function` - a reference to code chunk, created with the `fn/1` special form
    * `Integer` - whole numbers (not fractions)
    * `List` - collections of a variable number of elements (linked lists)
    * `Map` - collections of key-value pairs
    * `Process` - light-weight threads of execution
    * `Port` - mechanisms to interact with the external world
    * `Tuple` - collections of a fixed number of elements

  There are two data types without an accompanying module:

    * Bitstring - a sequence of bits, created with `Kernel.SpecialForms.<<>>/1`.
      When the number of bits is divisible by 8, they are called binaries and can
      be manipulated with Erlang's `:binary` module
    * Reference - a unique value in the runtime system, created with `make_ref/0`

  ### Data types

  Elixir also provides other data types that are built on top of the types
  listed above. Some of them are:

    * `Date` - `year-month-day` structs in a given calendar
    * `DateTime` - date and time with time zone in a given calendar
    * `Exception` - data raised from errors and unexpected scenarios
    * `MapSet` - unordered collections of unique elements
    * `NaiveDateTime` - date and time without time zone in a given calendar
    * `Keyword` - lists of two-element tuples, often representing optional values
    * `Range` - inclusive ranges between two integers
    * `Regex` - regular expressions
    * `String` - UTF-8 encoded binaries representing characters
    * `Time` - `hour:minute:second` structs in a given calendar
    * `URI` - representation of URIs that identify resources
    * `Version` - representation of versions and requirements

  ### System modules

  Modules that interface with the underlying system, such as:

    * `IO` - handles input and output
    * `File` - interacts with the underlying file system
    * `Path` - manipulates file system paths
    * `System` - reads and writes system information

  ### Protocols

  Protocols add polymorphic dispatch to Elixir. They are contracts
  implementable by data types. See `defprotocol/2` for more information on
  protocols. Elixir provides the following protocols in the standard library:

    * `Collectable` - collects data into a data type
    * `Enumerable` - handles collections in Elixir. The `Enum` module
      provides eager functions for working with collections, the `Stream`
      module provides lazy functions
    * `Inspect` - converts data types into their programming language
      representation
    * `List.Chars` - converts data types to their outside world
      representation as charlists (non-programming based)
    * `String.Chars` - converts data types to their outside world
      representation as strings (non-programming based)

  ### Process-based and application-centric functionality

  The following modules build on top of processes to provide concurrency,
  fault-tolerance, and more.

    * `Agent` - a process that encapsulates mutable state
    * `Application` - functions for starting, stopping and configuring
      applications
    * `GenServer` - a generic client-server API
    * `Registry` - a key-value process-based storage
    * `Supervisor` - a process that is responsible for starting,
      supervising and shutting down other processes
    * `Task` - a process that performs computations
    * `Task.Supervisor` - a supervisor for managing tasks exclusively

  ### Supporting documents

  Elixir documentation also includes supporting documents under the
  "Pages" section. Those are:

    * [Compatibility and Deprecations](compatibility-and-deprecations.html) - lists
      compatibility between every Elixir version and Erlang/OTP, release schema;
      lists all deprecated functions, when they were deprecated and alternatives
    * [Library Guidelines](library-guidelines.html) - general guidelines, anti-patterns,
      and rules for those writing libraries
    * [Naming Conventions](naming-conventions.html) - naming conventions for Elixir code
    * [Operators](operators.html) - lists all Elixir operators and their precedence
    * [Patterns and Guards](patterns-and-guards.html) - an introduction to patterns,
      guards, and extensions
    * [Syntax Reference](syntax-reference.html) - the language syntax reference
    * [Typespecs](typespecs.html)- types and function specifications, including list of types
    * [Unicode Syntax](unicode-syntax.html) - outlines Elixir support for Unicode
    * [Writing Documentation](writing-documentation.html) - guidelines for writing
      documentation in Elixir

  ## Guards

  This module includes the built-in guards used by Elixir developers.
  They are a predefined set of functions and macros that augment pattern
  matching, typically invoked after the `when` operator. For example:

      def drive(%User{age: age}) when age >= 16 do
        ...
      end

  The clause above will only be invoked if the user's age is more than
  or equal to 16. Guards also support joining multiple conditions with
  `and` and `or`. The whole guard is true if all guard expressions will
  evaluate to `true`. A more complete introduction to guards is available
  [in the "Patterns and Guards" page](patterns-and-guards.html).

  ## Inlining

  Some of the functions described in this module are inlined by
  the Elixir compiler into their Erlang counterparts in the
  [`:erlang` module](http://www.erlang.org/doc/man/erlang.html).
  Those functions are called BIFs (built-in internal functions)
  in Erlang-land and they exhibit interesting properties, as some
  of them are allowed in guards and others are used for compiler
  optimizations.

  Most of the inlined functions can be seen in effect when
  capturing the function:

      iex> &Kernel.is_atom/1
      &:erlang.is_atom/1

  Those functions will be explicitly marked in their docs as
  "inlined by the compiler".

  ## Truthy and falsy values

  Besides the booleans `true` and `false`, Elixir has the
  concept of a "truthy" or "falsy" value.

    *  a value is truthy when it is neither `false` nor `nil`
    *  a value is falsy when it is either `false` or `nil`

  Elixir has functions, like `and/2`, that *only* work with
  booleans, but also functions that work with these
  truthy/falsy values, like `&&/2` and `!/1`.

  ### Examples

  We can check the truthiness of a value by using the `!/1`
  function twice.

  Truthy values:

      iex> !!true
      true
      iex> !!5
      true
      iex> !![1,2]
      true
      iex> !!"foo"
      true

  Falsy values (of which there are exactly two):

      iex> !!false
      false
      iex> !!nil
      false

  """

  # We need this check only for bootstrap purposes.
  # Once Kernel is loaded and we recompile, it is a no-op.
  @compile {:inline, bootstrapped?: 1}
  case :code.ensure_loaded(Kernel) do
    {:module, _} ->
      defp bootstrapped?(_), do: true

    {:error, _} ->
      defp bootstrapped?(module), do: :code.ensure_loaded(module) == {:module, module}
  end

  ## Delegations to Erlang with inlining (macros)

  @doc """
  Returns an integer or float which is the arithmetical absolute value of `number`.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> abs(-3.33)
      3.33

      iex> abs(-3)
      3

  """
  @doc guard: true
  @spec abs(number) :: number
  def abs(number) do
    :erlang.abs(number)
  end

  @doc """
  Invokes the given anonymous function `fun` with the list of
  arguments `args`.

  Inlined by the compiler.

  ## Examples

      iex> apply(fn x -> x * 2 end, [2])
      4

  """
  @spec apply(fun, [any]) :: any
  def apply(fun, args) do
    :erlang.apply(fun, args)
  end

  @doc """
  Invokes the given function from `module` with the list of
  arguments `args`.

  `apply/3` is used to invoke functions where the module, function
  name or arguments are defined dynamically at runtime. For this
  reason, you can't invoke macros using `apply/3`, only functions.

  Inlined by the compiler.

  ## Examples

      iex> apply(Enum, :reverse, [[1, 2, 3]])
      [3, 2, 1]

  """
  @spec apply(module, function_name :: atom, [any]) :: any
  def apply(module, function_name, args) do
    :erlang.apply(module, function_name, args)
  end

  @doc """
  Extracts the part of the binary starting at `start` with length `length`.
  Binaries are zero-indexed.

  If `start` or `length` reference in any way outside the binary, an
  `ArgumentError` exception is raised.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> binary_part("foo", 1, 2)
      "oo"

  A negative `length` can be used to extract bytes that come *before* the byte
  at `start`:

      iex> binary_part("Hello", 5, -3)
      "llo"

  """
  @doc guard: true
  @spec binary_part(binary, non_neg_integer, integer) :: binary
  def binary_part(binary, start, length) do
    :erlang.binary_part(binary, start, length)
  end

  @doc """
  Returns an integer which is the size in bits of `bitstring`.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> bit_size(<<433::16, 3::3>>)
      19

      iex> bit_size(<<1, 2, 3>>)
      24

  """
  @doc guard: true
  @spec bit_size(bitstring) :: non_neg_integer
  def bit_size(bitstring) do
    :erlang.bit_size(bitstring)
  end

  @doc """
  Returns the number of bytes needed to contain `bitstring`.

  That is, if the number of bits in `bitstring` is not divisible by 8, the
  resulting number of bytes will be rounded up (by excess). This operation
  happens in constant time.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> byte_size(<<433::16, 3::3>>)
      3

      iex> byte_size(<<1, 2, 3>>)
      3

  """
  @doc guard: true
  @spec byte_size(bitstring) :: non_neg_integer
  def byte_size(bitstring) do
    :erlang.byte_size(bitstring)
  end

  @doc """
  Returns the smallest integer greater than or equal to `number`.

  If you want to perform ceil operation on other decimal places,
  use `Float.ceil/2` instead.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc since: "1.8.0", guard: true
  @spec ceil(number) :: integer
  def ceil(number) do
    :erlang.ceil(number)
  end

  @doc """
  Performs an integer division.

  Raises an `ArithmeticError` exception if one of the arguments is not an
  integer, or when the `divisor` is `0`.

  `div/2` performs *truncated* integer division. This means that
  the result is always rounded towards zero.

  If you want to perform floored integer division (rounding towards negative infinity),
  use `Integer.floor_div/2` instead.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      div(5, 2)
      #=> 2

      div(6, -4)
      #=> -1

      div(-99, 2)
      #=> -49

      div(100, 0)
      ** (ArithmeticError) bad argument in arithmetic expression

  """
  @doc guard: true
  @spec div(integer, neg_integer | pos_integer) :: integer
  def div(dividend, divisor) do
    :erlang.div(dividend, divisor)
  end

  @doc """
  Stops the execution of the calling process with the given reason.

  Since evaluating this function causes the process to terminate,
  it has no return value.

  Inlined by the compiler.

  ## Examples

  When a process reaches its end, by default it exits with
  reason `:normal`. You can also call `exit/1` explicitly if you
  want to terminate a process but not signal any failure:

      exit(:normal)

  In case something goes wrong, you can also use `exit/1` with
  a different reason:

      exit(:seems_bad)

  If the exit reason is not `:normal`, all the processes linked to the process
  that exited will crash (unless they are trapping exits).

  ## OTP exits

  Exits are used by the OTP to determine if a process exited abnormally
  or not. The following exits are considered "normal":

    * `exit(:normal)`
    * `exit(:shutdown)`
    * `exit({:shutdown, term})`

  Exiting with any other reason is considered abnormal and treated
  as a crash. This means the default supervisor behaviour kicks in,
  error reports are emitted, and so forth.

  This behaviour is relied on in many different places. For example,
  `ExUnit` uses `exit(:shutdown)` when exiting the test process to
  signal linked processes, supervision trees and so on to politely
  shut down too.

  ## CLI exits

  Building on top of the exit signals mentioned above, if the
  process started by the command line exits with any of the three
  reasons above, its exit is considered normal and the Operating
  System process will exit with status 0.

  It is, however, possible to customize the operating system exit
  signal by invoking:

      exit({:shutdown, integer})

  This will cause the operating system process to exit with the status given by
  `integer` while signaling all linked Erlang processes to politely
  shut down.

  Any other exit reason will cause the operating system process to exit with
  status `1` and linked Erlang processes to crash.
  """
  @spec exit(term) :: no_return
  def exit(reason) do
    :erlang.exit(reason)
  end

  @doc """
  Returns the largest integer smaller than or equal to `number`.

  If you want to perform floor operation on other decimal places,
  use `Float.floor/2` instead.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc since: "1.8.0", guard: true
  @spec floor(number) :: integer
  def floor(number) do
    :erlang.floor(number)
  end

  @doc """
  Returns the head of a list. Raises `ArgumentError` if the list is empty.

  It works with improper lists.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      hd([1, 2, 3, 4])
      #=> 1

      hd([])
      ** (ArgumentError) argument error

      hd([1 | 2])
      #=> 1

  """
  @doc guard: true
  @spec hd(nonempty_maybe_improper_list(elem, any)) :: elem when elem: term
  def hd(list) do
    :erlang.hd(list)
  end

  @doc """
  Returns `true` if `term` is an atom; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true
  @spec is_atom(term) :: boolean
  def is_atom(term) do
    :erlang.is_atom(term)
  end

  @doc """
  Returns `true` if `term` is a binary; otherwise returns `false`.

  A binary always contains a complete number of bytes.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> is_binary("foo")
      true
      iex> is_binary(<<1::3>>)
      false

  """
  @doc guard: true
  @spec is_binary(term) :: boolean
  def is_binary(term) do
    :erlang.is_binary(term)
  end

  @doc """
  Returns `true` if `term` is a bitstring (including a binary); otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> is_bitstring("foo")
      true
      iex> is_bitstring(<<1::3>>)
      true

  """
  @doc guard: true
  @spec is_bitstring(term) :: boolean
  def is_bitstring(term) do
    :erlang.is_bitstring(term)
  end

  @doc """
  Returns `true` if `term` is either the atom `true` or the atom `false` (i.e.,
  a boolean); otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true
  @spec is_boolean(term) :: boolean
  def is_boolean(term) do
    :erlang.is_boolean(term)
  end

  @doc """
  Returns `true` if `term` is a floating-point number; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true
  @spec is_float(term) :: boolean
  def is_float(term) do
    :erlang.is_float(term)
  end

  @doc """
  Returns `true` if `term` is a function; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true
  @spec is_function(term) :: boolean
  def is_function(term) do
    :erlang.is_function(term)
  end

  @doc """
  Returns `true` if `term` is a function that can be applied with `arity` number of arguments;
  otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> is_function(fn x -> x * 2 end, 1)
      true
      iex> is_function(fn x -> x * 2 end, 2)
      false

  """
  @doc guard: true
  @spec is_function(term, non_neg_integer) :: boolean
  def is_function(term, arity) do
    :erlang.is_function(term, arity)
  end

  @doc """
  Returns `true` if `term` is an integer; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true
  @spec is_integer(term) :: boolean
  def is_integer(term) do
    :erlang.is_integer(term)
  end

  @doc """
  Returns `true` if `term` is a list with zero or more elements; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true
  @spec is_list(term) :: boolean
  def is_list(term) do
    :erlang.is_list(term)
  end

  @doc """
  Returns `true` if `term` is either an integer or a floating-point number;
  otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true
  @spec is_number(term) :: boolean
  def is_number(term) do
    :erlang.is_number(term)
  end

  @doc """
  Returns `true` if `term` is a PID (process identifier); otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true
  @spec is_pid(term) :: boolean
  def is_pid(term) do
    :erlang.is_pid(term)
  end

  @doc """
  Returns `true` if `term` is a port identifier; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true
  @spec is_port(term) :: boolean
  def is_port(term) do
    :erlang.is_port(term)
  end

  @doc """
  Returns `true` if `term` is a reference; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true
  @spec is_reference(term) :: boolean
  def is_reference(term) do
    :erlang.is_reference(term)
  end

  @doc """
  Returns `true` if `term` is a tuple; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true
  @spec is_tuple(term) :: boolean
  def is_tuple(term) do
    :erlang.is_tuple(term)
  end

  @doc """
  Returns `true` if `term` is a map; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true
  @spec is_map(term) :: boolean
  def is_map(term) do
    :erlang.is_map(term)
  end

  @doc """
  Returns `true` if `key` is a key in `map`; otherwise returns `false`.

  It raises `BadMapError` if the first element is not a map.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true, since: "1.10.0"
  @spec is_map_key(map, term) :: boolean
  def is_map_key(map, key) do
    :erlang.is_map_key(key, map)
  end

  @doc """
  Returns the length of `list`.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> length([1, 2, 3, 4, 5, 6, 7, 8, 9])
      9

  """
  @doc guard: true
  @spec length(list) :: non_neg_integer
  def length(list) do
    :erlang.length(list)
  end

  @doc """
  Returns an almost unique reference.

  The returned reference will re-occur after approximately 2^82 calls;
  therefore it is unique enough for practical purposes.

  Inlined by the compiler.

  ## Examples

      make_ref()
      #=> #Reference<0.0.0.135>

  """
  @spec make_ref() :: reference
  def make_ref() do
    :erlang.make_ref()
  end

  @doc """
  Returns the size of a map.

  The size of a map is the number of key-value pairs that the map contains.

  This operation happens in constant time.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> map_size(%{a: "foo", b: "bar"})
      2

  """
  @doc guard: true
  @spec map_size(map) :: non_neg_integer
  def map_size(map) do
    :erlang.map_size(map)
  end

  @doc """
  Returns the biggest of the two given terms according to
  Erlang's term ordering.

  If the terms compare equal, the first one is returned.

  Inlined by the compiler.

  ## Examples

      iex> max(1, 2)
      2
      iex> max(:a, :b)
      :b

  Using Erlang's term ordering means that comparisons are
  structural and not semantic. For example, when comparing dates:

      iex> max(~D[2017-03-31], ~D[2017-04-01])
      ~D[2017-03-31]

  In the example above, `max/2` returned March 31st instead of April 1st
  because the structural comparison compares the day before the year. In
  such cases it is common for modules to provide functions such as
  `Date.compare/2` that perform semantic comparison.
  """
  @spec max(first, second) :: first | second when first: term, second: term
  def max(first, second) do
    :erlang.max(first, second)
  end

  @doc """
  Returns the smallest of the two given terms according to
  Erlang's term ordering.

  If the terms compare equal, the first one is returned.

  Inlined by the compiler.

  ## Examples

      iex> min(1, 2)
      1
      iex> min("foo", "bar")
      "bar"

  Using Erlang's term ordering means that comparisons are
  structural and not semantic. For example, when comparing dates:

      iex> min(~D[2017-03-31], ~D[2017-04-01])
      ~D[2017-04-01]

  In the example above, `min/2` returned April 1st instead of March 31st
  because the structural comparison compares the day before the year. In
  such cases it is common for modules to provide functions such as
  `Date.compare/2` that perform semantic comparison.
  """
  @spec min(first, second) :: first | second when first: term, second: term
  def min(first, second) do
    :erlang.min(first, second)
  end

  @doc """
  Returns an atom representing the name of the local node.
  If the node is not alive, `:nonode@nohost` is returned instead.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true
  @spec node() :: node
  def node do
    :erlang.node()
  end

  @doc """
  Returns the node where the given argument is located.
  The argument can be a PID, a reference, or a port.
  If the local node is not alive, `:nonode@nohost` is returned.

  Allowed in guard tests. Inlined by the compiler.
  """
  @doc guard: true
  @spec node(pid | reference | port) :: node
  def node(arg) do
    :erlang.node(arg)
  end

  @doc """
  Computes the remainder of an integer division.

  `rem/2` uses truncated division, which means that
  the result will always have the sign of the `dividend`.

  Raises an `ArithmeticError` exception if one of the arguments is not an
  integer, or when the `divisor` is `0`.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> rem(5, 2)
      1
      iex> rem(6, -4)
      2

  """
  @doc guard: true
  @spec rem(integer, neg_integer | pos_integer) :: integer
  def rem(dividend, divisor) do
    :erlang.rem(dividend, divisor)
  end

  @doc """
  Rounds a number to the nearest integer.

  If the number is equidistant to the two nearest integers, rounds away from zero.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> round(5.6)
      6

      iex> round(5.2)
      5

      iex> round(-9.9)
      -10

      iex> round(-9)
      -9

      iex> round(2.5)
      3

      iex> round(-2.5)
      -3

  """
  @doc guard: true
  @spec round(number) :: integer
  def round(number) do
    :erlang.round(number)
  end

  @doc """
  Sends a message to the given `dest` and returns the message.

  `dest` may be a remote or local PID, a local port, a locally
  registered name, or a tuple in the form of `{registered_name, node}` for a
  registered name at another node.

  Inlined by the compiler.

  ## Examples

      iex> send(self(), :hello)
      :hello

  """
  @spec send(dest :: Process.dest(), message) :: message when message: any
  def send(dest, message) do
    :erlang.send(dest, message)
  end

  @doc """
  Returns the PID (process identifier) of the calling process.

  Allowed in guard clauses. Inlined by the compiler.
  """
  @doc guard: true
  @spec self() :: pid
  def self() do
    :erlang.self()
  end

  @doc """
  Spawns the given function and returns its PID.

  Typically developers do not use the `spawn` functions, instead they use
  abstractions such as `Task`, `GenServer` and `Agent`, built on top of
  `spawn`, that spawns processes with more conveniences in terms of
  introspection and debugging.

  Check the `Process` module for more process-related functions.

  The anonymous function receives 0 arguments, and may return any value.

  Inlined by the compiler.

  ## Examples

      current = self()
      child = spawn(fn -> send(current, {self(), 1 + 2}) end)

      receive do
        {^child, 3} -> IO.puts("Received 3 back")
      end

  """
  @spec spawn((() -> any)) :: pid
  def spawn(fun) do
    :erlang.spawn(fun)
  end

  @doc """
  Spawns the given function `fun` from the given `module` passing it the given
  `args` and returns its PID.

  Typically developers do not use the `spawn` functions, instead they use
  abstractions such as `Task`, `GenServer` and `Agent`, built on top of
  `spawn`, that spawns processes with more conveniences in terms of
  introspection and debugging.

  Check the `Process` module for more process-related functions.

  Inlined by the compiler.

  ## Examples

      spawn(SomeModule, :function, [1, 2, 3])

  """
  @spec spawn(module, atom, list) :: pid
  def spawn(module, fun, args) do
    :erlang.spawn(module, fun, args)
  end

  @doc """
  Spawns the given function, links it to the current process, and returns its PID.

  Typically developers do not use the `spawn` functions, instead they use
  abstractions such as `Task`, `GenServer` and `Agent`, built on top of
  `spawn`, that spawns processes with more conveniences in terms of
  introspection and debugging.

  Check the `Process` module for more process-related functions. For more
  information on linking, check `Process.link/1`.

  The anonymous function receives 0 arguments, and may return any value.

  Inlined by the compiler.

  ## Examples

      current = self()
      child = spawn_link(fn -> send(current, {self(), 1 + 2}) end)

      receive do
        {^child, 3} -> IO.puts("Received 3 back")
      end

  """
  @spec spawn_link((() -> any)) :: pid
  def spawn_link(fun) do
    :erlang.spawn_link(fun)
  end

  @doc """
  Spawns the given function `fun` from the given `module` passing it the given
  `args`, links it to the current process, and returns its PID.

  Typically developers do not use the `spawn` functions, instead they use
  abstractions such as `Task`, `GenServer` and `Agent`, built on top of
  `spawn`, that spawns processes with more conveniences in terms of
  introspection and debugging.

  Check the `Process` module for more process-related functions. For more
  information on linking, check `Process.link/1`.

  Inlined by the compiler.

  ## Examples

      spawn_link(SomeModule, :function, [1, 2, 3])

  """
  @spec spawn_link(module, atom, list) :: pid
  def spawn_link(module, fun, args) do
    :erlang.spawn_link(module, fun, args)
  end

  @doc """
  Spawns the given function, monitors it and returns its PID
  and monitoring reference.

  Typically developers do not use the `spawn` functions, instead they use
  abstractions such as `Task`, `GenServer` and `Agent`, built on top of
  `spawn`, that spawns processes with more conveniences in terms of
  introspection and debugging.

  Check the `Process` module for more process-related functions.

  The anonymous function receives 0 arguments, and may return any value.

  Inlined by the compiler.

  ## Examples

      current = self()
      spawn_monitor(fn -> send(current, {self(), 1 + 2}) end)

  """
  @spec spawn_monitor((() -> any)) :: {pid, reference}
  def spawn_monitor(fun) do
    :erlang.spawn_monitor(fun)
  end

  @doc """
  Spawns the given module and function passing the given args,
  monitors it and returns its PID and monitoring reference.

  Typically developers do not use the `spawn` functions, instead they use
  abstractions such as `Task`, `GenServer` and `Agent`, built on top of
  `spawn`, that spawns processes with more conveniences in terms of
  introspection and debugging.

  Check the `Process` module for more process-related functions.

  Inlined by the compiler.

  ## Examples

      spawn_monitor(SomeModule, :function, [1, 2, 3])

  """
  @spec spawn_monitor(module, atom, list) :: {pid, reference}
  def spawn_monitor(module, fun, args) do
    :erlang.spawn_monitor(module, fun, args)
  end

  @doc """
  A non-local return from a function.

  Check `Kernel.SpecialForms.try/1` for more information.

  Inlined by the compiler.
  """
  @spec throw(term) :: no_return
  def throw(term) do
    :erlang.throw(term)
  end

  @doc """
  Returns the tail of a list. Raises `ArgumentError` if the list is empty.

  It works with improper lists.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      tl([1, 2, 3, :go])
      #=> [2, 3, :go]

      tl([])
      ** (ArgumentError) argument error

      tl([:one])
      #=> []

      tl([:a, :b | :c])
      #=> [:b | :c]

      tl([:a | %{b: 1}])
      #=> %{b: 1}

  """
  @doc guard: true
  @spec tl(nonempty_maybe_improper_list(elem, tail)) :: maybe_improper_list(elem, tail) | tail
        when elem: term, tail: term
  def tl(list) do
    :erlang.tl(list)
  end

  @doc """
  Returns the integer part of `number`.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> trunc(5.4)
      5

      iex> trunc(-5.99)
      -5

      iex> trunc(-5)
      -5

  """
  @doc guard: true
  @spec trunc(number) :: integer
  def trunc(number) do
    :erlang.trunc(number)
  end

  @doc """
  Returns the size of a tuple.

  This operation happens in constant time.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> tuple_size({:a, :b, :c})
      3

  """
  @doc guard: true
  @spec tuple_size(tuple) :: non_neg_integer
  def tuple_size(tuple) do
    :erlang.tuple_size(tuple)
  end

  @doc """
  Arithmetic addition.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 + 2
      3

  """
  @doc guard: true
  @spec integer + integer :: integer
  @spec float + float :: float
  @spec integer + float :: float
  @spec float + integer :: float
  def left + right do
    :erlang.+(left, right)
  end

  @doc """
  Arithmetic subtraction.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 - 2
      -1

  """
  @doc guard: true
  @spec integer - integer :: integer
  @spec float - float :: float
  @spec integer - float :: float
  @spec float - integer :: float
  def left - right do
    :erlang.-(left, right)
  end

  @doc """
  Arithmetic unary plus.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> +1
      1

  """
  @doc guard: true
  @spec +integer :: integer
  @spec +float :: float
  def +value do
    :erlang.+(value)
  end

  @doc """
  Arithmetic unary minus.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> -2
      -2

  """
  @doc guard: true
  @spec -0 :: 0
  @spec -pos_integer :: neg_integer
  @spec -neg_integer :: pos_integer
  @spec -float :: float
  def -value do
    :erlang.-(value)
  end

  @doc """
  Arithmetic multiplication.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 * 2
      2

  """
  @doc guard: true
  @spec integer * integer :: integer
  @spec float * float :: float
  @spec integer * float :: float
  @spec float * integer :: float
  def left * right do
    :erlang.*(left, right)
  end

  @doc """
  Arithmetic division.

  The result is always a float. Use `div/2` and `rem/2` if you want
  an integer division or the remainder.

  Raises `ArithmeticError` if `right` is 0 or 0.0.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      1 / 2
      #=> 0.5

      -3.0 / 2.0
      #=> -1.5

      5 / 1
      #=> 5.0

      7 / 0
      ** (ArithmeticError) bad argument in arithmetic expression

  """
  @doc guard: true
  @spec number / number :: float
  def left / right do
    :erlang./(left, right)
  end

  @doc """
  Concatenates a proper list and a term, returning a list.

  The complexity of `a ++ b` is proportional to `length(a)`, so avoid repeatedly
  appending to lists of arbitrary length, for example, `list ++ [element]`.
  Instead, consider prepending via `[element | rest]` and then reversing.

  If the `right` operand is not a proper list, it returns an improper list.
  If the `left` operand is not a proper list, it raises `ArgumentError`.

  Inlined by the compiler.

  ## Examples

      iex> [1] ++ [2, 3]
      [1, 2, 3]

      iex> 'foo' ++ 'bar'
      'foobar'

      # returns an improper list
      iex> [1] ++ 2
      [1 | 2]

      # returns a proper list
      iex> [1] ++ [2]
      [1, 2]

      # improper list on the right will return an improper list
      iex> [1] ++ [2 | 3]
      [1, 2 | 3]

  """
  @spec list ++ term :: maybe_improper_list
  def left ++ right do
    :erlang.++(left, right)
  end

  @doc """
  Removes the first occurrence of an element on the left list
  for each element on the right.

  The complexity of `a -- b` is proportional to `length(a) * length(b)`,
  meaning that it will be very slow if both `a` and `b` are long lists.
  In such cases, consider converting each list to a `MapSet` and using
  `MapSet.difference/2`.

  Inlined by the compiler.

  ## Examples

      iex> [1, 2, 3] -- [1, 2]
      [3]

      iex> [1, 2, 3, 2, 1] -- [1, 2, 2]
      [3, 1]

  """
  @spec list -- list :: list
  def left -- right do
    :erlang.--(left, right)
  end

  @doc """
  Boolean not.

  `arg` must be a boolean; if it's not, an `ArgumentError` exception is raised.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> not false
      true

  """
  @doc guard: true
  @spec not true :: false
  @spec not false :: true
  def not value do
    :erlang.not(value)
  end

  @doc """
  Returns `true` if left is less than right.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 < 2
      true

  """
  @doc guard: true
  @spec term < term :: boolean
  def left < right do
    :erlang.<(left, right)
  end

  @doc """
  Returns `true` if left is more than right.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 > 2
      false

  """
  @doc guard: true
  @spec term > term :: boolean
  def left > right do
    :erlang.>(left, right)
  end

  @doc """
  Returns `true` if left is less than or equal to right.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 <= 2
      true

  """
  @doc guard: true
  @spec term <= term :: boolean
  def left <= right do
    :erlang."=<"(left, right)
  end

  @doc """
  Returns `true` if left is more than or equal to right.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 >= 2
      false

  """
  @doc guard: true
  @spec term >= term :: boolean
  def left >= right do
    :erlang.>=(left, right)
  end

  @doc """
  Returns `true` if the two terms are equal.

  This operator considers 1 and 1.0 to be equal. For stricter
  semantics, use `===/2` instead.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 == 2
      false

      iex> 1 == 1.0
      true

  """
  @doc guard: true
  @spec term == term :: boolean
  def left == right do
    :erlang.==(left, right)
  end

  @doc """
  Returns `true` if the two terms are not equal.

  This operator considers 1 and 1.0 to be equal. For match
  comparison, use `!==/2` instead.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 != 2
      true

      iex> 1 != 1.0
      false

  """
  @doc guard: true
  @spec term != term :: boolean
  def left != right do
    :erlang."/="(left, right)
  end

  @doc """
  Returns `true` if the two terms are exactly equal.

  The terms are only considered to be exactly equal if they
  have the same value and are of the same type. For example,
  `1 == 1.0` returns `true`, but since they are of different
  types, `1 === 1.0` returns `false`.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 === 2
      false

      iex> 1 === 1.0
      false

  """
  @doc guard: true
  @spec term === term :: boolean
  def left === right do
    :erlang."=:="(left, right)
  end

  @doc """
  Returns `true` if the two terms are not exactly equal.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 !== 2
      true

      iex> 1 !== 1.0
      true

  """
  @doc guard: true
  @spec term !== term :: boolean
  def left !== right do
    :erlang."=/="(left, right)
  end

  @doc """
  Gets the element at the zero-based `index` in `tuple`.

  It raises `ArgumentError` when index is negative or it is out of range of the tuple elements.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      tuple = {:foo, :bar, 3}
      elem(tuple, 1)
      #=> :bar

      elem({}, 0)
      ** (ArgumentError) argument error

      elem({:foo, :bar}, 2)
      ** (ArgumentError) argument error

  """
  @doc guard: true
  @spec elem(tuple, non_neg_integer) :: term
  def elem(tuple, index) do
    :erlang.element(index + 1, tuple)
  end

  @doc """
  Puts `value` at the given zero-based `index` in `tuple`.

  Inlined by the compiler.

  ## Examples

      iex> tuple = {:foo, :bar, 3}
      iex> put_elem(tuple, 0, :baz)
      {:baz, :bar, 3}

  """
  @spec put_elem(tuple, non_neg_integer, term) :: tuple
  def put_elem(tuple, index, value) do
    :erlang.setelement(index + 1, tuple, value)
  end

  ## Implemented in Elixir

  defp optimize_boolean({:case, meta, args}) do
    {:case, [{:optimize_boolean, true} | meta], args}
  end

  @doc """
  Boolean or.

  If `left` is `true`, returns `true`; otherwise returns `right`.

  Requires only the `left` operand to be a boolean since it short-circuits.
  If the `left` operand is not a boolean, an `ArgumentError` exception is
  raised.

  Allowed in guard tests.

  ## Examples

      iex> true or false
      true
      iex> false or 42
      42

  """
  @doc guard: true
  defmacro left or right do
    case __CALLER__.context do
      nil -> build_boolean_check(:or, left, true, right)
      :match -> invalid_match!(:or)
      :guard -> quote(do: :erlang.orelse(unquote(left), unquote(right)))
    end
  end

  @doc """
  Boolean and.

  If `left` is `false`, returns `false`; otherwise returns `right`.

  Requires only the `left` operand to be a boolean since it short-circuits. If
  the `left` operand is not a boolean, an `ArgumentError` exception is raised.

  Allowed in guard tests.

  ## Examples

      iex> true and false
      false
      iex> true and "yay!"
      "yay!"

  """
  @doc guard: true
  defmacro left and right do
    case __CALLER__.context do
      nil -> build_boolean_check(:and, left, right, false)
      :match -> invalid_match!(:and)
      :guard -> quote(do: :erlang.andalso(unquote(left), unquote(right)))
    end
  end

  defp build_boolean_check(operator, check, true_clause, false_clause) do
    optimize_boolean(
      quote do
        case unquote(check) do
          false -> unquote(false_clause)
          true -> unquote(true_clause)
          other -> :erlang.error({:badbool, unquote(operator), other})
        end
      end
    )
  end

  @doc """
  Boolean not.

  Receives any argument (not just booleans) and returns `true` if the argument
  is `false` or `nil`; returns `false` otherwise.

  Not allowed in guard clauses.

  ## Examples

      iex> !Enum.empty?([])
      false

      iex> !List.first([])
      true

  """
  defmacro !value

  defmacro !{:!, _, [value]} do
    assert_no_match_or_guard_scope(__CALLER__.context, "!")

    optimize_boolean(
      quote do
        case unquote(value) do
          x when :"Elixir.Kernel".in(x, [false, nil]) -> false
          _ -> true
        end
      end
    )
  end

  defmacro !value do
    assert_no_match_or_guard_scope(__CALLER__.context, "!")

    optimize_boolean(
      quote do
        case unquote(value) do
          x when :"Elixir.Kernel".in(x, [false, nil]) -> true
          _ -> false
        end
      end
    )
  end

  @doc """
  Concatenates two binaries.

  ## Examples

      iex> "foo" <> "bar"
      "foobar"

  The `<>/2` operator can also be used in pattern matching (and guard clauses) as
  long as the left argument is a literal binary:

      iex> "foo" <> x = "foobar"
      iex> x
      "bar"

  `x <> "bar" = "foobar"` would have resulted in a `CompileError` exception.

  """
  defmacro left <> right do
    concats = extract_concatenations({:<>, [], [left, right]}, __CALLER__)
    quote(do: <<unquote_splicing(concats)>>)
  end

  # Extracts concatenations in order to optimize many
  # concatenations into one single clause.
  defp extract_concatenations({:<>, _, [left, right]}, caller) do
    [wrap_concatenation(left, :left, caller) | extract_concatenations(right, caller)]
  end

  defp extract_concatenations(other, caller) do
    [wrap_concatenation(other, :right, caller)]
  end

  defp wrap_concatenation(binary, _side, _caller) when is_binary(binary) do
    binary
  end

  defp wrap_concatenation(literal, _side, _caller)
       when is_list(literal) or is_atom(literal) or is_integer(literal) or is_float(literal) do
    :erlang.error(
      ArgumentError.exception(
        "expected binary argument in <> operator but got: #{Macro.to_string(literal)}"
      )
    )
  end

  defp wrap_concatenation(other, side, caller) do
    expanded = expand_concat_argument(other, side, caller)
    {:"::", [], [expanded, {:binary, [], nil}]}
  end

  defp expand_concat_argument(arg, :left, %{context: :match} = caller) do
    expanded_arg =
      case bootstrapped?(Macro) do
        true -> Macro.expand(arg, caller)
        false -> arg
      end

    case expanded_arg do
      {var, _, nil} when is_atom(var) ->
        invalid_concat_left_argument_error(Atom.to_string(var))

      {:^, _, [{var, _, nil}]} when is_atom(var) ->
        invalid_concat_left_argument_error("^#{Atom.to_string(var)}")

      _ ->
        expanded_arg
    end
  end

  defp expand_concat_argument(arg, _, _) do
    arg
  end

  defp invalid_concat_left_argument_error(arg) do
    :erlang.error(
      ArgumentError.exception(
        "the left argument of <> operator inside a match should always be a literal " <>
          "binary because its size can't be verified. Got: #{arg}"
      )
    )
  end

  @doc """
  Raises an exception.

  If the argument `msg` is a binary, it raises a `RuntimeError` exception
  using the given argument as message.

  If `msg` is an atom, it just calls `raise/2` with the atom as the first
  argument and `[]` as the second argument.

  If `msg` is an exception struct, it is raised as is.

  If `msg` is anything else, `raise` will fail with an `ArgumentError`
  exception.

  ## Examples

      iex> raise "oops"
      ** (RuntimeError) oops

      try do
        1 + :foo
      rescue
        x in [ArithmeticError] ->
          IO.puts("that was expected")
          raise x
      end

  """
  defmacro raise(message) do
    # Try to figure out the type at compilation time
    # to avoid dead code and make Dialyzer happy.
    message =
      case not is_binary(message) and bootstrapped?(Macro) do
        true -> Macro.expand(message, __CALLER__)
        false -> message
      end

    case message do
      message when is_binary(message) ->
        quote do
          :erlang.error(RuntimeError.exception(unquote(message)))
        end

      {:<<>>, _, _} = message ->
        quote do
          :erlang.error(RuntimeError.exception(unquote(message)))
        end

      alias when is_atom(alias) ->
        quote do
          :erlang.error(unquote(alias).exception([]))
        end

      _ ->
        quote do
          :erlang.error(Kernel.Utils.raise(unquote(message)))
        end
    end
  end

  @doc """
  Raises an exception.

  Calls the `exception/1` function on the given argument (which has to be a
  module name like `ArgumentError` or `RuntimeError`) passing `attrs` as the
  attributes in order to retrieve the exception struct.

  Any module that contains a call to the `defexception/1` macro automatically
  implements the `c:Exception.exception/1` callback expected by `raise/2`.
  For more information, see `defexception/1`.

  ## Examples

      iex> raise(ArgumentError, "Sample")
      ** (ArgumentError) Sample

  """
  defmacro raise(exception, attributes) do
    quote do
      :erlang.error(unquote(exception).exception(unquote(attributes)))
    end
  end

  @doc """
  Raises an exception preserving a previous stacktrace.

  Works like `raise/1` but does not generate a new stacktrace.

  Notice that `__STACKTRACE__` can be used inside catch/rescue
  to retrieve the current stacktrace.

  ## Examples

      try do
        raise "oops"
      rescue
        exception ->
          reraise exception, __STACKTRACE__
      end

  """
  defmacro reraise(message, stacktrace) do
    # Try to figure out the type at compilation time
    # to avoid dead code and make Dialyzer happy.
    case Macro.expand(message, __CALLER__) do
      message when is_binary(message) ->
        quote do
          :erlang.error(
            :erlang.raise(:error, RuntimeError.exception(unquote(message)), unquote(stacktrace))
          )
        end

      {:<<>>, _, _} = message ->
        quote do
          :erlang.error(
            :erlang.raise(:error, RuntimeError.exception(unquote(message)), unquote(stacktrace))
          )
        end

      alias when is_atom(alias) ->
        quote do
          :erlang.error(:erlang.raise(:error, unquote(alias).exception([]), unquote(stacktrace)))
        end

      message ->
        quote do
          :erlang.error(
            :erlang.raise(:error, Kernel.Utils.raise(unquote(message)), unquote(stacktrace))
          )
        end
    end
  end

  @doc """
  Raises an exception preserving a previous stacktrace.

  `reraise/3` works like `reraise/2`, except it passes arguments to the
  `exception/1` function as explained in `raise/2`.

  ## Examples

      try do
        raise "oops"
      rescue
        exception ->
          reraise WrapperError, [exception: exception], __STACKTRACE__
      end

  """
  defmacro reraise(exception, attributes, stacktrace) do
    quote do
      :erlang.raise(
        :error,
        unquote(exception).exception(unquote(attributes)),
        unquote(stacktrace)
      )
    end
  end

  @doc """
  Matches the term on the `left` against the regular expression or string on the
  `right`.

  Returns `true` if `left` matches `right` (if it's a regular expression)
  or contains `right` (if it's a string).

  ## Examples

      iex> "abcd" =~ ~r/c(d)/
      true

      iex> "abcd" =~ ~r/e/
      false

      iex> "abcd" =~ "bc"
      true

      iex> "abcd" =~ "ad"
      false

      iex> "abcd" =~ ""
      true

  """
  @spec String.t() =~ (String.t() | Regex.t()) :: boolean
  def left =~ "" when is_binary(left), do: true

  def left =~ right when is_binary(left) and is_binary(right) do
    :binary.match(left, right) != :nomatch
  end

  def left =~ right when is_binary(left) do
    Regex.match?(right, left)
  end

  @doc ~S"""
  Inspects the given argument according to the `Inspect` protocol.
  The second argument is a keyword list with options to control
  inspection.

  ## Options

  `inspect/2` accepts a list of options that are internally
  translated to an `Inspect.Opts` struct. Check the docs for
  `Inspect.Opts` to see the supported options.

  ## Examples

      iex> inspect(:foo)
      ":foo"

      iex> inspect([1, 2, 3, 4, 5], limit: 3)
      "[1, 2, 3, ...]"

      iex> inspect([1, 2, 3], pretty: true, width: 0)
      "[1,\n 2,\n 3]"

      iex> inspect("olá" <> <<0>>)
      "<<111, 108, 195, 161, 0>>"

      iex> inspect("olá" <> <<0>>, binaries: :as_strings)
      "\"olá\\0\""

      iex> inspect("olá", binaries: :as_binaries)
      "<<111, 108, 195, 161>>"

      iex> inspect('bar')
      "'bar'"

      iex> inspect([0 | 'bar'])
      "[0, 98, 97, 114]"

      iex> inspect(100, base: :octal)
      "0o144"

      iex> inspect(100, base: :hex)
      "0x64"

  Note that the `Inspect` protocol does not necessarily return a valid
  representation of an Elixir term. In such cases, the inspected result
  must start with `#`. For example, inspecting a function will return:

      inspect(fn a, b -> a + b end)
      #=> #Function<...>

  The `Inspect` protocol can be derived to hide certain fields
  from structs, so they don't show up in logs, inspects and similar.
  See the "Deriving" section of the documentation of the `Inspect`
  protocol for more information.
  """
  @spec inspect(Inspect.t(), keyword) :: String.t()
  def inspect(term, opts \\ []) when is_list(opts) do
    opts = struct(Inspect.Opts, opts)

    limit =
      case opts.pretty do
        true -> opts.width
        false -> :infinity
      end

    doc = Inspect.Algebra.group(Inspect.Algebra.to_doc(term, opts))
    IO.iodata_to_binary(Inspect.Algebra.format(doc, limit))
  end

  @doc """
  Creates and updates a struct.

  The `struct` argument may be an atom (which defines `defstruct`)
  or a `struct` itself. The second argument is any `Enumerable` that
  emits two-element tuples (key-value pairs) during enumeration.

  Keys in the `Enumerable` that don't exist in the struct are automatically
  discarded. Note that keys must be atoms, as only atoms are allowed when
  defining a struct. If keys in the `Enumerable` are duplicated, the last
  entry will be taken (same behaviour as `Map.new/1`).

  This function is useful for dynamically creating and updating structs, as
  well as for converting maps to structs; in the latter case, just inserting
  the appropriate `:__struct__` field into the map may not be enough and
  `struct/2` should be used instead.

  ## Examples

      defmodule User do
        defstruct name: "john"
      end

      struct(User)
      #=> %User{name: "john"}

      opts = [name: "meg"]
      user = struct(User, opts)
      #=> %User{name: "meg"}

      struct(user, unknown: "value")
      #=> %User{name: "meg"}

      struct(User, %{name: "meg"})
      #=> %User{name: "meg"}

      # String keys are ignored
      struct(User, %{"name" => "meg"})
      #=> %User{name: "john"}

  """
  @spec struct(module | struct, Enum.t()) :: struct
  def struct(struct, fields \\ []) do
    struct(struct, fields, fn
      {:__struct__, _val}, acc ->
        acc

      {key, val}, acc ->
        case acc do
          %{^key => _} -> %{acc | key => val}
          _ -> acc
        end
    end)
  end

  @doc """
  Similar to `struct/2` but checks for key validity.

  The function `struct!/2` emulates the compile time behaviour
  of structs. This means that:

    * when building a struct, as in `struct!(SomeStruct, key: :value)`,
      it is equivalent to `%SomeStruct{key: :value}` and therefore this
      function will check if every given key-value belongs to the struct.
      If the struct is enforcing any key via `@enforce_keys`, those will
      be enforced as well;

    * when updating a struct, as in `struct!(%SomeStruct{}, key: :value)`,
      it is equivalent to `%SomeStruct{struct | key: :value}` and therefore this
      function will check if every given key-value belongs to the struct.
      However, updating structs does not enforce keys, as keys are enforced
      only when building;

  """
  @spec struct!(module | struct, Enum.t()) :: struct
  def struct!(struct, fields \\ [])

  def struct!(struct, fields) when is_atom(struct) do
    validate_struct!(struct.__struct__(fields), struct, 1)
  end

  def struct!(struct, fields) when is_map(struct) do
    struct(struct, fields, fn
      {:__struct__, _}, acc ->
        acc

      {key, val}, acc ->
        Map.replace!(acc, key, val)
    end)
  end

  defp struct(struct, [], _fun) when is_atom(struct) do
    validate_struct!(struct.__struct__(), struct, 0)
  end

  defp struct(struct, fields, fun) when is_atom(struct) do
    struct(validate_struct!(struct.__struct__(), struct, 0), fields, fun)
  end

  defp struct(%_{} = struct, [], _fun) do
    struct
  end

  defp struct(%_{} = struct, fields, fun) do
    Enum.reduce(fields, struct, fun)
  end

  defp validate_struct!(%{__struct__: module} = struct, module, _arity) do
    struct
  end

  defp validate_struct!(%{__struct__: struct_name}, module, arity) when is_atom(struct_name) do
    error_message =
      "expected struct name returned by #{inspect(module)}.__struct__/#{arity} to be " <>
        "#{inspect(module)}, got: #{inspect(struct_name)}"

    :erlang.error(ArgumentError.exception(error_message))
  end

  defp validate_struct!(expr, module, arity) do
    error_message =
      "expected #{inspect(module)}.__struct__/#{arity} to return a map with a :__struct__ " <>
        "key that holds the name of the struct (atom), got: #{inspect(expr)}"

    :erlang.error(ArgumentError.exception(error_message))
  end

  @doc """
  Returns true if `term` is a struct; otherwise returns `false`.

  Allowed in guard tests.

  ## Examples

      iex> is_struct(URI.parse("/"))
      true

      iex> is_struct(%{})
      false

  """
  @doc since: "1.10.0", guard: true
  defmacro is_struct(term) do
    case __CALLER__.context do
      nil ->
        quote do
          case unquote(term) do
            %_{} -> true
            _ -> false
          end
        end

      :match ->
        invalid_match!(:is_struct)

      :guard ->
        quote do
          is_map(unquote(term)) and :erlang.is_map_key(:__struct__, unquote(term)) and
            is_atom(:erlang.map_get(:__struct__, unquote(term)))
        end
    end
  end

  @doc """
  Gets a value from a nested structure.

  Uses the `Access` module to traverse the structures
  according to the given `keys`, unless the `key` is a
  function, which is detailed in a later section.

  ## Examples

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> get_in(users, ["john", :age])
      27

  In case any of the keys returns `nil`, `nil` will be returned:

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> get_in(users, ["unknown", :age])
      nil

  ## Functions as keys

  If a key is a function, the function will be invoked passing three
  arguments:

    * the operation (`:get`)
    * the data to be accessed
    * a function to be invoked next

  This means `get_in/2` can be extended to provide custom lookups.
  In the example below, we use a function to get all the maps inside
  a list:

      iex> users = [%{name: "john", age: 27}, %{name: "meg", age: 23}]
      iex> all = fn :get, data, next -> Enum.map(data, next) end
      iex> get_in(users, [all, :age])
      [27, 23]

  If the previous value before invoking the function is `nil`,
  the function *will* receive `nil` as a value and must handle it
  accordingly.

  The `Access` module ships with many convenience accessor functions,
  like the `all` anonymous function defined above. See `Access.all/0`,
  `Access.key/2`, and others as examples.
  """
  @spec get_in(Access.t(), nonempty_list(term)) :: term
  def get_in(data, keys)

  def get_in(data, [h]) when is_function(h), do: h.(:get, data, & &1)
  def get_in(data, [h | t]) when is_function(h), do: h.(:get, data, &get_in(&1, t))

  def get_in(nil, [_]), do: nil
  def get_in(nil, [_ | t]), do: get_in(nil, t)

  def get_in(data, [h]), do: Access.get(data, h)
  def get_in(data, [h | t]), do: get_in(Access.get(data, h), t)

  @doc """
  Puts a value in a nested structure.

  Uses the `Access` module to traverse the structures
  according to the given `keys`, unless the `key` is a
  function. If the key is a function, it will be invoked
  as specified in `get_and_update_in/3`.

  ## Examples

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> put_in(users, ["john", :age], 28)
      %{"john" => %{age: 28}, "meg" => %{age: 23}}

  In case any of the entries in the middle returns `nil`,
  an error will be raised when trying to access it next.
  """
  @spec put_in(Access.t(), nonempty_list(term), term) :: Access.t()
  def put_in(data, [_ | _] = keys, value) do
    elem(get_and_update_in(data, keys, fn _ -> {nil, value} end), 1)
  end

  @doc """
  Updates a key in a nested structure.

  Uses the `Access` module to traverse the structures
  according to the given `keys`, unless the `key` is a
  function. If the key is a function, it will be invoked
  as specified in `get_and_update_in/3`.

  ## Examples

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> update_in(users, ["john", :age], &(&1 + 1))
      %{"john" => %{age: 28}, "meg" => %{age: 23}}

  In case any of the entries in the middle returns `nil`,
  an error will be raised when trying to access it next.
  """
  @spec update_in(Access.t(), nonempty_list(term), (term -> term)) :: Access.t()
  def update_in(data, [_ | _] = keys, fun) when is_function(fun) do
    elem(get_and_update_in(data, keys, fn x -> {nil, fun.(x)} end), 1)
  end

  @doc """
  Gets a value and updates a nested structure.

  `data` is a nested structure (that is, a map, keyword
  list, or struct that implements the `Access` behaviour).

  The `fun` argument receives the value of `key` (or `nil` if `key`
  is not present) and must return one of the following values:

    * a two-element tuple `{get_value, new_value}`. In this case,
      `get_value` is the retrieved value which can possibly be operated on before
      being returned. `new_value` is the new value to be stored under `key`.

    * `:pop`, which implies that the current value under `key`
      should be removed from the structure and returned.

  This function uses the `Access` module to traverse the structures
  according to the given `keys`, unless the `key` is a function,
  which is detailed in a later section.

  ## Examples

  This function is useful when there is a need to retrieve the current
  value (or something calculated in function of the current value) and
  update it at the same time. For example, it could be used to read the
  current age of a user while increasing it by one in one pass:

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> get_and_update_in(users, ["john", :age], &{&1, &1 + 1})
      {27, %{"john" => %{age: 28}, "meg" => %{age: 23}}}

  ## Functions as keys

  If a key is a function, the function will be invoked passing three
  arguments:

    * the operation (`:get_and_update`)
    * the data to be accessed
    * a function to be invoked next

  This means `get_and_update_in/3` can be extended to provide custom
  lookups. The downside is that functions cannot be stored as keys
  in the accessed data structures.

  When one of the keys is a function, the function is invoked.
  In the example below, we use a function to get and increment all
  ages inside a list:

      iex> users = [%{name: "john", age: 27}, %{name: "meg", age: 23}]
      iex> all = fn :get_and_update, data, next ->
      ...>   data |> Enum.map(next) |> Enum.unzip()
      ...> end
      iex> get_and_update_in(users, [all, :age], &{&1, &1 + 1})
      {[27, 23], [%{name: "john", age: 28}, %{name: "meg", age: 24}]}

  If the previous value before invoking the function is `nil`,
  the function *will* receive `nil` as a value and must handle it
  accordingly (be it by failing or providing a sane default).

  The `Access` module ships with many convenience accessor functions,
  like the `all` anonymous function defined above. See `Access.all/0`,
  `Access.key/2`, and others as examples.
  """
  @spec get_and_update_in(
          structure :: Access.t(),
          keys,
          (term -> {get_value, update_value} | :pop)
        ) :: {get_value, structure :: Access.t()}
        when keys: nonempty_list(any),
             get_value: var,
             update_value: term
  def get_and_update_in(data, keys, fun)

  def get_and_update_in(data, [head], fun) when is_function(head, 3),
    do: head.(:get_and_update, data, fun)

  def get_and_update_in(data, [head | tail], fun) when is_function(head, 3),
    do: head.(:get_and_update, data, &get_and_update_in(&1, tail, fun))

  def get_and_update_in(data, [head], fun) when is_function(fun, 1),
    do: Access.get_and_update(data, head, fun)

  def get_and_update_in(data, [head | tail], fun) when is_function(fun, 1),
    do: Access.get_and_update(data, head, &get_and_update_in(&1, tail, fun))

  @doc """
  Pops a key from the given nested structure.

  Uses the `Access` protocol to traverse the structures
  according to the given `keys`, unless the `key` is a
  function. If the key is a function, it will be invoked
  as specified in `get_and_update_in/3`.

  ## Examples

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> pop_in(users, ["john", :age])
      {27, %{"john" => %{}, "meg" => %{age: 23}}}

  In case any entry returns `nil`, its key will be removed
  and the deletion will be considered a success.

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> pop_in(users, ["jane", :age])
      {nil, %{"john" => %{age: 27}, "meg" => %{age: 23}}}

  """
  @spec pop_in(data, nonempty_list(Access.get_and_update_fun(term, data) | term)) :: {term, data}
        when data: Access.container()
  def pop_in(data, keys)

  def pop_in(nil, [key | _]) do
    raise ArgumentError, "could not pop key #{inspect(key)} on a nil value"
  end

  def pop_in(data, [_ | _] = keys) do
    pop_in_data(data, keys)
  end

  defp pop_in_data(nil, [_ | _]), do: :pop

  defp pop_in_data(data, [fun]) when is_function(fun),
    do: fun.(:get_and_update, data, fn _ -> :pop end)

  defp pop_in_data(data, [fun | tail]) when is_function(fun),
    do: fun.(:get_and_update, data, &pop_in_data(&1, tail))

  defp pop_in_data(data, [key]), do: Access.pop(data, key)

  defp pop_in_data(data, [key | tail]),
    do: Access.get_and_update(data, key, &pop_in_data(&1, tail))

  @doc """
  Puts a value in a nested structure via the given `path`.

  This is similar to `put_in/3`, except the path is extracted via
  a macro rather than passing a list. For example:

      put_in(opts[:foo][:bar], :baz)

  Is equivalent to:

      put_in(opts, [:foo, :bar], :baz)

  This also works with nested structs and the `struct.path.to.value` way to specify
  paths:

      put_in(struct.foo.bar, :baz)

  Note that in order for this macro to work, the complete path must always
  be visible by this macro. For more information about the supported path
  expressions, please check `get_and_update_in/2` docs.

  ## Examples

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> put_in(users["john"][:age], 28)
      %{"john" => %{age: 28}, "meg" => %{age: 23}}

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> put_in(users["john"].age, 28)
      %{"john" => %{age: 28}, "meg" => %{age: 23}}

  """
  defmacro put_in(path, value) do
    case unnest(path, [], true, "put_in/2") do
      {[h | t], true} ->
        nest_update_in(h, t, quote(do: fn _ -> unquote(value) end))

      {[h | t], false} ->
        expr = nest_get_and_update_in(h, t, quote(do: fn _ -> {nil, unquote(value)} end))
        quote(do: :erlang.element(2, unquote(expr)))
    end
  end

  @doc """
  Pops a key from the nested structure via the given `path`.

  This is similar to `pop_in/2`, except the path is extracted via
  a macro rather than passing a list. For example:

      pop_in(opts[:foo][:bar])

  Is equivalent to:

      pop_in(opts, [:foo, :bar])

  Note that in order for this macro to work, the complete path must always
  be visible by this macro. For more information about the supported path
  expressions, please check `get_and_update_in/2` docs.

  ## Examples

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> pop_in(users["john"][:age])
      {27, %{"john" => %{}, "meg" => %{age: 23}}}

      iex> users = %{john: %{age: 27}, meg: %{age: 23}}
      iex> pop_in(users.john[:age])
      {27, %{john: %{}, meg: %{age: 23}}}

  In case any entry returns `nil`, its key will be removed
  and the deletion will be considered a success.
  """
  defmacro pop_in(path) do
    {[h | t], _} = unnest(path, [], true, "pop_in/1")
    nest_pop_in(:map, h, t)
  end

  @doc """
  Updates a nested structure via the given `path`.

  This is similar to `update_in/3`, except the path is extracted via
  a macro rather than passing a list. For example:

      update_in(opts[:foo][:bar], &(&1 + 1))

  Is equivalent to:

      update_in(opts, [:foo, :bar], &(&1 + 1))

  This also works with nested structs and the `struct.path.to.value` way to specify
  paths:

      update_in(struct.foo.bar, &(&1 + 1))

  Note that in order for this macro to work, the complete path must always
  be visible by this macro. For more information about the supported path
  expressions, please check `get_and_update_in/2` docs.

  ## Examples

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> update_in(users["john"][:age], &(&1 + 1))
      %{"john" => %{age: 28}, "meg" => %{age: 23}}

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> update_in(users["john"].age, &(&1 + 1))
      %{"john" => %{age: 28}, "meg" => %{age: 23}}

  """
  defmacro update_in(path, fun) do
    case unnest(path, [], true, "update_in/2") do
      {[h | t], true} ->
        nest_update_in(h, t, fun)

      {[h | t], false} ->
        expr = nest_get_and_update_in(h, t, quote(do: fn x -> {nil, unquote(fun).(x)} end))
        quote(do: :erlang.element(2, unquote(expr)))
    end
  end

  @doc """
  Gets a value and updates a nested data structure via the given `path`.

  This is similar to `get_and_update_in/3`, except the path is extracted
  via a macro rather than passing a list. For example:

      get_and_update_in(opts[:foo][:bar], &{&1, &1 + 1})

  Is equivalent to:

      get_and_update_in(opts, [:foo, :bar], &{&1, &1 + 1})

  This also works with nested structs and the `struct.path.to.value` way to specify
  paths:

      get_and_update_in(struct.foo.bar, &{&1, &1 + 1})

  Note that in order for this macro to work, the complete path must always
  be visible by this macro. See the "Paths" section below.

  ## Examples

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> get_and_update_in(users["john"].age, &{&1, &1 + 1})
      {27, %{"john" => %{age: 28}, "meg" => %{age: 23}}}

  ## Paths

  A path may start with a variable, local or remote call, and must be
  followed by one or more:

    * `foo[bar]` - accesses the key `bar` in `foo`; in case `foo` is nil,
      `nil` is returned

    * `foo.bar` - accesses a map/struct field; in case the field is not
      present, an error is raised

  Here are some valid paths:

      users["john"][:age]
      users["john"].age
      User.all()["john"].age
      all_users()["john"].age

  Here are some invalid ones:

      # Does a remote call after the initial value
      users["john"].do_something(arg1, arg2)

      # Does not access any key or field
      users

  """
  defmacro get_and_update_in(path, fun) do
    {[h | t], _} = unnest(path, [], true, "get_and_update_in/2")
    nest_get_and_update_in(h, t, fun)
  end

  defp nest_update_in([], fun), do: fun

  defp nest_update_in(list, fun) do
    quote do
      fn x -> unquote(nest_update_in(quote(do: x), list, fun)) end
    end
  end

  defp nest_update_in(h, [{:map, key} | t], fun) do
    quote do
      Map.update!(unquote(h), unquote(key), unquote(nest_update_in(t, fun)))
    end
  end

  defp nest_get_and_update_in([], fun), do: fun

  defp nest_get_and_update_in(list, fun) do
    quote do
      fn x -> unquote(nest_get_and_update_in(quote(do: x), list, fun)) end
    end
  end

  defp nest_get_and_update_in(h, [{:access, key} | t], fun) do
    quote do
      Access.get_and_update(unquote(h), unquote(key), unquote(nest_get_and_update_in(t, fun)))
    end
  end

  defp nest_get_and_update_in(h, [{:map, key} | t], fun) do
    quote do
      Map.get_and_update!(unquote(h), unquote(key), unquote(nest_get_and_update_in(t, fun)))
    end
  end

  defp nest_pop_in(kind, list) do
    quote do
      fn x -> unquote(nest_pop_in(kind, quote(do: x), list)) end
    end
  end

  defp nest_pop_in(:map, h, [{:access, key}]) do
    quote do
      case unquote(h) do
        nil -> {nil, nil}
        h -> Access.pop(h, unquote(key))
      end
    end
  end

  defp nest_pop_in(_, _, [{:map, key}]) do
    raise ArgumentError,
          "cannot use pop_in when the last segment is a map/struct field. " <>
            "This would effectively remove the field #{inspect(key)} from the map/struct"
  end

  defp nest_pop_in(_, h, [{:map, key} | t]) do
    quote do
      Map.get_and_update!(unquote(h), unquote(key), unquote(nest_pop_in(:map, t)))
    end
  end

  defp nest_pop_in(_, h, [{:access, key}]) do
    quote do
      case unquote(h) do
        nil -> :pop
        h -> Access.pop(h, unquote(key))
      end
    end
  end

  defp nest_pop_in(_, h, [{:access, key} | t]) do
    quote do
      Access.get_and_update(unquote(h), unquote(key), unquote(nest_pop_in(:access, t)))
    end
  end

  defp unnest({{:., _, [Access, :get]}, _, [expr, key]}, acc, _all_map?, kind) do
    unnest(expr, [{:access, key} | acc], false, kind)
  end

  defp unnest({{:., _, [expr, key]}, _, []}, acc, all_map?, kind)
       when is_tuple(expr) and :erlang.element(1, expr) != :__aliases__ and
              :erlang.element(1, expr) != :__MODULE__ do
    unnest(expr, [{:map, key} | acc], all_map?, kind)
  end

  defp unnest(other, [], _all_map?, kind) do
    raise ArgumentError,
          "expected expression given to #{kind} to access at least one element, " <>
            "got: #{Macro.to_string(other)}"
  end

  defp unnest(other, acc, all_map?, kind) do
    case proper_start?(other) do
      true ->
        {[other | acc], all_map?}

      false ->
        raise ArgumentError,
              "expression given to #{kind} must start with a variable, local or remote call " <>
                "and be followed by an element access, got: #{Macro.to_string(other)}"
    end
  end

  defp proper_start?({{:., _, [expr, _]}, _, _args})
       when is_atom(expr)
       when :erlang.element(1, expr) == :__aliases__
       when :erlang.element(1, expr) == :__MODULE__,
       do: true

  defp proper_start?({atom, _, _args})
       when is_atom(atom),
       do: true

  defp proper_start?(other), do: not is_tuple(other)

  @doc """
  Converts the argument to a string according to the
  `String.Chars` protocol.

  This is the function invoked when there is string interpolation.

  ## Examples

      iex> to_string(:foo)
      "foo"

  """
  defmacro to_string(term) do
    quote(do: :"Elixir.String.Chars".to_string(unquote(term)))
  end

  @doc """
  Converts the given term to a charlist according to the `List.Chars` protocol.

  ## Examples

      iex> to_charlist(:foo)
      'foo'

  """
  defmacro to_charlist(term) do
    quote(do: List.Chars.to_charlist(unquote(term)))
  end

  @doc """
  Returns `true` if `term` is `nil`, `false` otherwise.

  Allowed in guard clauses.

  ## Examples

      iex> is_nil(1)
      false

      iex> is_nil(nil)
      true

  """
  @doc guard: true
  defmacro is_nil(term) do
    quote(do: unquote(term) == nil)
  end

  @doc """
  A convenience macro that checks if the right side (an expression) matches the
  left side (a pattern).

  ## Examples

      iex> match?(1, 1)
      true

      iex> match?({1, _}, {1, 2})
      true

      iex> map = %{a: 1, b: 2}
      iex> match?(%{a: _}, map)
      true

      iex> a = 1
      iex> match?(^a, 1)
      true

  `match?/2` is very useful when filtering or finding a value in an enumerable:

      iex> list = [a: 1, b: 2, a: 3]
      iex> Enum.filter(list, &match?({:a, _}, &1))
      [a: 1, a: 3]

  Guard clauses can also be given to the match:

      iex> list = [a: 1, b: 2, a: 3]
      iex> Enum.filter(list, &match?({:a, x} when x < 2, &1))
      [a: 1]

  However, variables assigned in the match will not be available
  outside of the function call (unlike regular pattern matching with the `=`
  operator):

      iex> match?(_x, 1)
      true
      iex> binding()
      []

  """
  defmacro match?(pattern, expr) do
    success =
      quote do
        unquote(pattern) -> true
      end

    failure =
      quote generated: true do
        _ -> false
      end

    {:case, [], [expr, [do: success ++ failure]]}
  end

  @doc """
  Reads and writes attributes of the current module.

  The canonical example for attributes is annotating that a module
  implements an OTP behaviour, such as `GenServer`:

      defmodule MyServer do
        @behaviour GenServer
        # ... callbacks ...
      end

  By default Elixir supports all the module attributes supported by Erlang, but
  custom attributes can be used as well:

      defmodule MyServer do
        @my_data 13
        IO.inspect(@my_data)
        #=> 13
      end

  Unlike Erlang, such attributes are not stored in the module by default since
  it is common in Elixir to use custom attributes to store temporary data that
  will be available at compile-time. Custom attributes may be configured to
  behave closer to Erlang by using `Module.register_attribute/3`.

  Finally, notice that attributes can also be read inside functions:

      defmodule MyServer do
        @my_data 11
        def first_data, do: @my_data
        @my_data 13
        def second_data, do: @my_data
      end

      MyServer.first_data()
      #=> 11

      MyServer.second_data()
      #=> 13

  It is important to note that reading an attribute takes a snapshot of
  its current value. In other words, the value is read at compilation
  time and not at runtime. Check the `Module` module for other functions
  to manipulate module attributes.
  """
  defmacro @expr

  defmacro @{:__aliases__, _meta, _args} do
    raise ArgumentError, "module attributes set via @ cannot start with an uppercase letter"
  end

  defmacro @{name, meta, args} do
    assert_module_scope(__CALLER__, :@, 1)
    function? = __CALLER__.function != nil

    cond do
      # Check for Macro as it is compiled later than Kernel
      not bootstrapped?(Macro) ->
        nil

      not function? and __CALLER__.context == :match ->
        raise ArgumentError,
              "invalid write attribute syntax, you probably meant to use: @#{name} expression"

      # Typespecs attributes are currently special cased by the compiler
      is_list(args) and typespec?(name) ->
        case bootstrapped?(Kernel.Typespec) do
          false ->
            :ok

          true ->
            pos = :elixir_locals.cache_env(__CALLER__)
            %{line: line, file: file, module: module} = __CALLER__

            quote do
              Kernel.Typespec.deftypespec(
                unquote(name),
                unquote(Macro.escape(hd(args), unquote: true)),
                unquote(line),
                unquote(file),
                unquote(module),
                unquote(pos)
              )
            end
        end

      true ->
        do_at(args, meta, name, function?, __CALLER__)
    end
  end

  # @attribute(value)
  defp do_at([arg], meta, name, function?, env) do
    line =
      case :lists.keymember(:context, 1, meta) do
        true -> nil
        false -> env.line
      end

    cond do
      function? ->
        raise ArgumentError, "cannot set attribute @#{name} inside function/macro"

      name == :behavior ->
        warn_message = "@behavior attribute is not supported, please use @behaviour instead"
        IO.warn(warn_message, Macro.Env.stacktrace(env))

      :lists.member(name, [:moduledoc, :typedoc, :doc]) ->
        arg = {env.line, arg}

        quote do
          Module.__put_attribute__(__MODULE__, unquote(name), unquote(arg), unquote(line))
        end

      true ->
        quote do
          Module.__put_attribute__(__MODULE__, unquote(name), unquote(arg), unquote(line))
        end
    end
  end

  # @attribute or @attribute()
  defp do_at(args, _meta, name, function?, env) when is_atom(args) or args == [] do
    line = env.line
    doc_attr? = :lists.member(name, [:moduledoc, :typedoc, :doc])

    case function? do
      true ->
        value =
          case Module.__get_attribute__(env.module, name, line) do
            {_, doc} when doc_attr? -> doc
            other -> other
          end

        try do
          :elixir_quote.escape(value, :default, false)
        rescue
          ex in [ArgumentError] ->
            raise ArgumentError,
                  "cannot inject attribute @#{name} into function/macro because " <>
                    Exception.message(ex)
        end

      false when doc_attr? ->
        quote do
          case Module.__get_attribute__(__MODULE__, unquote(name), unquote(line)) do
            {_, doc} -> doc
            other -> other
          end
        end

      false ->
        quote do
          Module.__get_attribute__(__MODULE__, unquote(name), unquote(line))
        end
    end
  end

  # All other cases
  defp do_at(args, _meta, name, _function?, _env) do
    raise ArgumentError, "expected 0 or 1 argument for @#{name}, got: #{length(args)}"
  end

  defp typespec?(:type), do: true
  defp typespec?(:typep), do: true
  defp typespec?(:opaque), do: true
  defp typespec?(:spec), do: true
  defp typespec?(:callback), do: true
  defp typespec?(:macrocallback), do: true
  defp typespec?(_), do: false

  @doc """
  Returns the binding for the given context as a keyword list.

  In the returned result, keys are variable names and values are the
  corresponding variable values.

  If the given `context` is `nil` (by default it is), the binding for the
  current context is returned.

  ## Examples

      iex> x = 1
      iex> binding()
      [x: 1]
      iex> x = 2
      iex> binding()
      [x: 2]

      iex> binding(:foo)
      []
      iex> var!(x, :foo) = 1
      1
      iex> binding(:foo)
      [x: 1]

  """
  defmacro binding(context \\ nil) do
    in_match? = Macro.Env.in_match?(__CALLER__)

    bindings =
      for {v, c} <- Macro.Env.vars(__CALLER__), c == context do
        {v, wrap_binding(in_match?, {v, [generated: true], c})}
      end

    :lists.sort(bindings)
  end

  defp wrap_binding(true, var) do
    quote(do: ^unquote(var))
  end

  defp wrap_binding(_, var) do
    var
  end

  @doc """
  Provides an `if/2` macro.

  This macro expects the first argument to be a condition and the second
  argument to be a keyword list.

  ## One-liner examples

      if(foo, do: bar)

  In the example above, `bar` will be returned if `foo` evaluates to
  a truthy value (neither `false` nor `nil`). Otherwise, `nil` will be
  returned.

  An `else` option can be given to specify the opposite:

      if(foo, do: bar, else: baz)

  ## Blocks examples

  It's also possible to pass a block to the `if/2` macro. The first
  example above would be translated to:

      if foo do
        bar
      end

  Note that `do/end` become delimiters. The second example would
  translate to:

      if foo do
        bar
      else
        baz
      end

  In order to compare more than two clauses, the `cond/1` macro has to be used.
  """
  defmacro if(condition, clauses) do
    build_if(condition, clauses)
  end

  defp build_if(condition, do: do_clause) do
    build_if(condition, do: do_clause, else: nil)
  end

  defp build_if(condition, do: do_clause, else: else_clause) do
    optimize_boolean(
      quote do
        case unquote(condition) do
          x when :"Elixir.Kernel".in(x, [false, nil]) -> unquote(else_clause)
          _ -> unquote(do_clause)
        end
      end
    )
  end

  defp build_if(_condition, _arguments) do
    raise ArgumentError,
          "invalid or duplicate keys for if, only \"do\" and an optional \"else\" are permitted"
  end

  @doc """
  Provides an `unless` macro.

  This macro evaluates and returns the `do` block passed in as the second
  argument if `condition` evaluates to a falsy value (`false` or `nil`).
  Otherwise, it returns the value of the `else` block if present or `nil` if not.

  See also `if/2`.

  ## Examples

      iex> unless(Enum.empty?([]), do: "Hello")
      nil

      iex> unless(Enum.empty?([1, 2, 3]), do: "Hello")
      "Hello"

      iex> unless Enum.sum([2, 2]) == 5 do
      ...>   "Math still works"
      ...> else
      ...>   "Math is broken"
      ...> end
      "Math still works"

  """
  defmacro unless(condition, clauses) do
    build_unless(condition, clauses)
  end

  defp build_unless(condition, do: do_clause) do
    build_unless(condition, do: do_clause, else: nil)
  end

  defp build_unless(condition, do: do_clause, else: else_clause) do
    quote do
      if(unquote(condition), do: unquote(else_clause), else: unquote(do_clause))
    end
  end

  defp build_unless(_condition, _arguments) do
    raise ArgumentError,
          "invalid or duplicate keys for unless, " <>
            "only \"do\" and an optional \"else\" are permitted"
  end

  @doc """
  Destructures two lists, assigning each term in the
  right one to the matching term in the left one.

  Unlike pattern matching via `=`, if the sizes of the left
  and right lists don't match, destructuring simply stops
  instead of raising an error.

  ## Examples

      iex> destructure([x, y, z], [1, 2, 3, 4, 5])
      iex> {x, y, z}
      {1, 2, 3}

  In the example above, even though the right list has more entries than the
  left one, destructuring works fine. If the right list is smaller, the
  remaining elements are simply set to `nil`:

      iex> destructure([x, y, z], [1])
      iex> {x, y, z}
      {1, nil, nil}

  The left-hand side supports any expression you would use
  on the left-hand side of a match:

      x = 1
      destructure([^x, y, z], [1, 2, 3])

  The example above will only work if `x` matches the first value in the right
  list. Otherwise, it will raise a `MatchError` (like the `=` operator would
  do).
  """
  defmacro destructure(left, right) when is_list(left) do
    quote do
      unquote(left) = Kernel.Utils.destructure(unquote(right), unquote(length(left)))
    end
  end

  @doc """
  Returns a range with the specified `first` and `last` integers.

  If last is larger than first, the range will be increasing from
  first to last. If first is larger than last, the range will be
  decreasing from first to last. If first is equal to last, the range
  will contain one element, which is the number itself.

  ## Examples

      iex> 0 in 1..3
      false

      iex> 1 in 1..3
      true

      iex> 2 in 1..3
      true

      iex> 3 in 1..3
      true

  """
  defmacro first..last do
    case bootstrapped?(Macro) do
      true ->
        first = Macro.expand(first, __CALLER__)
        last = Macro.expand(last, __CALLER__)
        range(__CALLER__.context, first, last)

      false ->
        range(__CALLER__.context, first, last)
    end
  end

  defp range(_context, first, last) when is_integer(first) and is_integer(last) do
    {:%{}, [], [__struct__: Elixir.Range, first: first, last: last]}
  end

  defp range(_context, first, last)
       when is_float(first) or is_float(last) or is_atom(first) or is_atom(last) or
              is_binary(first) or is_binary(last) or is_list(first) or is_list(last) do
    raise ArgumentError,
          "ranges (first..last) expect both sides to be integers, " <>
            "got: #{Macro.to_string({:.., [], [first, last]})}"
  end

  defp range(nil, first, last) do
    quote(do: Elixir.Range.new(unquote(first), unquote(last)))
  end

  defp range(_, first, last) do
    {:%{}, [], [__struct__: Elixir.Range, first: first, last: last]}
  end

  @doc """
  Provides a short-circuit operator that evaluates and returns
  the second expression only if the first one evaluates to a truthy value
  (neither `false` nor `nil`). Returns the first expression
  otherwise.

  Not allowed in guard clauses.

  ## Examples

      iex> Enum.empty?([]) && Enum.empty?([])
      true

      iex> List.first([]) && true
      nil

      iex> Enum.empty?([]) && List.first([1])
      1

      iex> false && throw(:bad)
      false

  Note that, unlike `and/2`, this operator accepts any expression
  as the first argument, not only booleans.
  """
  defmacro left && right do
    assert_no_match_or_guard_scope(__CALLER__.context, "&&")

    quote do
      case unquote(left) do
        x when :"Elixir.Kernel".in(x, [false, nil]) ->
          x

        _ ->
          unquote(right)
      end
    end
  end

  @doc """
  Provides a short-circuit operator that evaluates and returns the second
  expression only if the first one does not evaluate to a truthy value (that is,
  it is either `nil` or `false`). Returns the first expression otherwise.

  Not allowed in guard clauses.

  ## Examples

      iex> Enum.empty?([1]) || Enum.empty?([1])
      false

      iex> List.first([]) || true
      true

      iex> Enum.empty?([1]) || 1
      1

      iex> Enum.empty?([]) || throw(:bad)
      true

  Note that, unlike `or/2`, this operator accepts any expression
  as the first argument, not only booleans.
  """
  defmacro left || right do
    assert_no_match_or_guard_scope(__CALLER__.context, "||")

    quote do
      case unquote(left) do
        x when :"Elixir.Kernel".in(x, [false, nil]) ->
          unquote(right)

        x ->
          x
      end
    end
  end

  @doc """
  Pipe operator.

  This operator introduces the expression on the left-hand side as
  the first argument to the function call on the right-hand side.

  ## Examples

      iex> [1, [2], 3] |> List.flatten()
      [1, 2, 3]

  The example above is the same as calling `List.flatten([1, [2], 3])`.

  The `|>` operator is mostly useful when there is a desire to execute a series
  of operations resembling a pipeline:

      iex> [1, [2], 3] |> List.flatten() |> Enum.map(fn x -> x * 2 end)
      [2, 4, 6]

  In the example above, the list `[1, [2], 3]` is passed as the first argument
  to the `List.flatten/1` function, then the flattened list is passed as the
  first argument to the `Enum.map/2` function which doubles each element of the
  list.

  In other words, the expression above simply translates to:

      Enum.map(List.flatten([1, [2], 3]), fn x -> x * 2 end)

  ## Pitfalls

  There are two common pitfalls when using the pipe operator.

  The first one is related to operator precedence. For example,
  the following expression:

      String.graphemes "Hello" |> Enum.reverse

  Translates to:

      String.graphemes("Hello" |> Enum.reverse())

  which results in an error as the `Enumerable` protocol is not defined
  for binaries. Adding explicit parentheses resolves the ambiguity:

      String.graphemes("Hello") |> Enum.reverse()

  Or, even better:

      "Hello" |> String.graphemes() |> Enum.reverse()

  The second pitfall is that the `|>` operator works on calls.
  For example, when you write:

      "Hello" |> some_function()

  Elixir sees the right-hand side is a function call and pipes
  to it. This means that, if you want to pipe to an anonymous
  or captured function, it must also be explicitly called.

  Given the anonymous function:

      fun = fn x -> IO.puts(x) end
      fun.("Hello")

  This won't work as it will rather try to invoke the local
  function `fun`:

      "Hello" |> fun()

  This works:

      "Hello" |> fun.()

  As you can see, the `|>` operator retains the same semantics
  as when the pipe is not used since both require the `fun.(...)`
  notation.
  """
  defmacro left |> right do
    [{h, _} | t] = Macro.unpipe({:|>, [], [left, right]})

    fun = fn {x, pos}, acc ->
      Macro.pipe(acc, x, pos)
    end

    :lists.foldl(fun, h, t)
  end

  @doc """
  Returns `true` if `module` is loaded and contains a
  public `function` with the given `arity`, otherwise `false`.

  Note that this function does not load the module in case
  it is not loaded. Check `Code.ensure_loaded/1` for more
  information.

  Inlined by the compiler.

  ## Examples

      iex> function_exported?(Enum, :map, 2)
      true

      iex> function_exported?(Enum, :map, 10)
      false

      iex> function_exported?(List, :to_string, 1)
      true
  """
  @spec function_exported?(module, atom, arity) :: boolean
  def function_exported?(module, function, arity) do
    :erlang.function_exported(module, function, arity)
  end

  @doc """
  Returns `true` if `module` is loaded and contains a
  public `macro` with the given `arity`, otherwise `false`.

  Note that this function does not load the module in case
  it is not loaded. Check `Code.ensure_loaded/1` for more
  information.

  If `module` is an Erlang module (as opposed to an Elixir module), this
  function always returns `false`.

  ## Examples

      iex> macro_exported?(Kernel, :use, 2)
      true

      iex> macro_exported?(:erlang, :abs, 1)
      false

  """
  @spec macro_exported?(module, atom, arity) :: boolean
  def macro_exported?(module, macro, arity)
      when is_atom(module) and is_atom(macro) and is_integer(arity) and
             (arity >= 0 and arity <= 255) do
    function_exported?(module, :__info__, 1) and
      :lists.member({macro, arity}, module.__info__(:macros))
  end

  @doc """
  Checks if the element on the left-hand side is a member of the
  collection on the right-hand side.

  ## Examples

      iex> x = 1
      iex> x in [1, 2, 3]
      true

  This operator (which is a macro) simply translates to a call to
  `Enum.member?/2`. The example above would translate to:

      Enum.member?([1, 2, 3], x)

  Elixir also supports `left not in right`, which evaluates to
  `not(left in right)`:

      iex> x = 1
      iex> x not in [1, 2, 3]
      false

  ## Guards

  The `in/2` operator (as well as `not in`) can be used in guard clauses as
  long as the right-hand side is a range or a list. In such cases, Elixir will
  expand the operator to a valid guard expression. For example:

      when x in [1, 2, 3]

  translates to:

      when x === 1 or x === 2 or x === 3

  When using ranges:

      when x in 1..3

  translates to:

      when is_integer(x) and x >= 1 and x <= 3

  Note that only integers can be considered inside a range by `in`.

  ### AST considerations

  `left not in right` is parsed by the compiler into the AST:

      {:not, _, [{:in, _, [left, right]}]}

  This is the same AST as `not(left in right)`.

  Additionally, `Macro.to_string/2` will translate all occurrences of
  this AST to `left not in right`.
  """
  @doc guard: true
  defmacro left in right do
    in_module? = __CALLER__.context == nil

    expand =
      case bootstrapped?(Macro) do
        true -> &Macro.expand(&1, __CALLER__)
        false -> & &1
      end

    case expand.(right) do
      [] when not in_module? ->
        false

      [] ->
        quote do
          _ = unquote(left)
          false
        end

      [head | tail] = list when not in_module? ->
        in_var(in_module?, left, &in_list(&1, head, tail, expand, list, in_module?))

      [_ | _] = list when in_module? ->
        case ensure_evaled(list, {0, []}, expand) do
          {[head | tail], {_, []}} ->
            in_var(in_module?, left, &in_list(&1, head, tail, expand, list, in_module?))

          {[head | tail], {_, vars_values}} ->
            {vars, values} = :lists.unzip(:lists.reverse(vars_values))
            is_in_list = &in_list(&1, head, tail, expand, list, in_module?)

            quote do
              {unquote_splicing(vars)} = {unquote_splicing(values)}
              unquote(in_var(in_module?, left, is_in_list))
            end
        end

      {:%{}, _meta, [__struct__: Elixir.Range, first: first, last: last]} ->
        in_var(in_module?, left, &in_range(&1, expand.(first), expand.(last)))

      right when in_module? ->
        quote(do: Elixir.Enum.member?(unquote(right), unquote(left)))

      %{__struct__: Elixir.Range, first: _, last: _} ->
        raise ArgumentError, "non-literal range in guard should be escaped with Macro.escape/2"

      right ->
        raise_on_invalid_args_in_2(right)
    end
  end

  defp raise_on_invalid_args_in_2(right) do
    raise ArgumentError, <<
      "invalid right argument for operator \"in\", it expects a compile-time proper list ",
      "or compile-time range on the right side when used in guard expressions, got: ",
      Macro.to_string(right)::binary
    >>
  end

  defp in_var(false, ast, fun), do: fun.(ast)

  defp in_var(true, {atom, _, context} = var, fun) when is_atom(atom) and is_atom(context),
    do: fun.(var)

  defp in_var(true, ast, fun) do
    quote do
      var = unquote(ast)
      unquote(fun.(quote(do: var)))
    end
  end

  # Called as ensure_evaled(list, {0, []}). Note acc is reversed.
  defp ensure_evaled(list, acc, expand) do
    fun = fn
      {:|, meta, [head, tail]}, acc ->
        {head, acc} = ensure_evaled_element(head, acc)
        {tail, acc} = ensure_evaled_tail(expand.(tail), acc, expand)
        {{:|, meta, [head, tail]}, acc}

      elem, acc ->
        ensure_evaled_element(elem, acc)
    end

    :lists.mapfoldl(fun, acc, list)
  end

  defp ensure_evaled_element(elem, acc)
       when is_number(elem) or is_atom(elem) or is_binary(elem) do
    {elem, acc}
  end

  defp ensure_evaled_element(elem, acc) do
    ensure_evaled_var(elem, acc)
  end

  defp ensure_evaled_tail(elem, acc, expand) when is_list(elem) do
    ensure_evaled(elem, acc, expand)
  end

  defp ensure_evaled_tail(elem, acc, _expand) do
    ensure_evaled_var(elem, acc)
  end

  defp ensure_evaled_var(elem, {index, ast}) do
    var = {String.to_atom("arg" <> Integer.to_string(index)), [], __MODULE__}
    {var, {index + 1, [{var, elem} | ast]}}
  end

  defp in_range(left, first, last) do
    case is_integer(first) and is_integer(last) do
      true ->
        in_range_literal(left, first, last)

      false ->
        quote do
          :erlang.is_integer(unquote(left)) and :erlang.is_integer(unquote(first)) and
            :erlang.is_integer(unquote(last)) and
            ((:erlang."=<"(unquote(first), unquote(last)) and
                unquote(increasing_compare(left, first, last))) or
               (:erlang.<(unquote(last), unquote(first)) and
                  unquote(decreasing_compare(left, first, last))))
        end
    end
  end

  defp in_range_literal(left, first, first) do
    quote do
      :erlang."=:="(unquote(left), unquote(first))
    end
  end

  defp in_range_literal(left, first, last) when first < last do
    quote do
      :erlang.andalso(
        :erlang.is_integer(unquote(left)),
        unquote(increasing_compare(left, first, last))
      )
    end
  end

  defp in_range_literal(left, first, last) do
    quote do
      :erlang.andalso(
        :erlang.is_integer(unquote(left)),
        unquote(decreasing_compare(left, first, last))
      )
    end
  end

  defp in_list(left, head, tail, expand, right, in_module?) do
    [head | tail] =
      :lists.foldl(
        &[comp(left, &1, expand, right, in_module?) | &2],
        [],
        [head | tail]
      )

    :lists.foldl(&quote(do: :erlang.orelse(unquote(&1), unquote(&2))), head, tail)
  end

  defp comp(left, {:|, _, [head, tail]}, expand, right, in_module?) do
    case expand.(tail) do
      [] ->
        quote(do: :erlang."=:="(unquote(left), unquote(head)))

      [tail_head | tail] ->
        quote do
          :erlang.orelse(
            :erlang."=:="(unquote(left), unquote(head)),
            unquote(in_list(left, tail_head, tail, expand, right, in_module?))
          )
        end

      tail when in_module? ->
        quote do
          :erlang.orelse(
            :erlang."=:="(unquote(left), unquote(head)),
            :lists.member(unquote(left), unquote(tail))
          )
        end

      _ ->
        raise_on_invalid_args_in_2(right)
    end
  end

  defp comp(left, right, _expand, _right, _in_module?) do
    quote(do: :erlang."=:="(unquote(left), unquote(right)))
  end

  defp increasing_compare(var, first, last) do
    quote do
      :erlang.andalso(
        :erlang.>=(unquote(var), unquote(first)),
        :erlang."=<"(unquote(var), unquote(last))
      )
    end
  end

  defp decreasing_compare(var, first, last) do
    quote do
      :erlang.andalso(
        :erlang."=<"(unquote(var), unquote(first)),
        :erlang.>=(unquote(var), unquote(last))
      )
    end
  end

  @doc """
  When used inside quoting, marks that the given variable should
  not be hygienized.

  The argument can be either a variable unquoted or in standard tuple form
  `{name, meta, context}`.

  Check `Kernel.SpecialForms.quote/2` for more information.
  """
  defmacro var!(var, context \\ nil)

  defmacro var!({name, meta, atom}, context) when is_atom(name) and is_atom(atom) do
    # Remove counter and force them to be vars
    meta = :lists.keydelete(:counter, 1, meta)
    meta = :lists.keystore(:var, 1, meta, {:var, true})

    case Macro.expand(context, __CALLER__) do
      context when is_atom(context) ->
        {name, meta, context}

      other ->
        raise ArgumentError,
              "expected var! context to expand to an atom, got: #{Macro.to_string(other)}"
    end
  end

  defmacro var!(other, _context) do
    raise ArgumentError, "expected a variable to be given to var!, got: #{Macro.to_string(other)}"
  end

  @doc """
  When used inside quoting, marks that the given alias should not
  be hygienized. This means the alias will be expanded when
  the macro is expanded.

  Check `Kernel.SpecialForms.quote/2` for more information.
  """
  defmacro alias!(alias) when is_atom(alias) do
    alias
  end

  defmacro alias!({:__aliases__, meta, args}) do
    # Simply remove the alias metadata from the node
    # so it does not affect expansion.
    {:__aliases__, :lists.keydelete(:alias, 1, meta), args}
  end

  ## Definitions implemented in Elixir

  @doc ~S"""
  Defines a module given by name with the given contents.

  This macro defines a module with the given `alias` as its name and with the
  given contents. It returns a tuple with four elements:

    * `:module`
    * the module name
    * the binary contents of the module
    * the result of evaluating the contents block

  ## Examples

      defmodule Number do
        def one, do: 1
        def two, do: 2
      end
      #=> {:module, Number, <<70, 79, 82, ...>>, {:two, 0}}

      Number.one()
      #=> 1

      Number.two()
      #=> 2

  ## Nesting

  Nesting a module inside another module affects the name of the nested module:

      defmodule Foo do
        defmodule Bar do
        end
      end

  In the example above, two modules - `Foo` and `Foo.Bar` - are created.
  When nesting, Elixir automatically creates an alias to the inner module,
  allowing the second module `Foo.Bar` to be accessed as `Bar` in the same
  lexical scope where it's defined (the `Foo` module). This only happens
  if the nested module is defined via an alias.

  If the `Foo.Bar` module is moved somewhere else, the references to `Bar` in
  the `Foo` module need to be updated to the fully-qualified name (`Foo.Bar`) or
  an alias has to be explicitly set in the `Foo` module with the help of
  `Kernel.SpecialForms.alias/2`.

      defmodule Foo.Bar do
        # code
      end

      defmodule Foo do
        alias Foo.Bar
        # code here can refer to "Foo.Bar" as just "Bar"
      end

  ## Dynamic names

  Elixir module names can be dynamically generated. This is very
  useful when working with macros. For instance, one could write:

      defmodule String.to_atom("Foo#{1}") do
        # contents ...
      end

  Elixir will accept any module name as long as the expression passed as the
  first argument to `defmodule/2` evaluates to an atom.
  Note that, when a dynamic name is used, Elixir won't nest the name under
  the current module nor automatically set up an alias.

  ## Reserved module names

  If you attempt to define a module that already exists, you will get a
  warning saying that a module has been redefined.

  There are some modules that Elixir does not currently implement but it
  may be implement in the future. Those modules are reserved and defining
  them will result in a compilation error:

      defmodule Any do
        # code
      end
      ** (CompileError) iex:1: module Any is reserved and cannot be defined

  Elixir reserves the following module names: `Elixir`, `Any`, `BitString`,
  `PID`, and `Reference`.
  """
  defmacro defmodule(alias, do_block)

  defmacro defmodule(alias, do: block) do
    env = __CALLER__
    boot? = bootstrapped?(Macro)

    expanded =
      case boot? do
        true -> Macro.expand(alias, env)
        false -> alias
      end

    {expanded, with_alias} =
      case boot? and is_atom(expanded) do
        true ->
          # Expand the module considering the current environment/nesting
          {full, old, new} = expand_module(alias, expanded, env)
          meta = [defined: full, context: env.module] ++ alias_meta(alias)
          {full, {:alias, meta, [old, [as: new, warn: false]]}}

        false ->
          {expanded, nil}
      end

    # We do this so that the block is not tail-call optimized and stacktraces
    # are not messed up. Basically, we just insert something between the return
    # value of the block and what is returned by defmodule. Using just ":ok" or
    # similar doesn't work because it's likely optimized away by the compiler.
    block =
      quote do
        result = unquote(block)
        :elixir_utils.noop()
        result
      end

    escaped =
      case env do
        %{function: nil, lexical_tracker: pid} when is_pid(pid) ->
          integer = Kernel.LexicalTracker.write_cache(pid, block)
          quote(do: Kernel.LexicalTracker.read_cache(unquote(pid), unquote(integer)))

        %{} ->
          :elixir_quote.escape(block, :default, false)
      end

    module_vars = :lists.map(&module_var/1, :maps.keys(elem(env.current_vars, 0)))

    quote do
      unquote(with_alias)
      :elixir_module.compile(unquote(expanded), unquote(escaped), unquote(module_vars), __ENV__)
    end
  end

  defp alias_meta({:__aliases__, meta, _}), do: meta
  defp alias_meta(_), do: []

  # defmodule Elixir.Alias
  defp expand_module({:__aliases__, _, [:"Elixir", _ | _]}, module, _env),
    do: {module, module, nil}

  # defmodule Alias in root
  defp expand_module({:__aliases__, _, _}, module, %{module: nil}),
    do: {module, module, nil}

  # defmodule Alias nested
  defp expand_module({:__aliases__, _, [h | t]}, _module, env) when is_atom(h) do
    module = :elixir_aliases.concat([env.module, h])
    alias = String.to_atom("Elixir." <> Atom.to_string(h))

    case t do
      [] -> {module, module, alias}
      _ -> {String.to_atom(Enum.join([module | t], ".")), module, alias}
    end
  end

  # defmodule _
  defp expand_module(_raw, module, _env) do
    {module, module, nil}
  end

  defp module_var({name, kind}) when is_atom(kind), do: {name, [generated: true], kind}
  defp module_var({name, kind}), do: {name, [counter: kind, generated: true], nil}

  @doc ~S"""
  Defines a public function with the given name and body.

  ## Examples

      defmodule Foo do
        def bar, do: :baz
      end

      Foo.bar()
      #=> :baz

  A function that expects arguments can be defined as follows:

      defmodule Foo do
        def sum(a, b) do
          a + b
        end
      end

  In the example above, a `sum/2` function is defined; this function receives
  two arguments and returns their sum.

  ## Default arguments

  `\\` is used to specify a default value for a parameter of a function. For
  example:

      defmodule MyMath do
        def multiply_by(number, factor \\ 2) do
          number * factor
        end
      end

      MyMath.multiply_by(4, 3)
      #=> 12

      MyMath.multiply_by(4)
      #=> 8

  The compiler translates this into multiple functions with different arities,
  here `MyMath.multiply_by/1` and `MyMath.multiply_by/2`, that represent cases when
  arguments for parameters with default values are passed or not passed.

  When defining a function with default arguments as well as multiple
  explicitly declared clauses, you must write a function head that declares the
  defaults. For example:

      defmodule MyString do
        def join(string1, string2 \\ nil, separator \\ " ")

        def join(string1, nil, _separator) do
          string1
        end

        def join(string1, string2, separator) do
          string1 <> separator <> string2
        end
      end

  Note that `\\` can't be used with anonymous functions because they
  can only have a sole arity.

  ## Function and variable names

  Function and variable names have the following syntax:
  A _lowercase ASCII letter_ or an _underscore_, followed by any number of
  _lowercase or uppercase ASCII letters_, _numbers_, or _underscores_.
  Optionally they can end in either an _exclamation mark_ or a _question mark_.

  For variables, any identifier starting with an underscore should indicate an
  unused variable. For example:

      def foo(bar) do
        []
      end
      #=> warning: variable bar is unused

      def foo(_bar) do
        []
      end
      #=> no warning

      def foo(_bar) do
        _bar
      end
      #=> warning: the underscored variable "_bar" is used after being set

  ## `rescue`/`catch`/`after`/`else`

  Function bodies support `rescue`, `catch`, `after`, and `else` as `Kernel.SpecialForms.try/1`
  does. For example, the following two functions are equivalent:

      def format(value) do
        try do
          format!(value)
        catch
          :exit, reason -> {:error, reason}
        end
      end

      def format(value) do
        format!(value)
      catch
        :exit, reason -> {:error, reason}
      end

  """
  defmacro def(call, expr \\ nil) do
    define(:def, call, expr, __CALLER__)
  end

  @doc """
  Defines a private function with the given name and body.

  Private functions are only accessible from within the module in which they are
  defined. Trying to access a private function from outside the module it's
  defined in results in an `UndefinedFunctionError` exception.

  Check `def/2` for more information.

  ## Examples

      defmodule Foo do
        def bar do
          sum(1, 2)
        end

        defp sum(a, b), do: a + b
      end

      Foo.bar()
      #=> 3

      Foo.sum(1, 2)
      ** (UndefinedFunctionError) undefined function Foo.sum/2

  """
  defmacro defp(call, expr \\ nil) do
    define(:defp, call, expr, __CALLER__)
  end

  @doc """
  Defines a public macro with the given name and body.

  Macros must be defined before its usage.

  Check `def/2` for rules on naming and default arguments.

  ## Examples

      defmodule MyLogic do
        defmacro unless(expr, opts) do
          quote do
            if !unquote(expr), unquote(opts)
          end
        end
      end

      require MyLogic

      MyLogic.unless false do
        IO.puts("It works")
      end

  """
  defmacro defmacro(call, expr \\ nil) do
    define(:defmacro, call, expr, __CALLER__)
  end

  @doc """
  Defines a private macro with the given name and body.

  Private macros are only accessible from the same module in which they are
  defined.

  Private macros must be defined before its usage.

  Check `defmacro/2` for more information, and check `def/2` for rules on
  naming and default arguments.

  """
  defmacro defmacrop(call, expr \\ nil) do
    define(:defmacrop, call, expr, __CALLER__)
  end

  defp define(kind, call, expr, env) do
    module = assert_module_scope(env, kind, 2)
    assert_no_function_scope(env, kind, 2)

    unquoted_call = :elixir_quote.has_unquotes(call)
    unquoted_expr = :elixir_quote.has_unquotes(expr)
    escaped_call = :elixir_quote.escape(call, :default, true)

    escaped_expr =
      case unquoted_expr do
        true ->
          :elixir_quote.escape(expr, :default, true)

        false ->
          key = :erlang.unique_integer()
          :elixir_module.write_cache(module, key, expr)
          quote(do: :elixir_module.read_cache(unquote(module), unquote(key)))
      end

    # Do not check clauses if any expression was unquoted
    check_clauses = not (unquoted_expr or unquoted_call)
    pos = :elixir_locals.cache_env(env)

    quote do
      :elixir_def.store_definition(
        unquote(kind),
        unquote(check_clauses),
        unquote(escaped_call),
        unquote(escaped_expr),
        unquote(pos)
      )
    end
  end

  @doc """
  Defines a struct.

  A struct is a tagged map that allows developers to provide
  default values for keys, tags to be used in polymorphic
  dispatches and compile time assertions.

  To define a struct, a developer must define both `__struct__/0` and
  `__struct__/1` functions. `defstruct/1` is a convenience macro which
  defines such functions with some conveniences.

  For more information about structs, please check `Kernel.SpecialForms.%/2`.

  ## Examples

      defmodule User do
        defstruct name: nil, age: nil
      end

  Struct fields are evaluated at compile-time, which allows
  them to be dynamic. In the example below, `10 + 11` is
  evaluated at compile-time and the age field is stored
  with value `21`:

      defmodule User do
        defstruct name: nil, age: 10 + 11
      end

  The `fields` argument is usually a keyword list with field names
  as atom keys and default values as corresponding values. `defstruct/1`
  also supports a list of atoms as its argument: in that case, the atoms
  in the list will be used as the struct's field names and they will all
  default to `nil`.

      defmodule Post do
        defstruct [:title, :content, :author]
      end

  ## Deriving

  Although structs are maps, by default structs do not implement
  any of the protocols implemented for maps. For example, attempting
  to use a protocol with the `User` struct leads to an error:

      john = %User{name: "John"}
      MyProtocol.call(john)
      ** (Protocol.UndefinedError) protocol MyProtocol not implemented for %User{...}

  `defstruct/1`, however, allows protocol implementations to be
  *derived*. This can be done by defining a `@derive` attribute as a
  list before invoking `defstruct/1`:

      defmodule User do
        @derive [MyProtocol]
        defstruct name: nil, age: 10 + 11
      end

      MyProtocol.call(john) # it works!

  For each protocol in the `@derive` list, Elixir will assert the protocol has
  been implemented for `Any`. If the `Any` implementation defines a
  `__deriving__/3` callback, the callback will be invoked and it should define
  the implementation module. Otherwise an implementation that simply points to
  the `Any` implementation is automatically derived. For more information on
  the `__deriving__/3` callback, see `Protocol.derive/3`.

  ## Enforcing keys

  When building a struct, Elixir will automatically guarantee all keys
  belongs to the struct:

      %User{name: "john", unknown: :key}
      ** (KeyError) key :unknown not found in: %User{age: 21, name: nil}

  Elixir also allows developers to enforce certain keys must always be
  given when building the struct:

      defmodule User do
        @enforce_keys [:name]
        defstruct name: nil, age: 10 + 11
      end

  Now trying to build a struct without the name key will fail:

      %User{age: 21}
      ** (ArgumentError) the following keys must also be given when building struct User: [:name]

  Keep in mind `@enforce_keys` is a simple compile-time guarantee
  to aid developers when building structs. It is not enforced on
  updates and it does not provide any sort of value-validation.

  ## Types

  It is recommended to define types for structs. By convention such type
  is called `t`. To define a struct inside a type, the struct literal syntax
  is used:

      defmodule User do
        defstruct name: "John", age: 25
        @type t :: %__MODULE__{name: String.t(), age: non_neg_integer}
      end

  It is recommended to only use the struct syntax when defining the struct's
  type. When referring to another struct it's better to use `User.t` instead of
  `%User{}`.

  The types of the struct fields that are not included in `%User{}` default to
  `term()` (see `t:term/0`).

  Structs whose internal structure is private to the local module (pattern
  matching them or directly accessing their fields should not be allowed) should
  use the `@opaque` attribute. Structs whose internal structure is public should
  use `@type`.
  """
  defmacro defstruct(fields) do
    builder =
      case bootstrapped?(Enum) do
        true ->
          quote do
            case @enforce_keys do
              [] ->
                def __struct__(kv) do
                  Enum.reduce(kv, @struct, fn {key, val}, map ->
                    Map.replace!(map, key, val)
                  end)
                end

              _ ->
                def __struct__(kv) do
                  {map, keys} =
                    Enum.reduce(kv, {@struct, @enforce_keys}, fn {key, val}, {map, keys} ->
                      {Map.replace!(map, key, val), List.delete(keys, key)}
                    end)

                  case keys do
                    [] ->
                      map

                    _ ->
                      raise ArgumentError,
                            "the following keys must also be given when building " <>
                              "struct #{inspect(__MODULE__)}: #{inspect(keys)}"
                  end
                end
            end
          end

        false ->
          quote do
            _ = @enforce_keys

            def __struct__(kv) do
              :lists.foldl(fn {key, val}, acc -> Map.replace!(acc, key, val) end, @struct, kv)
            end
          end
      end

    quote do
      if Module.has_attribute?(__MODULE__, :struct) do
        raise ArgumentError,
              "defstruct has already been called for " <>
                "#{Kernel.inspect(__MODULE__)}, defstruct can only be called once per module"
      end

      {struct, keys, derive} = Kernel.Utils.defstruct(__MODULE__, unquote(fields))
      @struct struct
      @enforce_keys keys

      case derive do
        [] -> :ok
        _ -> Protocol.__derive__(derive, __MODULE__, __ENV__)
      end

      def __struct__() do
        @struct
      end

      unquote(builder)
      Kernel.Utils.announce_struct(__MODULE__)
      struct
    end
  end

  @doc ~S"""
  Defines an exception.

  Exceptions are structs backed by a module that implements
  the `Exception` behaviour. The `Exception` behaviour requires
  two functions to be implemented:

    * [`exception/1`](`c:Exception.exception/1`) - receives the arguments given to `raise/2`
      and returns the exception struct. The default implementation
      accepts either a set of keyword arguments that is merged into
      the struct or a string to be used as the exception's message.

    * [`message/1`](`c:Exception.message/1`) - receives the exception struct and must return its
      message. Most commonly exceptions have a message field which
      by default is accessed by this function. However, if an exception
      does not have a message field, this function must be explicitly
      implemented.

  Since exceptions are structs, the API supported by `defstruct/1`
  is also available in `defexception/1`.

  ## Raising exceptions

  The most common way to raise an exception is via `raise/2`:

      defmodule MyAppError do
        defexception [:message]
      end

      value = [:hello]

      raise MyAppError,
        message: "did not get what was expected, got: #{inspect(value)}"

  In many cases it is more convenient to pass the expected value to
  `raise/2` and generate the message in the `c:Exception.exception/1` callback:

      defmodule MyAppError do
        defexception [:message]

        @impl true
        def exception(value) do
          msg = "did not get what was expected, got: #{inspect(value)}"
          %MyAppError{message: msg}
        end
      end

      raise MyAppError, value

  The example above shows the preferred strategy for customizing
  exception messages.
  """
  defmacro defexception(fields) do
    quote bind_quoted: [fields: fields] do
      @behaviour Exception
      struct = defstruct([__exception__: true] ++ fields)

      if Map.has_key?(struct, :message) do
        @impl true
        def message(exception) do
          exception.message
        end

        defoverridable message: 1

        @impl true
        def exception(msg) when Kernel.is_binary(msg) do
          exception(message: msg)
        end
      end

      # Calls to Kernel functions must be fully-qualified to ensure
      # reproducible builds; otherwise, this macro will generate ASTs
      # with different metadata (:import, :context) depending on if
      # it is the bootstrapped version or not.
      # TODO: Change the implementation on v2.0 to simply call Kernel.struct!/2
      @impl true
      def exception(args) when Kernel.is_list(args) do
        struct = __struct__()
        {valid, invalid} = Enum.split_with(args, fn {k, _} -> Map.has_key?(struct, k) end)

        case invalid do
          [] ->
            :ok

          _ ->
            IO.warn(
              "the following fields are unknown when raising " <>
                "#{Kernel.inspect(__MODULE__)}: #{Kernel.inspect(invalid)}. " <>
                "Please make sure to only give known fields when raising " <>
                "or redefine #{Kernel.inspect(__MODULE__)}.exception/1 to " <>
                "discard unknown fields. Future Elixir versions will raise on " <>
                "unknown fields given to raise/2"
            )
        end

        Kernel.struct!(struct, valid)
      end

      defoverridable exception: 1
    end
  end

  @doc """
  Defines a protocol.

  See the `Protocol` module for more information.
  """
  defmacro defprotocol(name, do_block)

  defmacro defprotocol(name, do: block) do
    Protocol.__protocol__(name, do: block)
  end

  @doc """
  Defines an implementation for the given protocol.

  See the `Protocol` module for more information.
  """
  defmacro defimpl(name, opts, do_block \\ []) do
    merged = Keyword.merge(opts, do_block)
    merged = Keyword.put_new(merged, :for, __CALLER__.module)
    Protocol.__impl__(name, merged)
  end

  @doc """
  Makes the given functions in the current module overridable.

  An overridable function is lazily defined, allowing a developer to override
  it.

  Macros cannot be overridden as functions and vice-versa.

  ## Example

      defmodule DefaultMod do
        defmacro __using__(_opts) do
          quote do
            def test(x, y) do
              x + y
            end

            defoverridable test: 2
          end
        end
      end

      defmodule InheritMod do
        use DefaultMod

        def test(x, y) do
          x * y + super(x, y)
        end
      end

  As seen as in the example above, `super` can be used to call the default
  implementation.

  If `@behaviour` has been defined, `defoverridable` can also be called with a
  module as an argument. All implemented callbacks from the behaviour above the
  call to `defoverridable` will be marked as overridable.

  ## Example

      defmodule Behaviour do
        @callback foo :: any
      end

      defmodule DefaultMod do
        defmacro __using__(_opts) do
          quote do
            @behaviour Behaviour

            def foo do
              "Override me"
            end

            defoverridable Behaviour
          end
        end
      end

      defmodule InheritMod do
        use DefaultMod

        def foo do
          "Overridden"
        end
      end

  """
  defmacro defoverridable(keywords_or_behaviour) do
    quote do
      Module.make_overridable(__MODULE__, unquote(keywords_or_behaviour))
    end
  end

  @doc """
  Generates a macro suitable for use in guard expressions.

  It raises at compile time if the definition uses expressions that aren't
  allowed in guards, and otherwise creates a macro that can be used both inside
  or outside guards.

  Note the convention in Elixir is to name functions/macros allowed in
  guards with the `is_` prefix, such as `is_list/1`. If, however, the
  function/macro returns a boolean and is not allowed in guards, it should
  have no prefix and end with a question mark, such as `Keyword.keyword?/1`.

  ## Example

      defmodule Integer.Guards do
        defguard is_even(value) when is_integer(value) and rem(value, 2) == 0
      end

      defmodule Collatz do
        @moduledoc "Tools for working with the Collatz sequence."
        import Integer.Guards

        @doc "Determines the number of steps `n` takes to reach `1`."
        # If this function never converges, please let me know what `n` you used.
        def converge(n) when n > 0, do: step(n, 0)

        defp step(1, step_count) do
          step_count
        end

        defp step(n, step_count) when is_even(n) do
          step(div(n, 2), step_count + 1)
        end

        defp step(n, step_count) do
          step(3 * n + 1, step_count + 1)
        end
      end

  """
  @doc since: "1.6.0"
  @spec defguard(Macro.t()) :: Macro.t()
  defmacro defguard(guard) do
    define_guard(:defmacro, guard, __CALLER__)
  end

  @doc """
  Generates a private macro suitable for use in guard expressions.

  It raises at compile time if the definition uses expressions that aren't
  allowed in guards, and otherwise creates a private macro that can be used
  both inside or outside guards in the current module.

  Similar to `defmacrop/2`, `defguardp/1` must be defined before its use
  in the current module.
  """
  @doc since: "1.6.0"
  @spec defguardp(Macro.t()) :: Macro.t()
  defmacro defguardp(guard) do
    define_guard(:defmacrop, guard, __CALLER__)
  end

  defp define_guard(kind, guard, env) do
    case :elixir_utils.extract_guards(guard) do
      {call, [_, _ | _]} ->
        raise ArgumentError,
              "invalid syntax in defguard #{Macro.to_string(call)}, " <>
                "only a single when clause is allowed"

      {call, impls} ->
        case Macro.decompose_call(call) do
          {_name, args} ->
            validate_variable_only_args!(call, args)

            macro_definition =
              case impls do
                [] ->
                  define(kind, call, nil, env)

                [guard] ->
                  quoted =
                    quote do
                      require Kernel.Utils
                      Kernel.Utils.defguard(unquote(args), unquote(guard))
                    end

                  define(kind, call, [do: quoted], env)
              end

            quote do
              @doc guard: true
              unquote(macro_definition)
            end

          _invalid_definition ->
            raise ArgumentError, "invalid syntax in defguard #{Macro.to_string(call)}"
        end
    end
  end

  defp validate_variable_only_args!(call, args) do
    Enum.each(args, fn
      {ref, _meta, context} when is_atom(ref) and is_atom(context) ->
        :ok

      {:\\, _m1, [{ref, _m2, context}, _default]} when is_atom(ref) and is_atom(context) ->
        :ok

      _match ->
        raise ArgumentError, "invalid syntax in defguard #{Macro.to_string(call)}"
    end)
  end

  @doc """
  Uses the given module in the current context.

  When calling:

      use MyModule, some: :options

  the `__using__/1` macro from the `MyModule` module is invoked with the second
  argument passed to `use` as its argument. Since `__using__/1` is a macro, all
  the usual macro rules apply, and its return value should be quoted code
  that is then inserted where `use/2` is called.

  ## Examples

  For example, in order to write test cases using the `ExUnit` framework
  provided with Elixir, a developer should `use` the `ExUnit.Case` module:

      defmodule AssertionTest do
        use ExUnit.Case, async: true

        test "always pass" do
          assert true
        end
      end

  In this example, `ExUnit.Case.__using__/1` is called with the keyword list
  `[async: true]` as its argument; `use/2` translates to:

      defmodule AssertionTest do
        require ExUnit.Case
        ExUnit.Case.__using__(async: true)

        test "always pass" do
          assert true
        end
      end

  `ExUnit.Case` will then define the `__using__/1` macro:

      defmodule ExUnit.Case do
        defmacro __using__(opts) do
          # do something with opts
          quote do
            # return some code to inject in the caller
          end
        end
      end

  ## Best practices

  `__using__/1` is typically used when there is a need to set some state (via
  module attributes) or callbacks (like `@before_compile`, see the documentation
  for `Module` for more information) into the caller.

  `__using__/1` may also be used to alias, require, or import functionality
  from different modules:

      defmodule MyModule do
        defmacro __using__(_opts) do
          quote do
            import MyModule.Foo
            import MyModule.Bar
            import MyModule.Baz

            alias MyModule.Repo
          end
        end
      end

  However, do not provide `__using__/1` if all it does is to import,
  alias or require the module itself. For example, avoid this:

      defmodule MyModule do
        defmacro __using__(_opts) do
          quote do
            import MyModule
          end
        end
      end

  In such cases, developers should instead import or alias the module
  directly, so that they can customize those as they wish,
  without the indirection behind `use/2`.

  Finally, developers should also avoid defining functions inside
  the `__using__/1` callback, unless those functions are the default
  implementation of a previously defined `@callback` or are functions
  meant to be overridden (see `defoverridable/1`). Even in these cases,
  defining functions should be seen as a "last resort".

  In case you want to provide some existing functionality to the user module,
  please define it in a module which will be imported accordingly; for example,
  `ExUnit.Case` doesn't define the `test/3` macro in the module that calls
  `use ExUnit.Case`, but it defines `ExUnit.Case.test/3` and just imports that
  into the caller when used.
  """
  defmacro use(module, opts \\ []) do
    calls =
      Enum.map(expand_aliases(module, __CALLER__), fn
        expanded when is_atom(expanded) ->
          quote do
            require unquote(expanded)
            unquote(expanded).__using__(unquote(opts))
          end

        _otherwise ->
          raise ArgumentError,
                "invalid arguments for use, " <>
                  "expected a compile time atom or alias, got: #{Macro.to_string(module)}"
      end)

    quote(do: (unquote_splicing(calls)))
  end

  defp expand_aliases({{:., _, [base, :{}]}, _, refs}, env) do
    base = Macro.expand(base, env)

    Enum.map(refs, fn
      {:__aliases__, _, ref} ->
        Module.concat([base | ref])

      ref when is_atom(ref) ->
        Module.concat(base, ref)

      other ->
        other
    end)
  end

  defp expand_aliases(module, env) do
    [Macro.expand(module, env)]
  end

  @doc """
  Defines a function that delegates to another module.

  Functions defined with `defdelegate/2` are public and can be invoked from
  outside the module they're defined in, as if they were defined using `def/2`.
  Therefore, `defdelegate/2` is about extending the current module's public API.
  If what you want is to invoke a function defined in another module without
  using its full module name, then use `alias/2` to shorten the module name or use
  `import/2` to be able to invoke the function without the module name altogether.

  Delegation only works with functions; delegating macros is not supported.

  Check `def/2` for rules on naming and default arguments.

  ## Options

    * `:to` - the module to dispatch to.

    * `:as` - the function to call on the target given in `:to`.
      This parameter is optional and defaults to the name being
      delegated (`funs`).

  ## Examples

      defmodule MyList do
        defdelegate reverse(list), to: Enum
        defdelegate other_reverse(list), to: Enum, as: :reverse
      end

      MyList.reverse([1, 2, 3])
      #=> [3, 2, 1]

      MyList.other_reverse([1, 2, 3])
      #=> [3, 2, 1]

  """
  defmacro defdelegate(funs, opts) do
    funs = Macro.escape(funs, unquote: true)

    # don't add compile-time dependency on :to
    opts =
      with true <- is_list(opts),
           {:ok, target} <- Keyword.fetch(opts, :to),
           {:__aliases__, _, _} <- target do
        target = Macro.expand(target, %{__CALLER__ | function: {:__info__, 1}})
        Keyword.replace!(opts, :to, target)
      else
        _ ->
          opts
      end

    quote bind_quoted: [funs: funs, opts: opts] do
      target =
        Keyword.get(opts, :to) || raise ArgumentError, "expected to: to be given as argument"

      if is_list(funs) do
        IO.warn(
          "passing a list to Kernel.defdelegate/2 is deprecated, please define each delegate separately",
          Macro.Env.stacktrace(__ENV__)
        )
      end

      if Keyword.has_key?(opts, :append_first) do
        IO.warn(
          "Kernel.defdelegate/2 :append_first option is deprecated",
          Macro.Env.stacktrace(__ENV__)
        )
      end

      for fun <- List.wrap(funs) do
        {name, args, as, as_args} = Kernel.Utils.defdelegate(fun, opts)

        @doc delegate_to: {target, as, :erlang.length(as_args)}

        def unquote(name)(unquote_splicing(args)) do
          unquote(target).unquote(as)(unquote_splicing(as_args))
        end
      end
    end
  end

  ## Sigils

  @doc ~S"""
  Handles the sigil `~S` for strings.

  It returns a string without interpolations and without escape
  characters, except for the escaping of the closing sigil character
  itself.

  ## Examples

      iex> ~S(foo)
      "foo"
      iex> ~S(f#{o}o)
      "f\#{o}o"
      iex> ~S(\o/)
      "\\o/"

  However, if you want to re-use the sigil character itself on
  the string, you need to escape it:

      iex> ~S((\))
      "()"

  """
  defmacro sigil_S(term, modifiers)
  defmacro sigil_S({:<<>>, _, [binary]}, []) when is_binary(binary), do: binary

  @doc ~S"""
  Handles the sigil `~s` for strings.

  It returns a string as if it was a double quoted string, unescaping characters
  and replacing interpolations.

  ## Examples

      iex> ~s(foo)
      "foo"

      iex> ~s(f#{:o}o)
      "foo"

      iex> ~s(f\#{:o}o)
      "f\#{:o}o"

  """
  defmacro sigil_s(term, modifiers)

  defmacro sigil_s({:<<>>, _, [piece]}, []) when is_binary(piece) do
    :elixir_interpolation.unescape_chars(piece)
  end

  defmacro sigil_s({:<<>>, line, pieces}, []) do
    {:<<>>, line, unescape_tokens(pieces)}
  end

  @doc ~S"""
  Handles the sigil `~C` for charlists.

  It returns a charlist without interpolations and without escape
  characters, except for the escaping of the closing sigil character
  itself.

  ## Examples

      iex> ~C(foo)
      'foo'

      iex> ~C(f#{o}o)
      'f\#{o}o'

  """
  defmacro sigil_C(term, modifiers)

  defmacro sigil_C({:<<>>, _meta, [string]}, []) when is_binary(string) do
    String.to_charlist(string)
  end

  @doc ~S"""
  Handles the sigil `~c` for charlists.

  It returns a charlist as if it was a single quoted string, unescaping
  characters and replacing interpolations.

  ## Examples

      iex> ~c(foo)
      'foo'

      iex> ~c(f#{:o}o)
      'foo'

      iex> ~c(f\#{:o}o)
      'f\#{:o}o'

  """
  defmacro sigil_c(term, modifiers)

  # We can skip the runtime conversion if we are
  # creating a binary made solely of series of chars.
  defmacro sigil_c({:<<>>, _meta, [string]}, []) when is_binary(string) do
    String.to_charlist(:elixir_interpolation.unescape_chars(string))
  end

  defmacro sigil_c({:<<>>, _meta, pieces}, []) do
    quote(do: List.to_charlist(unquote(unescape_list_tokens(pieces))))
  end

  @doc """
  Handles the sigil `~r` for regular expressions.

  It returns a regular expression pattern, unescaping characters and replacing
  interpolations.

  More information on regular expressions can be found in the `Regex` module.

  ## Examples

      iex> Regex.match?(~r(foo), "foo")
      true

      iex> Regex.match?(~r/a#{:b}c/, "abc")
      true

  """
  defmacro sigil_r(term, modifiers)

  defmacro sigil_r({:<<>>, _meta, [string]}, options) when is_binary(string) do
    binary = :elixir_interpolation.unescape_chars(string, &Regex.unescape_map/1)
    regex = Regex.compile!(binary, :binary.list_to_bin(options))
    Macro.escape(regex)
  end

  defmacro sigil_r({:<<>>, meta, pieces}, options) do
    binary = {:<<>>, meta, unescape_tokens(pieces, &Regex.unescape_map/1)}
    quote(do: Regex.compile!(unquote(binary), unquote(:binary.list_to_bin(options))))
  end

  @doc ~S"""
  Handles the sigil `~R` for regular expressions.

  It returns a regular expression pattern without interpolations and
  without escape characters. Note it still supports escape of Regex
  tokens (such as escaping `+` or `?`) and it also requires you to
  escape the closing sigil character itself if it appears on the Regex.

  More information on regexes can be found in the `Regex` module.

  ## Examples

      iex> Regex.match?(~R(f#{1,3}o), "f#o")
      true

  """
  defmacro sigil_R(term, modifiers)

  defmacro sigil_R({:<<>>, _meta, [string]}, options) when is_binary(string) do
    regex = Regex.compile!(string, :binary.list_to_bin(options))
    Macro.escape(regex)
  end

  @doc ~S"""
  Handles the sigil `~D` for dates.

  By default, this sigil uses the built-in `Calendar.ISO`, which
  requires dates to be written in the ISO8601 format:

      ~D[yyyy-mm-dd]

  such as:

      ~D[2015-01-13]

  If you are using alternative calendars, any representation can
  be used as long as you follow the representation by a single space
  and the calendar name:

      ~D[SOME-REPRESENTATION My.Alternative.Calendar]

  The lower case `~d` variant does not exist as interpolation
  and escape characters are not useful for date sigils.

  More information on dates can be found in the `Date` module.

  ## Examples

      iex> ~D[2015-01-13]
      ~D[2015-01-13]

  """
  defmacro sigil_D(date_string, modifiers)

  defmacro sigil_D({:<<>>, _, [string]}, []) do
    {{:ok, {year, month, day}}, calendar} = parse_with_calendar!(string, :parse_date, "Date")
    to_calendar_struct(Date, calendar: calendar, year: year, month: month, day: day)
  end

  @doc ~S"""
  Handles the sigil `~T` for times.

  By default, this sigil uses the built-in `Calendar.ISO`, which
  requires times to be written in the ISO8601 format:

      ~T[hh:mm:ss]
      ~T[hh:mm:ss.ssssss]

  such as:

      ~T[13:00:07]
      ~T[13:00:07.123]

  If you are using alternative calendars, any representation can
  be used as long as you follow the representation by a single space
  and the calendar name:

      ~T[SOME-REPRESENTATION My.Alternative.Calendar]

  The lower case `~t` variant does not exist as interpolation
  and escape characters are not useful for time sigils.

  More information on times can be found in the `Time` module.

  ## Examples

      iex> ~T[13:00:07]
      ~T[13:00:07]
      iex> ~T[13:00:07.001]
      ~T[13:00:07.001]

  """
  defmacro sigil_T(time_string, modifiers)

  defmacro sigil_T({:<<>>, _, [string]}, []) do
    {{:ok, {hour, minute, second, microsecond}}, calendar} =
      parse_with_calendar!(string, :parse_time, "Time")

    to_calendar_struct(Time,
      calendar: calendar,
      hour: hour,
      minute: minute,
      second: second,
      microsecond: microsecond
    )
  end

  @doc ~S"""
  Handles the sigil `~N` for naive date times.

  By default, this sigil uses the built-in `Calendar.ISO`, which
  requires naive date times to be written in the ISO8601 format:

      ~N[yyyy-mm-dd hh:mm:ss]
      ~N[yyyy-mm-dd hh:mm:ss.ssssss]
      ~N[yyyy-mm-ddThh:mm:ss.ssssss]

  such as:

      ~N[2015-01-13 13:00:07]
      ~N[2015-01-13T13:00:07.123]

  If you are using alternative calendars, any representation can
  be used as long as you follow the representation by a single space
  and the calendar name:

      ~N[SOME-REPRESENTATION My.Alternative.Calendar]

  The lower case `~n` variant does not exist as interpolation
  and escape characters are not useful for date time sigils.

  More information on naive date times can be found in the
  `NaiveDateTime` module.

  ## Examples

      iex> ~N[2015-01-13 13:00:07]
      ~N[2015-01-13 13:00:07]
      iex> ~N[2015-01-13T13:00:07.001]
      ~N[2015-01-13 13:00:07.001]

  """
  defmacro sigil_N(naive_datetime_string, modifiers)

  defmacro sigil_N({:<<>>, _, [string]}, []) do
    {{:ok, {year, month, day, hour, minute, second, microsecond}}, calendar} =
      parse_with_calendar!(string, :parse_naive_datetime, "NaiveDateTime")

    to_calendar_struct(NaiveDateTime,
      calendar: calendar,
      year: year,
      month: month,
      day: day,
      hour: hour,
      minute: minute,
      second: second,
      microsecond: microsecond
    )
  end

  @doc ~S"""
  Handles the sigil `~U` to create a UTC `DateTime`.

  By default, this sigil uses the built-in `Calendar.ISO`, which
  requires UTC date times to be written in the ISO8601 format:

      ~U[yyyy-mm-dd hh:mm:ssZ]
      ~U[yyyy-mm-dd hh:mm:ss.ssssssZ]
      ~U[yyyy-mm-ddThh:mm:ss.ssssss+00:00]

  such as:

      ~U[2015-01-13 13:00:07Z]
      ~U[2015-01-13T13:00:07.123+00:00]

  If you are using alternative calendars, any representation can
  be used as long as you follow the representation by a single space
  and the calendar name:

      ~U[SOME-REPRESENTATION My.Alternative.Calendar]

  The given `datetime_string` must include "Z" or "00:00" offset
  which marks it as UTC, otherwise an error is raised.

  The lower case `~u` variant does not exist as interpolation
  and escape characters are not useful for date time sigils.

  More information on date times can be found in the `DateTime` module.

  ## Examples

      iex> ~U[2015-01-13 13:00:07Z]
      ~U[2015-01-13 13:00:07Z]
      iex> ~U[2015-01-13T13:00:07.001+00:00]
      ~U[2015-01-13 13:00:07.001Z]

  """
  @doc since: "1.9.0"
  defmacro sigil_U(datetime_string, modifiers)

  defmacro sigil_U({:<<>>, _, [string]}, []) do
    {{:ok, {year, month, day, hour, minute, second, microsecond}, offset}, calendar} =
      parse_with_calendar!(string, :parse_utc_datetime, "UTC DateTime")

    if offset != 0 do
      raise ArgumentError,
            "cannot parse #{inspect(string)} as UTC DateTime for #{inspect(calendar)}, reason: :non_utc_offset"
    end

    to_calendar_struct(DateTime,
      calendar: calendar,
      year: year,
      month: month,
      day: day,
      hour: hour,
      minute: minute,
      second: second,
      microsecond: microsecond,
      time_zone: "Etc/UTC",
      zone_abbr: "UTC",
      utc_offset: 0,
      std_offset: 0
    )
  end

  defp parse_with_calendar!(string, fun, context) do
    {calendar, string} = extract_calendar(string)
    result = apply(calendar, fun, [string])
    {maybe_raise!(result, calendar, context, string), calendar}
  end

  defp extract_calendar(string) do
    case :binary.split(string, " ", [:global]) do
      [_] -> {Calendar.ISO, string}
      parts -> maybe_atomize_calendar(List.last(parts), string)
    end
  end

  defp maybe_atomize_calendar(<<alias, _::binary>> = last_part, string)
       when alias >= ?A and alias <= ?Z do
    string = binary_part(string, 0, byte_size(string) - byte_size(last_part) - 1)
    {String.to_atom("Elixir." <> last_part), string}
  end

  defp maybe_atomize_calendar(_last_part, string) do
    {Calendar.ISO, string}
  end

  defp maybe_raise!({:error, reason}, calendar, type, string) do
    raise ArgumentError,
          "cannot parse #{inspect(string)} as #{type} for #{inspect(calendar)}, " <>
            "reason: #{inspect(reason)}"
  end

  defp maybe_raise!(other, _calendar, _type, _string), do: other

  defp to_calendar_struct(type, fields) do
    quote do
      %{unquote_splicing([__struct__: type] ++ fields)}
    end
  end

  @doc ~S"""
  Handles the sigil `~w` for list of words.

  It returns a list of "words" split by whitespace. Character unescaping and
  interpolation happens for each word.

  ## Modifiers

    * `s`: words in the list are strings (default)
    * `a`: words in the list are atoms
    * `c`: words in the list are charlists

  ## Examples

      iex> ~w(foo #{:bar} baz)
      ["foo", "bar", "baz"]

      iex> ~w(foo #{" bar baz "})
      ["foo", "bar", "baz"]

      iex> ~w(--source test/enum_test.exs)
      ["--source", "test/enum_test.exs"]

      iex> ~w(foo bar baz)a
      [:foo, :bar, :baz]

  """
  defmacro sigil_w(term, modifiers)

  defmacro sigil_w({:<<>>, _meta, [string]}, modifiers) when is_binary(string) do
    split_words(:elixir_interpolation.unescape_chars(string), modifiers, __CALLER__)
  end

  defmacro sigil_w({:<<>>, meta, pieces}, modifiers) do
    binary = {:<<>>, meta, unescape_tokens(pieces)}
    split_words(binary, modifiers, __CALLER__)
  end

  @doc ~S"""
  Handles the sigil `~W` for list of words.

  It returns a list of "words" split by whitespace without interpolations
  and without escape characters, except for the escaping of the closing
  sigil character itself.

  ## Modifiers

    * `s`: words in the list are strings (default)
    * `a`: words in the list are atoms
    * `c`: words in the list are charlists

  ## Examples

      iex> ~W(foo #{bar} baz)
      ["foo", "\#{bar}", "baz"]

  """
  defmacro sigil_W(term, modifiers)

  defmacro sigil_W({:<<>>, _meta, [string]}, modifiers) when is_binary(string) do
    split_words(string, modifiers, __CALLER__)
  end

  defp split_words(string, [], caller) do
    split_words(string, [?s], caller)
  end

  defp split_words(string, [mod], caller)
       when mod == ?s or mod == ?a or mod == ?c do
    case is_binary(string) do
      true ->
        parts = String.split(string)

        parts_with_trailing_comma =
          :lists.filter(&(byte_size(&1) > 1 and :binary.last(&1) == ?,), parts)

        if parts_with_trailing_comma != [] do
          stacktrace = Macro.Env.stacktrace(caller)

          IO.warn(
            "the sigils ~w/~W do not allow trailing commas at the end of each word. " <>
              "If the comma is necessary, define a regular list with [...], otherwise remove the comma.",
            stacktrace
          )
        end

        case mod do
          ?s -> parts
          ?a -> :lists.map(&String.to_atom/1, parts)
          ?c -> :lists.map(&String.to_charlist/1, parts)
        end

      false ->
        parts = quote(do: String.split(unquote(string)))

        case mod do
          ?s -> parts
          ?a -> quote(do: :lists.map(&String.to_atom/1, unquote(parts)))
          ?c -> quote(do: :lists.map(&String.to_charlist/1, unquote(parts)))
        end
    end
  end

  defp split_words(_string, _mods, _caller) do
    raise ArgumentError, "modifier must be one of: s, a, c"
  end

  ## Shared functions

  defp assert_module_scope(env, fun, arity) do
    case env.module do
      nil -> raise ArgumentError, "cannot invoke #{fun}/#{arity} outside module"
      mod -> mod
    end
  end

  defp assert_no_function_scope(env, fun, arity) do
    case env.function do
      nil -> :ok
      _ -> raise ArgumentError, "cannot invoke #{fun}/#{arity} inside function/macro"
    end
  end

  defp assert_no_match_or_guard_scope(context, exp) do
    case context do
      :match ->
        invalid_match!(exp)

      :guard ->
        raise ArgumentError,
              "invalid expression in guard, #{exp} is not allowed in guards. " <>
                "To learn more about guards, visit: https://hexdocs.pm/elixir/patterns-and-guards.html"

      _ ->
        :ok
    end
  end

  defp invalid_match!(exp) do
    raise ArgumentError,
          "invalid expression in match, #{exp} is not allowed in patterns " <>
            "such as function clauses, case clauses or on the left side of the = operator"
  end

  # Helper to handle the :ok | :error tuple returned from :elixir_interpolation.unescape_tokens
  defp unescape_tokens(tokens) do
    case :elixir_interpolation.unescape_tokens(tokens) do
      {:ok, unescaped_tokens} -> unescaped_tokens
      {:error, reason} -> raise ArgumentError, to_string(reason)
    end
  end

  defp unescape_tokens(tokens, unescape_map) do
    case :elixir_interpolation.unescape_tokens(tokens, unescape_map) do
      {:ok, unescaped_tokens} -> unescaped_tokens
      {:error, reason} -> raise ArgumentError, to_string(reason)
    end
  end

  defp unescape_list_tokens(tokens) do
    escape = fn
      {:"::", _, [expr, _]} -> expr
      binary when is_binary(binary) -> :elixir_interpolation.unescape_chars(binary)
    end

    :lists.map(escape, tokens)
  end

  @doc false
  # TODO: Remove on v2.0 (also hard-coded in elixir_dispatch)
  @deprecated "Use Kernel.to_charlist/1 instead"
  defmacro to_char_list(arg) do
    quote(do: Kernel.to_charlist(unquote(arg)))
  end
end