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defmodule Kernel.SpecialForms do
@moduledoc """
Special forms are the basic building blocks of Elixir, and therefore
cannot be overridden by the developer.
The `Kernel.SpecialForms` module consists solely of macros that can be
invoked anywhere in Elixir code without the use of the
`Kernel.SpecialForms.` prefix. This is possible because they all have
been automatically imported, in the same fashion as the functions and
macros from the `Kernel` module.
These building blocks are defined in this module. Some of these special forms are lexical (like
`alias/2`, `case/2`, etc.). The macros `{}/1` and `<<>>/1` are also special
forms used to define tuple and binary data structures respectively.
This module also documents macros that return information about Elixir's
compilation environment, such as (`__ENV__/0`, `__MODULE__/0`, `__DIR__/0` and `__CALLER__/0`).
Additionally, it documents two special forms, `__block__/1` and
`__aliases__/1`, which are not intended to be called directly by the
developer but they appear in quoted contents since they are essential
in Elixir's constructs.
"""
defmacrop error!(args) do
quote do
_ = unquote(args)
message =
"Elixir's special forms are expanded by the compiler and must not be invoked directly"
:erlang.error(RuntimeError.exception(message))
end
end
@doc """
Creates a tuple.
More information about the tuple data type and about functions to manipulate
tuples can be found in the `Tuple` module; some functions for working with
tuples are also available in `Kernel` (such as `Kernel.elem/2` or
`Kernel.tuple_size/1`).
## AST representation
Only two-element tuples are considered literals in Elixir and return themselves
when quoted. Therefore, all other tuples are represented in the AST as calls to
the `:{}` special form.
iex> quote do
...> {1, 2}
...> end
{1, 2}
iex> quote do
...> {1, 2, 3}
...> end
{:{}, [], [1, 2, 3]}
"""
defmacro unquote(:{})(args), do: error!([args])
@doc """
Creates a map.
See the `Map` module for more information about maps, their syntax, and ways to
access and manipulate them.
## AST representation
Regardless of whether `=>` or the keyword syntax is used, key-value pairs in
maps are always represented internally as a list of two-element tuples for
simplicity:
iex> quote do
...> %{"a" => :b, c: :d}
...> end
{:%{}, [], [{"a", :b}, {:c, :d}]}
"""
defmacro unquote(:%{})(args), do: error!([args])
@doc """
Matches on or builds 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.
Structs are usually defined with the `Kernel.defstruct/1` macro:
defmodule User do
defstruct name: "john", age: 27
end
Now a struct can be created as follows:
%User{}
Underneath a struct is just a map with a `:__struct__` key
pointing to the `User` module:
%User{} == %{__struct__: User, name: "john", age: 27}
The struct fields can be given when building the struct:
%User{age: 31}
#=> %{__struct__: User, name: "john", age: 31}
Or also on pattern matching to extract values out:
%User{age: age} = user
An update operation specific for structs is also available:
%User{user | age: 28}
The advantage of structs is that they validate that the given
keys are part of the defined struct. The example below will fail
because there is no key `:full_name` in the `User` struct:
%User{full_name: "john doe"}
The syntax above will guarantee the given keys are valid at
compilation time and it will guarantee at runtime the given
argument is a struct, failing with `BadStructError` otherwise.
Although structs are maps, by default structs do not implement
any of the protocols implemented for maps. Check
`Kernel.defprotocol/2` for more information on how structs
can be used with protocols for polymorphic dispatch. Also
see `Kernel.struct/2` and `Kernel.struct!/2` for examples on
how to create and update structs dynamically.
## Pattern matching on struct names
Besides allowing pattern matching on struct fields, such as:
%User{age: age} = user
Structs also allow pattern matching on the struct name:
%struct_name{} = user
struct_name #=> User
You can also assign the struct name to `_` when you want to
check if something is a struct but you are not interested in
its name:
%_{} = user
"""
defmacro unquote(:%)(struct, map), do: error!([struct, map])
@doc """
Defines a new bitstring.
## Examples
iex> <<1, 2, 3>>
<<1, 2, 3>>
## Types
A bitstring is made of many segments and each segment has a
type. There are 9 types used in bitstrings:
- `integer`
- `float`
- `bits` (alias for `bitstring`)
- `bitstring`
- `binary`
- `bytes` (alias for `binary`)
- `utf8`
- `utf16`
- `utf32`
When no type is specified, the default is `integer`:
iex> <<1, 2, 3>>
<<1, 2, 3>>
Elixir also accepts by default the segment to be a literal
string or a literal charlist, which are by default expanded to integers:
iex> <<0, "foo">>
<<0, 102, 111, 111>>
Variables or any other type need to be explicitly tagged:
iex> rest = "oo"
iex> <<102, rest>>
** (ArgumentError) argument error
We can solve this by explicitly tagging it as `binary`:
iex> rest = "oo"
iex> <<102, rest::binary>>
"foo"
The `utf8`, `utf16`, and `utf32` types are for Unicode code points. They
can also be applied to literal strings and charlists:
iex> <<"foo"::utf16>>
<<0, 102, 0, 111, 0, 111>>
iex> <<"foo"::utf32>>
<<0, 0, 0, 102, 0, 0, 0, 111, 0, 0, 0, 111>>
## Options
Many options can be given by using `-` as separator. Order is
arbitrary, so the following are all equivalent:
<<102::integer-native, rest::binary>>
<<102::native-integer, rest::binary>>
<<102::unsigned-big-integer, rest::binary>>
<<102::unsigned-big-integer-size(8), rest::binary>>
<<102::unsigned-big-integer-8, rest::binary>>
<<102::8-integer-big-unsigned, rest::binary>>
<<102, rest::binary>>
### Unit and Size
The length of the match is equal to the `unit` (a number of bits) times the
`size` (the number of repeated segments of length `unit`).
Type | Default Unit
--------- | ------------
`integer` | 1 bit
`float` | 1 bit
`binary` | 8 bits
Sizes for types are a bit more nuanced. The default size for integers is 8.
For floats, it is 64. For floats, `size * unit` must result in 32 or 64,
corresponding to [IEEE 754](https://en.wikipedia.org/wiki/IEEE_floating_point)
binary32 and binary64, respectively.
For binaries, the default is the size of the binary. Only the last binary in a
match can use the default size. All others must have their size specified
explicitly, even if the match is unambiguous. For example:
iex> <<name::binary-size(5), " the ", species::binary>> = <<"Frank the Walrus">>
"Frank the Walrus"
iex> {name, species}
{"Frank", "Walrus"}
The size can be a variable:
iex> name_size = 5
iex> <<name::binary-size(name_size), " the ", species::binary>> = <<"Frank the Walrus">>
iex> {name, species}
{"Frank", "Walrus"}
And the variable can be defined in the match itself (prior to its use):
iex> <<name_size::size(8), name::binary-size(name_size), " the ", species::binary>> = <<5, "Frank the Walrus">>
iex> {name, species}
{"Frank", "Walrus"}
However, the size cannot be defined in the match outside the binary/bitstring match:
{name_size, <<name::binary-size(name_size), _rest::binary>>} = {5, <<"Frank the Walrus">>}
** (CompileError): undefined variable "name_size" in bitstring segment
Failing to specify the size for the non-last causes compilation to fail:
<<name::binary, " the ", species::binary>> = <<"Frank the Walrus">>
** (CompileError): a binary field without size is only allowed at the end of a binary pattern
#### Shortcut Syntax
Size and unit can also be specified using a syntax shortcut
when passing integer values:
iex> x = 1
iex> <<x::8>> == <<x::size(8)>>
true
iex> <<x::8*4>> == <<x::size(8)-unit(4)>>
true
This syntax reflects the fact the effective size is given by
multiplying the size by the unit.
### Modifiers
Some types have associated modifiers to clear up ambiguity in byte
representation.
Modifier | Relevant Type(s)
-------------------- | ----------------
`signed` | `integer`
`unsigned` (default) | `integer`
`little` | `integer`, `float`, `utf16`, `utf32`
`big` (default) | `integer`, `float`, `utf16`, `utf32`
`native` | `integer`, `utf16`, `utf32`
### Sign
Integers can be `signed` or `unsigned`, defaulting to `unsigned`.
iex> <<int::integer>> = <<-100>>
<<156>>
iex> int
156
iex> <<int::integer-signed>> = <<-100>>
<<156>>
iex> int
-100
`signed` and `unsigned` are only used for matching binaries (see below) and
are only used for integers.
iex> <<-100::signed, _rest::binary>> = <<-100, "foo">>
<<156, 102, 111, 111>>
### Endianness
Elixir has three options for endianness: `big`, `little`, and `native`.
The default is `big`:
iex> <<number::little-integer-size(16)>> = <<0, 1>>
<<0, 1>>
iex> number
256
iex> <<number::big-integer-size(16)>> = <<0, 1>>
<<0, 1>>
iex> number
1
`native` is determined by the VM at startup and will depend on the
host operating system.
## Binary/Bitstring Matching
Binary matching is a powerful feature in Elixir that is useful for extracting
information from binaries as well as pattern matching.
Binary matching can be used by itself to extract information from binaries:
iex> <<"Hello, ", place::binary>> = "Hello, World"
"Hello, World"
iex> place
"World"
Or as a part of function definitions to pattern match:
defmodule ImageTyper do
@png_signature <<137::size(8), 80::size(8), 78::size(8), 71::size(8),
13::size(8), 10::size(8), 26::size(8), 10::size(8)>>
@jpg_signature <<255::size(8), 216::size(8)>>
def type(<<@png_signature, _rest::binary>>), do: :png
def type(<<@jpg_signature, _rest::binary>>), do: :jpg
def type(_), do: :unknown
end
### Performance & Optimizations
The Erlang compiler can provide a number of optimizations on binary creation
and matching. To see optimization output, set the `bin_opt_info` compiler
option:
ERL_COMPILER_OPTIONS=bin_opt_info mix compile
To learn more about specific optimizations and performance considerations,
check out
[Erlang's Efficiency Guide on handling binaries](http://www.erlang.org/doc/efficiency_guide/binaryhandling.html).
"""
defmacro unquote(:<<>>)(args), do: error!([args])
@doc """
Defines a remote call, a call to an anonymous function, or an alias.
The dot (`.`) in Elixir can be used for remote calls:
iex> String.downcase("FOO")
"foo"
In this example above, we have used `.` to invoke `downcase` in the
`String` module, passing `"FOO"` as argument.
The dot may be used to invoke anonymous functions too:
iex> (fn n -> n end).(7)
7
in which case there is a function on the left hand side.
We can also use the dot for creating aliases:
iex> Hello.World
Hello.World
This time, we have joined two aliases, defining the final alias
`Hello.World`.
## Syntax
The right side of `.` may be a word starting with an uppercase letter, which represents
an alias, a word starting with lowercase or underscore, any valid language
operator or any name wrapped in single- or double-quotes. Those are all valid
examples:
iex> Kernel.Sample
Kernel.Sample
iex> Kernel.length([1, 2, 3])
3
iex> Kernel.+(1, 2)
3
iex> Kernel."+"(1, 2)
3
Wrapping the function name in single- or double-quotes is always a
remote call. Therefore `Kernel."Foo"` will attempt to call the function "Foo"
and not return the alias `Kernel.Foo`. This is done by design as module names
are more strict than function names.
When the dot is used to invoke an anonymous function there is only one
operand, but it is still written using a postfix notation:
iex> negate = fn n -> -n end
iex> negate.(7)
-7
## Quoted expression
When `.` is used, the quoted expression may take two distinct
forms. When the right side starts with a lowercase letter (or
underscore):
iex> quote do
...> String.downcase("FOO")
...> end
{{:., [], [{:__aliases__, [alias: false], [:String]}, :downcase]}, [], ["FOO"]}
Notice we have an inner tuple, containing the atom `:.` representing
the dot as first element:
{:., [], [{:__aliases__, [alias: false], [:String]}, :downcase]}
This tuple follows the general quoted expression structure in Elixir,
with the name as first argument, some keyword list as metadata as second,
and the list of arguments as third. In this case, the arguments are the
alias `String` and the atom `:downcase`. The second argument in a remote call
is **always** an atom.
In the case of calls to anonymous functions, the inner tuple with the dot
special form has only one argument, reflecting the fact that the operator is
unary:
iex> quote do
...> negate.(0)
...> end
{{:., [], [{:negate, [], __MODULE__}]}, [], [0]}
When the right side is an alias (i.e. starts with uppercase), we get instead:
iex> quote do
...> Hello.World
...> end
{:__aliases__, [alias: false], [:Hello, :World]}
We go into more details about aliases in the `__aliases__/1` special form
documentation.
## Unquoting
We can also use unquote to generate a remote call in a quoted expression:
iex> x = :downcase
iex> quote do
...> String.unquote(x)("FOO")
...> end
{{:., [], [{:__aliases__, [alias: false], [:String]}, :downcase]}, [], ["FOO"]}
Similar to `Kernel."FUNCTION_NAME"`, `unquote(x)` will always generate a remote call,
independent of the value of `x`. To generate an alias via the quoted expression,
one needs to rely on `Module.concat/2`:
iex> x = Sample
iex> quote do
...> Module.concat(String, unquote(x))
...> end
{{:., [], [{:__aliases__, [alias: false], [:Module]}, :concat]}, [],
[{:__aliases__, [alias: false], [:String]}, Sample]}
"""
defmacro unquote(:.)(left, right), do: error!([left, right])
@doc """
`alias/2` is used to set up aliases, often useful with modules' names.
## Examples
`alias/2` can be used to set up an alias for any module:
defmodule Math do
alias MyKeyword, as: Keyword
end
In the example above, we have set up `MyKeyword` to be aliased
as `Keyword`. So now, any reference to `Keyword` will be
automatically replaced by `MyKeyword`.
In case one wants to access the original `Keyword`, it can be done
by accessing `Elixir`:
Keyword.values #=> uses MyKeyword.values
Elixir.Keyword.values #=> uses Keyword.values
Notice that calling `alias` without the `:as` option automatically
sets an alias based on the last part of the module. For example:
alias Foo.Bar.Baz
Is the same as:
alias Foo.Bar.Baz, as: Baz
We can also alias multiple modules in one line:
alias Foo.{Bar, Baz, Biz}
Is the same as:
alias Foo.Bar
alias Foo.Baz
alias Foo.Biz
## Lexical scope
`import/2`, `require/2` and `alias/2` are called directives and all
have lexical scope. This means you can set up aliases inside
specific functions and it won't affect the overall scope.
## Warnings
If you alias a module and you don't use the alias, Elixir is
going to issue a warning implying the alias is not being used.
In case the alias is generated automatically by a macro,
Elixir won't emit any warnings though, since the alias
was not explicitly defined.
Both warning behaviours could be changed by explicitly
setting the `:warn` option to `true` or `false`.
"""
defmacro alias(module, opts), do: error!([module, opts])
@doc """
Requires a module in order to use its macros.
## Examples
Public functions in modules are globally available, but in order to use
macros, you need to opt-in by requiring the module they are defined in.
Let's suppose you created your own `if/2` implementation in the module
`MyMacros`. If you want to invoke it, you need to first explicitly
require the `MyMacros`:
defmodule Math do
require MyMacros
MyMacros.if do_something, it_works
end
An attempt to call a macro that was not loaded will raise an error.
## Alias shortcut
`require/2` also accepts `:as` as an option so it automatically sets
up an alias. Please check `alias/2` for more information.
"""
defmacro require(module, opts), do: error!([module, opts])
@doc """
Imports functions and macros from other modules.
`import/2` allows one to easily access functions or macros from
other modules without using the qualified name.
## Examples
If you are using several functions from a given module, you can
import those functions and reference them as local functions,
for example:
iex> import List
iex> flatten([1, [2], 3])
[1, 2, 3]
## Selector
By default, Elixir imports functions and macros from the given
module, except the ones starting with underscore (which are
usually callbacks):
import List
A developer can filter to import only macros or functions via
the only option:
import List, only: :functions
import List, only: :macros
Alternatively, Elixir allows a developer to pass pairs of
name/arities to `:only` or `:except` as a fine grained control
on what to import (or not):
import List, only: [flatten: 1]
import String, except: [split: 2]
Notice that calling `except` is always exclusive on a previously
declared `import/2`. If there is no previous import, then it applies
to all functions and macros in the module. For example:
import List, only: [flatten: 1, keyfind: 4]
import List, except: [flatten: 1]
After the two import calls above, only `List.keyfind/4` will be
imported.
## Underscore functions
By default functions starting with `_` are not imported. If you really want
to import a function starting with `_` you must explicitly include it in the
`:only` selector.
import File.Stream, only: [__build__: 3]
## Lexical scope
It is important to notice that `import/2` is lexical. This means you
can import specific macros inside specific functions:
defmodule Math do
def some_function do
# 1) Disable "if/2" from Kernel
import Kernel, except: [if: 2]
# 2) Require the new "if/2" macro from MyMacros
import MyMacros
# 3) Use the new macro
if do_something, it_works
end
end
In the example above, we imported macros from `MyMacros`,
replacing the original `if/2` implementation by our own
within that specific function. All other functions in that
module will still be able to use the original one.
## Warnings
If you import a module and you don't use any of the imported
functions or macros from this module, Elixir is going to issue
a warning implying the import is not being used.
In case the import is generated automatically by a macro,
Elixir won't emit any warnings though, since the import
was not explicitly defined.
Both warning behaviours could be changed by explicitly
setting the `:warn` option to `true` or `false`.
## Ambiguous function/macro names
If two modules `A` and `B` are imported and they both contain
a `foo` function with an arity of `1`, an error is only emitted
if an ambiguous call to `foo/1` is actually made; that is, the
errors are emitted lazily, not eagerly.
"""
defmacro import(module, opts), do: error!([module, opts])
@doc """
Returns the current environment information as a `Macro.Env` struct.
In the environment you can access the current filename,
line numbers, set up aliases, the current function and others.
"""
defmacro __ENV__, do: error!([])
@doc """
Returns the current module name as an atom or `nil` otherwise.
Although the module can be accessed in the `__ENV__/0`, this macro
is a convenient shortcut.
"""
defmacro __MODULE__, do: error!([])
@doc """
Returns the absolute path of the directory of the current file as a binary.
Although the directory can be accessed as `Path.dirname(__ENV__.file)`,
this macro is a convenient shortcut.
"""
defmacro __DIR__, do: error!([])
@doc """
Returns the current calling environment as a `Macro.Env` struct.
In the environment you can access the filename, line numbers,
set up aliases, the function and others.
"""
defmacro __CALLER__, do: error!([])
@doc """
Returns the stacktrace for the currently handled exception.
It is available only in the `catch` and `rescue` clauses of `try/1`
expressions.
To retrieve the stacktrace of the current process, use
`Process.info(self(), :current_stacktrace)` instead.
"""
defmacro __STACKTRACE__, do: error!([])
@doc """
Accesses an already bound variable in match clauses. Also known as the pin operator.
## Examples
Elixir allows variables to be rebound via static single assignment:
iex> x = 1
iex> x = x + 1
iex> x
2
However, in some situations, it is useful to match against an existing
value, instead of rebinding. This can be done with the `^` special form,
colloquially known as the pin operator:
iex> x = 1
iex> ^x = List.first([1])
iex> ^x = List.first([2])
** (MatchError) no match of right hand side value: 2
Note that `^x` always refers to the value of `x` prior to the match. The
following example will match:
iex> x = 0
iex> {x, ^x} = {1, 0}
iex> x
1
"""
defmacro ^var, do: error!([var])
@doc """
Matches the value on the right against the pattern on the left.
"""
defmacro left = right, do: error!([left, right])
@doc """
Used by types and bitstrings to specify types.
This operator is used in two distinct occasions in Elixir.
It is used in typespecs to specify the type of a variable,
function or of a type itself:
@type number :: integer | float
@spec add(number, number) :: number
It may also be used in bit strings to specify the type
of a given bit segment:
<<int::integer-little, rest::bits>> = bits
Read the documentation on the `Typespec` page and
`<<>>/1` for more information on typespecs and
bitstrings respectively.
"""
defmacro left :: right, do: error!([left, right])
@doc ~S"""
Gets the representation of any expression.
## Examples
iex> quote do
...> sum(1, 2, 3)
...> end
{:sum, [], [1, 2, 3]}
## Elixir's AST (Abstract Syntax Tree)
Any Elixir code can be represented using Elixir data structures.
The building block of Elixir macros is a tuple with three elements,
for example:
{:sum, [], [1, 2, 3]}
The tuple above represents a function call to `sum` passing 1, 2 and
3 as arguments. The tuple elements are:
* The first element of the tuple is always an atom or
another tuple in the same representation.
* The second element of the tuple represents [metadata](t:Macro.metadata/0).
* The third element of the tuple are the arguments for the
function call. The third argument may be an atom, which is
usually a variable (or a local call).
Besides the tuple described above, Elixir has a few literals that
are also part of its AST. Those literals return themselves when
quoted. They are:
:sum #=> Atoms
1 #=> Integers
2.0 #=> Floats
[1, 2] #=> Lists
"strings" #=> Strings
{key, value} #=> Tuples with two elements
Any other value, such as a map or a four-element tuple, must be escaped
(`Macro.escape/1`) before being introduced into an AST.
## Options
* `:bind_quoted` - passes a binding to the macro. Whenever a binding is
given, `unquote/1` is automatically disabled.
* `:context` - sets the resolution context.
* `:generated` - marks the given chunk as generated so it does not emit warnings.
Currently it only works on special forms (for example, you can annotate a `case`
but not an `if`).
* `:line` - sets the quoted expressions to have the given line.
* `:location` - when set to `:keep`, keeps the current line and file from
quote. Read the "Stacktrace information" section below for more information.
* `:unquote` - when `false`, disables unquoting. This means any `unquote`
call will be kept as is in the AST, instead of replaced by the `unquote`
arguments. For example:
iex> quote do
...> unquote("hello")
...> end
"hello"
iex> quote unquote: false do
...> unquote("hello")
...> end
{:unquote, [], ["hello"]}
## Quote and macros
`quote/2` is commonly used with macros for code generation. As an exercise,
let's define a macro that multiplies a number by itself (squared). In practice,
there is no reason to define such a macro (and it would actually be
seen as a bad practice), but it is simple enough that it allows us to focus
on the important aspects of quotes and macros:
defmodule Math do
defmacro squared(x) do
quote do
unquote(x) * unquote(x)
end
end
end
We can invoke it as:
import Math
IO.puts("Got #{squared(5)}")
At first, there is nothing in this example that actually reveals it is a
macro. But what is happening is that, at compilation time, `squared(5)`
becomes `5 * 5`. The argument `5` is duplicated in the produced code, we
can see this behaviour in practice though because our macro actually has
a bug:
import Math
my_number = fn ->
IO.puts("Returning 5")
5
end
IO.puts("Got #{squared(my_number.())}")
The example above will print:
Returning 5
Returning 5
Got 25
Notice how "Returning 5" was printed twice, instead of just once. This is
because a macro receives an expression and not a value (which is what we
would expect in a regular function). This means that:
squared(my_number.())
Actually expands to:
my_number.() * my_number.()
Which invokes the function twice, explaining why we get the printed value
twice! In the majority of the cases, this is actually unexpected behaviour,
and that's why one of the first things you need to keep in mind when it
comes to macros is to **not unquote the same value more than once**.
Let's fix our macro:
defmodule Math do
defmacro squared(x) do
quote do
x = unquote(x)
x * x
end
end
end
Now invoking `squared(my_number.())` as before will print the value just
once.
In fact, this pattern is so common that most of the times you will want
to use the `bind_quoted` option with `quote/2`:
defmodule Math do
defmacro squared(x) do
quote bind_quoted: [x: x] do
x * x
end
end
end
`:bind_quoted` will translate to the same code as the example above.
`:bind_quoted` can be used in many cases and is seen as good practice,
not only because it helps prevent us from running into common mistakes, but also
because it allows us to leverage other tools exposed by macros, such as
unquote fragments discussed in some sections below.
Before we finish this brief introduction, you will notice that, even though
we defined a variable `x` inside our quote:
quote do
x = unquote(x)
x * x
end
When we call:
import Math
squared(5)
x
** (CompileError) undefined variable x or undefined function x/0
We can see that `x` did not leak to the user context. This happens
because Elixir macros are hygienic, a topic we will discuss at length
in the next sections as well.
## Hygiene in variables
Consider the following example:
defmodule Hygiene do
defmacro no_interference do
quote do
a = 1
end
end
end
require Hygiene
a = 10
Hygiene.no_interference()
a
#=> 10
In the example above, `a` returns 10 even if the macro
is apparently setting it to 1 because variables defined
in the macro do not affect the context the macro is executed in.
If you want to set or get a variable in the caller's context, you
can do it with the help of the `var!` macro:
defmodule NoHygiene do
defmacro interference do
quote do
var!(a) = 1
end
end
end
require NoHygiene
a = 10
NoHygiene.interference()
a
#=> 1
You cannot even access variables defined in the same module unless
you explicitly give it a context:
defmodule Hygiene do
defmacro write do
quote do
a = 1
end
end
defmacro read do
quote do
a
end
end
end
Hygiene.write()
Hygiene.read()
** (RuntimeError) undefined variable a or undefined function a/0
For such, you can explicitly pass the current module scope as
argument:
defmodule ContextHygiene do
defmacro write do
quote do
var!(a, ContextHygiene) = 1
end
end
defmacro read do
quote do
var!(a, ContextHygiene)
end
end
end
ContextHygiene.write()
ContextHygiene.read()
#=> 1
## Hygiene in aliases
Aliases inside quote are hygienic by default.
Consider the following example:
defmodule Hygiene do
alias Map, as: M
defmacro no_interference do
quote do
M.new()
end
end
end
require Hygiene
Hygiene.no_interference()
#=> %{}
Notice that, even though the alias `M` is not available
in the context the macro is expanded, the code above works
because `M` still expands to `Map`.
Similarly, even if we defined an alias with the same name
before invoking a macro, it won't affect the macro's result:
defmodule Hygiene do
alias Map, as: M
defmacro no_interference do
quote do
M.new()
end
end
end
require Hygiene
alias SomethingElse, as: M
Hygiene.no_interference()
#=> %{}
In some cases, you want to access an alias or a module defined
in the caller. For such, you can use the `alias!` macro:
defmodule Hygiene do
# This will expand to Elixir.Nested.hello()
defmacro no_interference do
quote do
Nested.hello()
end
end
# This will expand to Nested.hello() for
# whatever is Nested in the caller
defmacro interference do
quote do
alias!(Nested).hello()
end
end
end
defmodule Parent do
defmodule Nested do
def hello, do: "world"
end
require Hygiene
Hygiene.no_interference()
** (UndefinedFunctionError) ...
Hygiene.interference()
#=> "world"
end
## Hygiene in imports
Similar to aliases, imports in Elixir are hygienic. Consider the
following code:
defmodule Hygiene do
defmacrop get_length do
quote do
length([1, 2, 3])
end
end
def return_length do
import Kernel, except: [length: 1]
get_length
end
end
Hygiene.return_length()
#=> 3
Notice how `Hygiene.return_length/0` returns `3` even though the `Kernel.length/1`
function is not imported. In fact, even if `return_length/0`
imported a function with the same name and arity from another
module, it wouldn't affect the function result:
def return_length do
import String, only: [length: 1]
get_length
end
Calling this new `return_length/0` will still return `3` as result.
Elixir is smart enough to delay the resolution to the latest
possible moment. So, if you call `length([1, 2, 3])` inside quote,
but no `length/1` function is available, it is then expanded in
the caller:
defmodule Lazy do
defmacrop get_length do
import Kernel, except: [length: 1]
quote do
length("hello")
end
end
def return_length do
import Kernel, except: [length: 1]
import String, only: [length: 1]
get_length
end
end
Lazy.return_length()
#=> 5
## Stacktrace information
When defining functions via macros, developers have the option of
choosing if runtime errors will be reported from the caller or from
inside the quote. Let's see an example:
# adder.ex
defmodule Adder do
@doc "Defines a function that adds two numbers"
defmacro defadd do
quote location: :keep do
def add(a, b), do: a + b
end
end
end
# sample.ex
defmodule Sample do
import Adder
defadd
end
require Sample
Sample.add(:one, :two)
** (ArithmeticError) bad argument in arithmetic expression
adder.ex:5: Sample.add/2
When using `location: :keep` and invalid arguments are given to
`Sample.add/2`, the stacktrace information will point to the file
and line inside the quote. Without `location: :keep`, the error is
reported to where `defadd` was invoked. `location: :keep` affects
only definitions inside the quote.
## Binding and unquote fragments
Elixir quote/unquote mechanisms provide a functionality called
unquote fragments. Unquote fragments provide an easy way to generate
functions on the fly. Consider this example:
kv = [foo: 1, bar: 2]
Enum.each(kv, fn {k, v} ->
def unquote(k)(), do: unquote(v)
end)
In the example above, we have generated the functions `foo/0` and
`bar/0` dynamically. Now, imagine that we want to convert this
functionality into a macro:
defmacro defkv(kv) do
Enum.map(kv, fn {k, v} ->
quote do
def unquote(k)(), do: unquote(v)
end
end)
end
We can invoke this macro as:
defkv [foo: 1, bar: 2]
However, we can't invoke it as follows:
kv = [foo: 1, bar: 2]
defkv kv
This is because the macro is expecting its arguments to be a
keyword list at **compilation** time. Since in the example above
we are passing the representation of the variable `kv`, our
code fails.
This is actually a common pitfall when developing macros. We are
assuming a particular shape in the macro. We can work around it
by unquoting the variable inside the quoted expression:
defmacro defkv(kv) do
quote do
Enum.each(unquote(kv), fn {k, v} ->
def unquote(k)(), do: unquote(v)
end)
end
end
If you try to run our new macro, you will notice it won't
even compile, complaining that the variables `k` and `v`
do not exist. This is because of the ambiguity: `unquote(k)`
can either be an unquote fragment, as previously, or a regular
unquote as in `unquote(kv)`.
One solution to this problem is to disable unquoting in the
macro, however, doing that would make it impossible to inject the
`kv` representation into the tree. That's when the `:bind_quoted`
option comes to the rescue (again!). By using `:bind_quoted`, we
can automatically disable unquoting while still injecting the
desired variables into the tree:
defmacro defkv(kv) do
quote bind_quoted: [kv: kv] do
Enum.each(kv, fn {k, v} ->
def unquote(k)(), do: unquote(v)
end)
end
end
In fact, the `:bind_quoted` option is recommended every time
one desires to inject a value into the quote.
"""
defmacro quote(opts, block), do: error!([opts, block])
@doc """
Unquotes the given expression inside a quoted expression.
This function expects a valid Elixir AST, also known as
quoted expression, as argument. If you would like to `unquote`
any value, such as a map or a four-element tuple, you should
call `Macro.escape/1` before unquoting.
## Examples
Imagine the situation you have a quoted expression and
you want to inject it inside some quote. The first attempt
would be:
value =
quote do
13
end
quote do
sum(1, value, 3)
end
Which would then return:
{:sum, [], [1, {:value, [], Elixir}, 3]}
Which is not the expected result. For this, we use `unquote`:
iex> value =
...> quote do
...> 13
...> end
iex> quote do
...> sum(1, unquote(value), 3)
...> end
{:sum, [], [1, 13, 3]}
If you want to unquote a value that is not a quoted expression,
such as a map, you need to call `Macro.escape/1` before:
iex> value = %{foo: :bar}
iex> quote do
...> process_map(unquote(Macro.escape(value)))
...> end
{:process_map, [], [{:%{}, [], [foo: :bar]}]}
If you forget to escape it, Elixir will raise an error
when compiling the code.
"""
defmacro unquote(:unquote)(expr), do: error!([expr])
@doc """
Unquotes the given list expanding its arguments.
Similar to `unquote/1`.
## Examples
iex> values = [2, 3, 4]
iex> quote do
...> sum(1, unquote_splicing(values), 5)
...> end
{:sum, [], [1, 2, 3, 4, 5]}
"""
defmacro unquote(:unquote_splicing)(expr), do: error!([expr])
@doc ~S"""
Comprehensions allow you to quickly build a data structure from
an enumerable or a bitstring.
Let's start with an example:
iex> for n <- [1, 2, 3, 4], do: n * 2
[2, 4, 6, 8]
A comprehension accepts many generators and filters. Enumerable
generators are defined using `<-`:
# A list generator:
iex> for n <- [1, 2, 3, 4], do: n * 2
[2, 4, 6, 8]
# A comprehension with two generators
iex> for x <- [1, 2], y <- [2, 3], do: x * y
[2, 3, 4, 6]
Filters can also be given:
# A comprehension with a generator and a filter
iex> for n <- [1, 2, 3, 4, 5, 6], rem(n, 2) == 0, do: n
[2, 4, 6]
Generators can also be used to filter as it removes any value
that doesn't match the pattern on the left side of `<-`:
iex> users = [user: "john", admin: "meg", guest: "barbara"]
iex> for {type, name} when type != :guest <- users do
...> String.upcase(name)
...> end
["JOHN", "MEG"]
Bitstring generators are also supported and are very useful when you
need to organize bitstring streams:
iex> pixels = <<213, 45, 132, 64, 76, 32, 76, 0, 0, 234, 32, 15>>
iex> for <<r::8, g::8, b::8 <- pixels>>, do: {r, g, b}
[{213, 45, 132}, {64, 76, 32}, {76, 0, 0}, {234, 32, 15}]
Variable assignments inside the comprehension, be it in generators,
filters or inside the block, are not reflected outside of the
comprehension.
## The `:into` and `:uniq` options
In the examples above, the result returned by the comprehension was
always a list. The returned result can be configured by passing an
`:into` option, that accepts any structure as long as it implements
the `Collectable` protocol.
For example, we can use bitstring generators with the `:into` option
to easily remove all spaces in a string:
iex> for <<c <- " hello world ">>, c != ?\s, into: "", do: <<c>>
"helloworld"
The `IO` module provides streams, that are both `Enumerable` and
`Collectable`, here is an upcase echo server using comprehensions:
for line <- IO.stream(:stdio, :line), into: IO.stream(:stdio, :line) do
String.upcase(line)
end
Similarly, `uniq: true` can also be given to comprehensions to guarantee
the results are only added to the collection if they were not returned
before. For example:
iex> for x <- [1, 1, 2, 3], uniq: true, do: x * 2
[2, 4, 6]
iex> for <<x <- "abcabc">>, uniq: true, into: "", do: <<x - 32>>
"ABC"
## The `:reduce` option
While the `:into` option allows us to customize the comprehension behaviour
to a given data type, such as putting all of the values inside a map or inside
a binary, it is not always enough.
For example, imagine that you have a binary with letters where you want to
count how many times each lowercase letter happens, ignoring all uppercase
ones. For instance, for the string `"AbCabCABc"`, we want to return the map
`%{"a" => 1, "b" => 2, "c" => 1}`.
If we were to use `:into`, we would need a data type that computes the
frequency of each element it holds. While there is no such data type in
Elixir, you could implement one yourself.
A simpler option would be to use comprehensions for the mapping and
filtering of letters, and then we invoke `Enum.reduce/3` to build a map,
for example:
iex> letters = for <<x <- "AbCabCABc">>, x in ?a..?z, do: <<x>>
iex> Enum.reduce(letters, %{}, fn x, acc -> Map.update(acc, x, 1, & &1 + 1) end)
%{"a" => 1, "b" => 2, "c" => 1}
While the above is straight-forward, it has the downside of traversing the
data at least twice. If you are expecting long strings as inputs, this can
be quite expensive.
Luckily, comprehensions also support the `:reduce` option, which would allow
us to fuse both steps above into a single step:
iex> for <<x <- "AbCabCABc">>, x in ?a..?z, reduce: %{} do
...> acc -> Map.update(acc, <<x>>, 1, & &1 + 1)
...> end
%{"a" => 1, "b" => 2, "c" => 1}
When the `:reduce` key is given, its value is used as the initial accumulator
and the `do` block must be changed to use `->` clauses, where the left side
of `->` receives the accumulated value of the previous iteration and the
expression on the right side must return the new accumulator value. Once there are no more
elements, the final accumulated value is returned. If there are no elements
at all, then the initial accumulator value is returned.
"""
defmacro for(args), do: error!([args])
@doc """
Used to combine matching clauses.
Let's start with an example:
iex> opts = %{width: 10, height: 15}
iex> with {:ok, width} <- Map.fetch(opts, :width),
...> {:ok, height} <- Map.fetch(opts, :height) do
...> {:ok, width * height}
...> end
{:ok, 150}
If all clauses match, the `do` block is executed, returning its result.
Otherwise the chain is aborted and the non-matched value is returned:
iex> opts = %{width: 10}
iex> with {:ok, width} <- Map.fetch(opts, :width),
...> {:ok, height} <- Map.fetch(opts, :height) do
...> {:ok, width * height}
...> end
:error
Guards can be used in patterns as well:
iex> users = %{"melany" => "guest", "bob" => :admin}
iex> with {:ok, role} when not is_binary(role) <- Map.fetch(users, "bob") do
...> {:ok, to_string(role)}
...> end
{:ok, "admin"}
As in `for/1`, variables bound inside `with/1` won't leak.
Expressions without `<-` may also be used in clauses. For instance,
you can perform regular matches with the `=` operator:
iex> width = nil
iex> opts = %{width: 10, height: 15}
iex> with {:ok, width} <- Map.fetch(opts, :width),
...> double_width = width * 2,
...> {:ok, height} <- Map.fetch(opts, :height) do
...> {:ok, double_width * height}
...> end
{:ok, 300}
iex> width
nil
The behaviour of any expression in a clause is the same as outside.
For example, `=` will raise a `MatchError` instead of returning the
non-matched value:
with :foo = :bar, do: :ok
** (MatchError) no match of right hand side value: :bar
As with any other function or macro call in Elixir, explicit parens can
also be used around the arguments before the `do`/`end` block:
iex> opts = %{width: 10, height: 15}
iex> with(
...> {:ok, width} <- Map.fetch(opts, :width),
...> {:ok, height} <- Map.fetch(opts, :height)
...> ) do
...> {:ok, width * height}
...> end
{:ok, 150}
The choice between parens and no parens is a matter of preference.
An `else` option can be given to modify what is being returned from
`with` in the case of a failed match:
iex> opts = %{width: 10}
iex> with {:ok, width} <- Map.fetch(opts, :width),
...> {:ok, height} <- Map.fetch(opts, :height) do
...> {:ok, width * height}
...> else
...> :error ->
...> {:error, :wrong_data}
...> end
{:error, :wrong_data}
If an `else` block is used and there are no matching clauses, a `WithClauseError`
exception is raised.
"""
defmacro with(args), do: error!([args])
@doc """
Defines an anonymous function.
## Examples
iex> add = fn a, b -> a + b end
iex> add.(1, 2)
3
Anonymous functions can also have multiple clauses. All clauses
should expect the same number of arguments:
iex> negate = fn
...> true -> false
...> false -> true
...> end
iex> negate.(false)
true
"""
defmacro unquote(:fn)(clauses), do: error!([clauses])
@doc """
Internal special form for block expressions.
This is the special form used whenever we have a block
of expressions in Elixir. This special form is private
and should not be invoked directly:
iex> quote do
...> 1
...> 2
...> 3
...> end
{:__block__, [], [1, 2, 3]}
"""
defmacro unquote(:__block__)(args), do: error!([args])
@doc """
Captures or creates an anonymous function.
## Capture
The capture operator is most commonly used to capture a
function with given name and arity from a module:
iex> fun = &Kernel.is_atom/1
iex> fun.(:atom)
true
iex> fun.("string")
false
In the example above, we captured `Kernel.is_atom/1` as an
anonymous function and then invoked it.
The capture operator can also be used to capture local functions,
including private ones, and imported functions by omitting the
module name:
&local_function/1
See also `Function.capture/3`.
## Anonymous functions
The capture operator can also be used to partially apply
functions, where `&1`, `&2` and so on can be used as value
placeholders. For example:
iex> double = &(&1 * 2)
iex> double.(2)
4
In other words, `&(&1 * 2)` is equivalent to `fn x -> x * 2 end`.
We can partially apply a remote function with placeholder:
iex> take_five = &Enum.take(&1, 5)
iex> take_five.(1..10)
[1, 2, 3, 4, 5]
Another example while using an imported or local function:
iex> first_elem = &elem(&1, 0)
iex> first_elem.({0, 1})
0
The `&` operator can be used with more complex expressions:
iex> fun = &(&1 + &2 + &3)
iex> fun.(1, 2, 3)
6
As well as with lists and tuples:
iex> fun = &{&1, &2}
iex> fun.(1, 2)
{1, 2}
iex> fun = &[&1 | &2]
iex> fun.(1, [2, 3])
[1, 2, 3]
The only restrictions when creating anonymous functions is that at
least one placeholder must be present, i.e. it must contain at least
`&1`, and that block expressions are not supported:
# No placeholder, fails to compile.
&(:foo)
# Block expression, fails to compile.
&(&1; &2)
"""
defmacro unquote(:&)(expr), do: error!([expr])
@doc """
Internal special form to hold aliases information.
It is usually compiled to an atom:
iex> quote do
...> Foo.Bar
...> end
{:__aliases__, [alias: false], [:Foo, :Bar]}
Elixir represents `Foo.Bar` as `__aliases__` so calls can be
unambiguously identified by the operator `:.`. For example:
iex> quote do
...> Foo.bar()
...> end
{{:., [], [{:__aliases__, [alias: false], [:Foo]}, :bar]}, [], []}
Whenever an expression iterator sees a `:.` as the tuple key,
it can be sure that it represents a call and the second argument
in the list is an atom.
On the other hand, aliases hold some properties:
1. The head element of aliases can be any term that must expand to
an atom at compilation time.
2. The tail elements of aliases are guaranteed to always be atoms.
3. When the head element of aliases is the atom `:Elixir`, no expansion happens.
"""
defmacro unquote(:__aliases__)(args), do: error!([args])
@doc """
Calls the overridden function when overriding it with `Kernel.defoverridable/1`.
See `Kernel.defoverridable/1` for more information and documentation.
"""
defmacro super(args), do: error!([args])
@doc ~S"""
Matches the given expression against the given clauses.
## Examples
case thing do
{:selector, i, value} when is_integer(i) ->
value
value ->
value
end
In the example above, we match `thing` against each clause "head"
and execute the clause "body" corresponding to the first clause
that matches.
If no clause matches, an error is raised.
For this reason, it may be necessary to add a final catch-all clause (like `_`)
which will always match.
x = 10
case x do
0 ->
"This clause won't match"
_ ->
"This clause would match any value (x = #{x})"
end
#=> "This clause would match any value (x = 10)"
## Variable handling
Notice that variables bound in a clause do not leak to the outer context:
case data do
{:ok, value} -> value
:error -> nil
end
value
#=> unbound variable value
When binding variables with the same names as variables in the outer context,
the variables in the outer context are not affected.
value = 7
case lucky? do
false -> value = 13
true -> true
end
value
#=> 7
In the example above, `value` is going to be `7` regardless of the value of
`lucky?`. The variable `value` bound in the clause and the variable `value`
bound in the outer context are two entirely separate variables.
If you want to pattern match against an existing variable,
you need to use the `^/1` operator:
x = 1
case 10 do
^x -> "Won't match"
_ -> "Will match"
end
#=> "Will match"
"""
defmacro case(condition, clauses), do: error!([condition, clauses])
@doc """
Evaluates the expression corresponding to the first clause that
evaluates to a truthy value.
cond do
hd([1, 2, 3]) ->
"1 is considered as true"
end
#=> "1 is considered as true"
Raises an error if all conditions evaluate to `nil` or `false`.
For this reason, it may be necessary to add a final always-truthy condition
(anything non-`false` and non-`nil`), which will always match.
## Examples
cond do
1 + 1 == 1 ->
"This will never match"
2 * 2 != 4 ->
"Nor this"
true ->
"This will"
end
#=> "This will"
"""
defmacro cond(clauses), do: error!([clauses])
@doc ~S"""
Evaluates the given expressions and handles any error, exit,
or throw that may have happened.
## Examples
try do
do_something_that_may_fail(some_arg)
rescue
ArgumentError ->
IO.puts("Invalid argument given")
catch
value ->
IO.puts("Caught #{inspect(value)}")
else
value ->
IO.puts("Success! The result was #{inspect(value)}")
after
IO.puts("This is printed regardless if it failed or succeeded")
end
The `rescue` clause is used to handle exceptions while the `catch`
clause can be used to catch thrown values and exits.
The `else` clause can be used to control flow based on the result of
the expression. `catch`, `rescue`, and `else` clauses work based on
pattern matching (similar to the `case` special form).
Calls inside `try/1` are not tail recursive since the VM needs to keep
the stacktrace in case an exception happens. To retrieve the stacktrace,
access `__STACKTRACE__/0` inside the `rescue` or `catch` clause.
## `rescue` clauses
Besides relying on pattern matching, `rescue` clauses provide some
conveniences around exceptions that allow one to rescue an
exception by its name. All the following formats are valid patterns
in `rescue` clauses:
# Rescue a single exception without binding the exception
# to a variable
try do
UndefinedModule.undefined_function
rescue
UndefinedFunctionError -> nil
end
# Rescue any of the given exception without binding
try do
UndefinedModule.undefined_function
rescue
[UndefinedFunctionError, ArgumentError] -> nil
end
# Rescue and bind the exception to the variable "x"
try do
UndefinedModule.undefined_function
rescue
x in [UndefinedFunctionError] -> nil
end
# Rescue all kinds of exceptions and bind the rescued exception
# to the variable "x"
try do
UndefinedModule.undefined_function
rescue
x -> nil
end
### Erlang errors
Erlang errors are transformed into Elixir ones when rescuing:
try do
:erlang.error(:badarg)
rescue
ArgumentError -> :ok
end
#=> :ok
The most common Erlang errors will be transformed into their
Elixir counterpart. Those which are not will be transformed
into the more generic `ErlangError`:
try do
:erlang.error(:unknown)
rescue
ErlangError -> :ok
end
#=> :ok
In fact, `ErlangError` can be used to rescue any error that is
not a proper Elixir error. For example, it can be used to rescue
the earlier `:badarg` error too, prior to transformation:
try do
:erlang.error(:badarg)
rescue
ErlangError -> :ok
end
#=> :ok
## `catch` clauses
The `catch` clause can be used to catch thrown values, exits, and errors.
### Catching thrown values
`catch` can be used to catch values thrown by `Kernel.throw/1`:
try do
throw(:some_value)
catch
thrown_value ->
IO.puts("A value was thrown: #{inspect(thrown_value)}")
end
### Catching values of any kind
The `catch` clause also supports catching exits and errors. To do that, it
allows matching on both the *kind* of the caught value as well as the value
itself:
try do
exit(:shutdown)
catch
:exit, value ->
IO.puts("Exited with value #{inspect(value)}")
end
try do
exit(:shutdown)
catch
kind, value when kind in [:exit, :throw] ->
IO.puts("Caught exit or throw with value #{inspect(value)}")
end
The `catch` clause also supports `:error` alongside `:exit` and `:throw` as
in Erlang, although this is commonly avoided in favor of `raise`/`rescue` control
mechanisms. One reason for this is that when catching `:error`, the error is
not automatically transformed into an Elixir error:
try do
:erlang.error(:badarg)
catch
:error, :badarg -> :ok
end
#=> :ok
## `after` clauses
An `after` clause allows you to define cleanup logic that will be invoked both
when the block of code passed to `try/1` succeeds and also when an error is raised. Note
that the process will exit as usual when receiving an exit signal that causes
it to exit abruptly and so the `after` clause is not guaranteed to be executed.
Luckily, most resources in Elixir (such as open files, ETS tables, ports, sockets,
and so on) are linked to or monitor the owning process and will automatically clean
themselves up if that process exits.
File.write!("tmp/story.txt", "Hello, World")
try do
do_something_with("tmp/story.txt")
after
File.rm("tmp/story.txt")
end
## `else` clauses
`else` clauses allow the result of the body passed to `try/1` to be pattern
matched on:
x = 2
try do
1 / x
rescue
ArithmeticError ->
:infinity
else
y when y < 1 and y > -1 ->
:small
_ ->
:large
end
If an `else` clause is not present and no exceptions are raised,
the result of the expression will be returned:
x = 1
^x =
try do
1 / x
rescue
ArithmeticError ->
:infinity
end
However, when an `else` clause is present but the result of the expression
does not match any of the patterns then an exception will be raised. This
exception will not be caught by a `catch` or `rescue` in the same `try`:
x = 1
try do
try do
1 / x
rescue
# The TryClauseError cannot be rescued here:
TryClauseError ->
:error_a
else
0 ->
:small
end
rescue
# The TryClauseError is rescued here:
TryClauseError ->
:error_b
end
Similarly, an exception inside an `else` clause is not caught or rescued
inside the same `try`:
try do
try do
nil
catch
# The exit(1) call below can not be caught here:
:exit, _ ->
:exit_a
else
_ ->
exit(1)
end
catch
# The exit is caught here:
:exit, _ ->
:exit_b
end
This means the VM no longer needs to keep the stacktrace once inside
an `else` clause and so tail recursion is possible when using a `try`
with a tail call as the final call inside an `else` clause. The same
is true for `rescue` and `catch` clauses.
Only the result of the tried expression falls down to the `else` clause.
If the `try` ends up in the `rescue` or `catch` clauses, their result
will not fall down to `else`:
try do
throw(:catch_this)
catch
:throw, :catch_this ->
:it_was_caught
else
# :it_was_caught will not fall down to this "else" clause.
other ->
{:else, other}
end
## Variable handling
Since an expression inside `try` may not have been evaluated
due to an exception, any variable created inside `try` cannot
be accessed externally. For instance:
try do
x = 1
do_something_that_may_fail(same_arg)
:ok
catch
_, _ -> :failed
end
x
#=> unbound variable "x"
In the example above, `x` cannot be accessed since it was defined
inside the `try` clause. A common practice to address this issue
is to return the variables defined inside `try`:
x =
try do
x = 1
do_something_that_may_fail(same_arg)
x
catch
_, _ -> :failed
end
"""
defmacro try(args), do: error!([args])
@doc """
Checks if there is a message matching the given clauses
in the current process mailbox.
In case there is no such message, the current process hangs
until a message arrives or waits until a given timeout value.
## Examples
receive do
{:selector, number, name} when is_integer(number) ->
name
name when is_atom(name) ->
name
_ ->
IO.puts(:stderr, "Unexpected message received")
end
An optional `after` clause can be given in case the message was not
received after the given timeout period, specified in milliseconds:
receive do
{:selector, number, name} when is_integer(number) ->
name
name when is_atom(name) ->
name
_ ->
IO.puts(:stderr, "Unexpected message received")
after
5000 ->
IO.puts(:stderr, "No message in 5 seconds")
end
The `after` clause can be specified even if there are no match clauses.
The timeout value given to `after` can be any expression evaluating to
one of the allowed values:
* `:infinity` - the process should wait indefinitely for a matching
message, this is the same as not using the after clause
* `0` - if there is no matching message in the mailbox, the timeout
will occur immediately
* positive integer smaller than or equal to `4_294_967_295` (`0xFFFFFFFF`
in hexadecimal notation) - it should be possible to represent the timeout
value as an unsigned 32-bit integer.
## Variable handling
The `receive/1` special form handles variables exactly as the `case/2`
special macro. For more information, check the docs for `case/2`.
"""
defmacro receive(args), do: error!([args])
end
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