Syntax.md

Syntax

Whitespace Rules

Syntax is whitespace sensitive. The general rule of thumb is that declarations which span multiple lines should be indented past the column on which they were first defined on their subsequent lines.

That is, the following is valid:

foo = bar +
  baz

But this is not:

foo = bar +
baz

Comments

A single line comment starts with --:

-- This is a comment

Multi-line comments are enclosed in {- and -}:

{-
  Comment
  continued comment
-}

Comments that start with a pipe character, |, are considered documentation, and will appear in the output of tools like psc-docs and Pursuit. For example:

-- | `bool` performs case analysis for the `Boolean` data type, like an `if` statement.
bool :: forall a. Boolean -> a -> a -> a
bool true x _ = x
bool false _ x = x

Note that, unlike Haskell, every line which should be considered documentation must start with a pipe. This allows you to do things like:

-- | Sort an array based on its `Ord` instance.
-- |
-- | This implementation runs in `O(n^2)` time, where `n` is the length of the
-- | input array.
-- TODO: try to optimise this?
sort :: forall a. (Ord a) => Array a -> Array a
sort xs = [...]

Top-level declarations

Values at the top level of a module are defined by providing a name followed by an equals sign and then the value to associate:

one = 1

Functions can also be defined at the top level by providing a list of patterns on the left hand side of the equals sign:

add x y = x + y

See the section on pattern matching for more details about the kinds of patterns that can be used here.

Functions using pattern matching may be defined multiple times to handle different pattern matches:

isEmpty [] = true
isEmpty _ = false

This does not mean functions can be arbitrarily overloaded with different numbers or types of arguments though.

Guards can also be used in these definitions:

isEmptyAlt xs | length xs == 0 = true
isEmptyAlt _ = false

A top level declaration is generally defined with a type signature:

multiply :: Number -> Number -> Number
multiply x y = x * y

Type signatures are not required for top-level declarations in general, but is good practice to do so. See the section on types for more details.

Function application

Function application is indicated by just the juxtaposition of a function with its arguments:

add 10 20

PureScript functions are defined as curried, so partial application has no special syntax:

add10 = add 10

In fact, add 10 20 is parsed as (add 10) 20.

Literals

Numbers

Numeric literals can be integers (type Int) or floating point numbers (type Number). Floating point numbers are identified by a decimal point. Integers in hexadecimal notation should be preceded by the characters 0x:

16 :: Int
0xF0 :: Int
16.0 :: Number

Strings

String literals are enclosed in double-quotes and may extend over multiple lines. Line breaks should be surrounded by slashes as follows:

"Hello World"

"Hello \
\World"

Line breaks will be omitted from the string when written this way.

Triple-quote Strings

If line breaks are required in the output, they can be inserted with \n. Alternatively, you can use triple double-quotes to prevent special parsing of escaped symbols. This also allows the use of double quotes within the string with no need to escape them:

jsIsHello :: String
jsIsHello = """
function isHello(greeting) {
  return greeting === "Hello";
}
"""

This method of declaring strings is especially useful when writing regular expression strings.

regex ".+@.+\\..+" noFlags
regex """.+@.+\..+""" noFlags

The example regular expression above is a very simple email address validator. Both are equivalent, but the second one, using triple double-quotes, is much easier to write and maintain. This is even more true when writing complex regular expressions, as many are.

Booleans

The boolean literals are true and false.

Functions

Function values (sometimes called lambdas) are introduced by using a backslash followed by a list of argument names:

\a b -> a + b

which would correspond to the following JavaScript:

function (a) {
  return function (b) {
    return a + b;
  }
}

Arrays

Array literals are surrounded by square brackets, as in JavaScript:

[]
[1, 2, 3]

Records

Record literals are surrounded by braces, as in JavaScript:

{}
{ foo: "Foo", bar: 1 }

Record literals with wildcards can be used to create a function that produces the record instead:

{ foo: _, bar: _ }

is equivalent to:

\foo bar -> { foo: foo, bar: bar }

Additional forms with Records

Property Accessors

To access a property of a record, use a dot followed by the property name, as in JavaScript:

rec.propertyName

There are also partially applied accessors, where an underscore is followed by a property name:

_.propertyName

This is equivalent to:

\rec -> rec.propertyName

These work with any number of levels:

_.nested.property.name

Record Updates

Properties on records can be updated using the following syntax:

rec { key1 = value1, ..., keyN = valueN, nestedKey { subKey = value, ... } }

Some or all of the keys may be updated at once, and records inside of records can also be updated.

For example, the following function increments the foo property on its argument:

\rec -> rec { foo = rec.foo + 1 }

Nested record updates look like this:

r = { val: -1
    , level1: { val: -1
              , level2: { val: -1 }
              }
    }
r' = r { level1 { val = 1 } }

Wildcards can also be used in updates to produce a partially applied update:

rec { foo = _ }

This is equivalent to:

\foo -> rec { foo = foo }

An underscore can also appear in the object position in an updater:

_ { foo = 1 }

This is equivalent to:

\rec -> rec { foo = 1 }

Binary Operators

Operators in PureScript are just regular binary functions. In particular, no operators are built into the language; an overview of operators defined in libraries such as the Prelude is therefore outside the scope of this reference.

Operators can be defined by providing an operator alias for an existing function (which must be binary, i.e. its type must be of the form a -> b -> c). For example:

data List a = Nil | Cons a (List a)

append :: forall a. List a -> List a -> List a
append xs Nil = xs
append Nil ys = ys
append (Cons x xs) ys = Cons x (append xs ys)

infixr 5 append as <>

This function can be used as follows::

oneToThree = Cons 1 (Cons 2 (Cons 3 Nil))
fourToSix = Cons 4 (Cons 5 (Cons 6 Nil))

oneToSix = oneToThree <> fourToSix

Operator alias declarations are made up of four parts:

• The associativity: either infixl, infixr, or infix.
• The precedence: an integer, between 0 and 9. Here, it is 5.
• The function to alias: here, append
• The operator: here, <>.

The declaration determines how expressions involving this operator are bracketed.

Associativity

infixl means that repeated applications are bracketed starting from the left. For example, # from Prelude is left-associative, meaning that an expression such as:

products # filter isInStock # groupBy productCategory # length

is bracketed as:

((products # filter isInStock) # groupBy productCategory) # length

Similarly, infixr means "right-associative", and repeated applications are bracketed starting from the right. For example, $ from Prelude is right-associative, so an expression like this:

length $ groupBy productCategory $ filter isInStock $ products

is bracketed as:

length $ (groupBy productCategory $ (filter isInStock $ products))

infix means "non-associative". Repeated use of a non-associative operator is disallowed. For example, == from Prelude is non-associative, which means that

true == true == true

results in a NonAssociativeError:

Cannot parse an expression that uses multiple instances of the non-associative operator Data.Eq.(==).
Use parentheses to resolve this ambiguity.

Non-associative parsing via infix is most appropriate for operators f for which x `f` (y `f` z) is not necessarily the same as (x `f` y) `f` z), that is, operators which do not satisfy the algebraic property of associativity.

Precedence

Precedence determines the order in which operators are bracketed. Operators with a higher precedence will be bracketed earlier. For example, take * and + from Prelude. * is precedence 7, whereas + is precedence 6. Therefore, if we write:

2 * 3 + 4

then this is bracketed as follows:

(2 * 3) + 4

Operators of different associativities may appear together as long as they do not have the same precedence. This restriction exists because the compiler is not able to make a sensible choice as to how to bracket such expressions. For example, the operators == and <$> from the Prelude have the fixities infix 4 and infixl 4 respectively, which means that given the expression

f <$> x == f <$> y

the compiler does not know whether to bracket it as

(f <$> x) == (f <$> y)

or

f <$> (x == f) <$> y

Therefore, we get a MixedAssociativityError:

Cannot parse an expression that uses operators of the same precedence but mixed associativity:

  Data.Functor.(<$>) is infixl
  Data.Eq.(==) is infix

Use parentheses to resolve this ambiguity.

Operators as values

Operators can be used as normal values by surrounding them with parentheses:

and = (&&)

Operator sections

Operators can be partially applied by surrounding them with parentheses and using _ as one of the operands:

half = (_ / 2)
double = (2 * _)

Functions as operators

Functions can be used as infix operators when they are surrounded by backticks:

foo x y = x * y + y
test = 10 `foo` 20

Operator sections also work for functions used this way:

fooBy2 = (_ `foo` 2)

Case expressions

The case and of keywords are used to deconstruct values to create logic based on the value's constructors. You can match on multiple values by delimiting them with , in the head and cases.

f :: Maybe Boolean -> Either Boolean Boolean -> String
f a b = case a, b of
  Just true, Right true -> "Both true"
  Just true, Left _ -> "Just is true"
  Nothing, Right true -> "Right is true"
  _, _ -> "Both are false"
f (Just true) (Right true)

Like top-level declarations, case expressions support guards.

f :: Either Int Unit -> String
f x = case x of
  Left x | x == 0 -> "Left zero"
         | x < 0 -> "Left negative"
         | otherwise -> "Left positive"
  Right _ -> "Right"

A binding can be avoided by using a single underscore in place of the expression to match on; in this context the underscore represents an anonymous argument.

case _ of
  0 -> "None"
  1 -> "One"
  _ -> "Some"

This is equivalent to

\x -> case x of
  0 -> "None"
  1 -> "One"
  _ -> "Some"

If-Then-Else expressions

The if, then and else keywords can be used to create conditional expressions similar to a JavaScript ternary expression. The else block is always required:

conditional = if 2 > 1 then "ok" else "oops"

Let and where bindings

The let keyword introduces a collection of local declarations, which may be mutually recursive, and which may include type declarations:

factorial :: Int -> Int
factorial =
  let
    go :: Int -> Int -> Int
    go acc 1 = acc
    go acc n = go (acc * n) (n - 1)
  in
    go 1

The where keyword can also be used to introduce local declarations at the end of a value declaration:

factorial :: Int -> Int
factorial = go 1
  where
  go :: Int -> Int -> Int
  go acc 1 = acc
  go acc n = go (acc * n) (n - 1)

Indentation in binding blocks

Indentation of a binding's body is significant. If defining multiple bindings, as in a let-in block, each binding must have the same level of indentation. The body of the binding's definition, then, must be further indented. To illustrate:

f =
  -- The `let-in` block starts at an indentation of 2 spaces, so
  --   the bindings in it must start at an indentation greater than 2.
  let
    -- Because `x` is indented 4 spaces, `y` must also be indented 4 spaces.
    -- Its body, then, must have indentation greater than 4 spaces.
    x :: Int -> Int
    x a =
      -- This body is indented 2 spaces.
      a
    y :: Int -> Int
    y c =
        -- This body is indented 4 spaces.
        c
  in do
    -- Because `m` is indented 4 spaces from the start of the `let`,
    --   `n` must also be indented 4 spaces.
    -- Its body, then, must be greater than 4 spaces.
    let m =
          -- This body is indented 2 spaces.
          x (y 1)
        n =
            -- This body is indented 4 spaces.
            x 1
    log "test"

Do notation

The do keyword introduces simple syntactic sugar for monadic expressions.

Here is an example, using the monad for the Maybe type:

maybeSum :: Maybe Number -> Maybe Number -> Maybe Number
maybeSum a b = do
  n <- a
  m <- b
  let result = n + m
  pure result

maybeSum takes two values of type Maybe Number and returns their sum if neither value is Nothing.

When using do notation, there must be a corresponding instance of the Monad type class for the return type.

Statements can have the following form:

• a <- x which desugars to x >>= \a -> ...
• x which desugars to x >>= \_ -> ... or just x if this is the last statement.
• A let binding let a = x. Note the lack of the in keyword.

The example maybeSum desugars to::

maybeSum a b =
  a >>= \n ->
    b >>= \m ->
      let result = n + m
      in pure result

Note: (>>=) is the bind function for the Bind type as defined in the Prelude package.