Source file: functions.fut

Functions

Functions are defined with def, where we can also annotate both the parameter and return types.

def plus2 (x: i32) : i32 =
  x + 2

Type inference is supported, so in many cases the types can be left off, in which case the correct type will be inferred from the definition.

def plus2_inferred x =
  x + 2

Functions are (almost) first-class values, with the type of a function from type a to type b written as a -> b. This means we can write higher-order functions:

def on_i32 (f: i32 -> i32) (x: i32) =
  f x

When passing an argument to a higher-order function, it is often most practical to use an anonymous function (sometimes called a lambda), like this:

def four = on_i32 (\x -> x + 2) 2

A shortcut notation, called operator sections, allows partial application of infix operators:

def another_four = on_i32 (+2) 2

By enclosing an operator in parentheses we can treat it like any variable name:

def plus = (+)

And we can treat a variable as an infix operator by enclosing it in backticks:

def yet_another_four = 2 `plus` 2

Function values are restricted as follows:

Arrays of functions are not permitted.
A function cannot be returned from an if expression.
A loop parameter cannot be a function.

However, it is fine to collect functions in tuples or records.

def fun_tuple : (i32 -> i32, i32 -> i32) =
  (plus2, plus2_inferred)

Polymorphic functions are also supported.