The Wyn Language Specification

Higher-order Functions

At a high level, Wyn functions are values, and can be used as any other value. However, to ensure that the compiler is able to compile the higher-order functions efficiently via defunctionalisation, certain type-driven restrictions exist on how functions can be used. These also apply to any record or tuple containing a function (a functional type):

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

Further, type parameters are divided into non-lifted (bound with an apostrophe, e.g. 't), size-lifted ('~t), and fully lifted ('^t). Only fully lifted type parameters may be instantiated with a functional type. Within a function, a lifted type parameter is treated as a functional type.

See also In-place updates for details on how consumption interacts with higher-order functions.

Function Arity and Partial Application

Wyn functions are not curried by default. Every function has a fixed arity (number of arguments) and must be called with exactly that many arguments. Partial application is not allowed.

def add(x: i32, y: i32) i32 = x + y

-- Valid: fully applied
def result = add(1, 2)

-- INVALID: partial application
def add_one = add(1)  -- Error: function requires 2 arguments

This restriction applies uniformly:

• Top-level function definitions
• Lambda expressions
• Built-in functions
• Functions passed as higher-order arguments

The restriction exists to simplify compilation to GPU targets and ensure predictable performance.

Explicit Currying with Placeholder Syntax

When you need to create a partially applied function, use explicit placeholder syntax with $:

def add(x: i32, y: i32) i32 = x + y

-- Create a 2-arity function that calls add with middle arg fixed
def add_with_5 = $add(_, 5, _)  -- Produces (i32, i32) -> i32

-- Create a 1-arity function
def add_one = $add(_, 1)  -- Produces i32 -> i32

-- Usage
def result = add_one(5)  -- Returns 6

The $func(args...) syntax:

• _ marks placeholder positions that become parameters of the new function
• Non-placeholder arguments are captured at the definition site
• The resulting function has arity equal to the number of _ placeholders
• The resulting function is itself non-curried (requires all placeholders filled at once)