@parameter

You can add the @parameter decorator on a nested function to create a parametric closure.

The @parameter decorator also works to create compile-time if and for statements, but this usage is deprecated in favor of the comptime if and comptime for statements.

Parametric closure

You can add @parameter on a nested function to create a "parametric" capturing closure. This means you can create a closure function that captures values from the outer scope (regardless of whether they are variables or parameters), and then use that closure as a parameter. For example:

def use_closure[func: fn(Int) capturing [_] -> Int](num: Int) -> Int:
    return func(num)

def create_closure():
    var x = 1

    @parameter
    def add(i: Int) -> Int:
        return x + i

    var y = use_closure[add](2)
    print(y)

create_closure()

Note the [_] in the function type:

def use_closure[func: fn(Int) capturing [_] -> Int](num: Int) -> Int:

This origin specifier represents the set of origins for the values captured by the parametric closure. This allows the compiler to correctly extend the lifetimes of those values. For more information on lifetimes and origins, see Lifetimes, origins and references.

If you leave the @parameter decorator off the add() function above, you'll get a compiler error:

invalid call to 'use_closure': failed to infer parameter '__origins__'

Parametric `if` statement

The @parameter if syntax is deprecated, please use comptime if instead. For more information, see the section on compile-time conditionals.

Parametric `for` statement

The @parameter for syntax is deprecated, please use comptime for instead. For details, see the section on compile-time loop unrolling.

Parametric closure​

Parametric if statement​

Parametric for statement​

Parametric closure

Parametric `if` statement

Parametric `for` statement