Executes a function or if statement at compile time.

You can add the @parameter decorator on an if statement or on a nested function to run that code at compile time.

Parametric if statement

You can add @parameter to any if condition that’s based on a valid parameter expression (it’s an expression that evaluates at compile time). This ensures that only the live branch of the if statement is compiled into the program, which can reduce your final binary size. For example:

if True:
    print("this will be included in the binary")
    print("this will be eliminated at compile time")
this will be included in the binary

Parametric closure

You can add @parameter on a nested function to create a “parametric” capturing closure. This means you to 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:

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

fn create_closure():
    var x = 1

    fn add(i: Int) -> Int:
        return x + i

    let y = use_closure[add](2)


Without the @parameter decorator, you’ll get a compiler error that says you “cannot use a dynamic value in call parameter”—referring to the use_closure[add](2) call—because the add() closure would still be dynamic.

This is an unsafe feature because we currently do not model the lifetimes of capture-by-reference.