Closure Syntax

Closures are created with vertical bars: |..| ...

# // Copyright 2025 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn main() {
    // Argument and return type can be inferred for lightweight syntax:
    let double_it = |n| n * 2;
    dbg!(double_it(50));

    // Or we can specify types and bracket the body to be fully explicit:
    let add_1f32 = |x: f32| -> f32 { x + 1.0 };
    dbg!(add_1f32(50.));
}

• The arguments go between the |..|. The body can be surrounded by { .. }, but if it is a single expression these can be omitted.

• Argument types are optional, and are inferred if not given. The return type is also optional, but can only be written if using { .. } around the body.

• The examples can both be written as mere nested functions instead -- they do not capture any variables from their lexical environment. We will see captures next.

More to Explore

• The ability to store functions in variables doesn't just apply to closures, regular functions can be put in variables and then invoked the same way that closures can: Example in the playground.

  • The linked example also demonstrates that closures that don't capture anything can also coerce to a regular function pointer.