Rust by Example

Closures succinctly capture variables from enclosing scopes. Does this have any consequences? It surely does. Observe how using a closure as a function parameter requires generics, which is necessary because of how they are defined:

#![allow(unused)]

fn main() {

// `F` must be generic.

fn apply(f: F) where

F: FnOnce() {

f();

}

}

When a closure is defined, the compiler implicitly creates a new anonymous structure to store the captured variables inside, meanwhile implementing the functionality via one of the traits: Fn, FnMut, or FnOnce for this unknown type. This type is assigned to the variable which is stored until calling.

Since this new type is of unknown type, any usage in a function will require generics. However, an unbounded type parameter would still be ambiguous and not be allowed. Thus, bounding by one of the traits: Fn, FnMut, or FnOnce (which it implements) is sufficient to specify its type.

// `F` must implement `Fn` for a closure which takes no

// inputs and returns nothing - exactly what is required

// for `print`.

fn apply(f: F) where

F: Fn() {

f();

}

fn main() {

let x = 7;

// Capture `x` into an anonymous type and implement

// `Fn` for it. Store it in `print`.

let print = || println!("{}", x);

apply(print);

}

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