Type anonymity - Rust By Example
Description: A description

# Type anonymity

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: 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 `<T>` 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: 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);
    }

### See also:

A thorough analysis, `Fn`, `FnMut`, and `FnOnce`