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:

1. // `F` must be generic.
2. fn apply<F>(f: F) where
3. F: FnOnce() {
4. f();
5. }

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.

1. // `F` must implement `Fn` for a closure which takes no
2. // inputs and returns nothing - exactly what is required
3. // for `print`.
4. fn apply<F>(f: F) where
5. F: Fn() {
6. f();
7. }

9. fn main() {
10. let x = 7;

12. // Capture `x` into an anonymous type and implement
13. // `Fn` for it. Store it in `print`.
14. let print = || println!("{}", x);

16. apply(print);
17. }

See also:

A thorough analysis, Fn, FnMut, and FnOnce