# As input parameters

While Rust chooses how to capture variables on the fly mostly without type annotation, this ambiguity is not allowed when writing functions. When taking a closure as an input parameter, the closure's complete type must be annotated using one of a few `traits`. In order of decreasing restriction, they are:

- `Fn`: the closure captures by reference (`&T`)
- `FnMut`: the closure captures by mutable reference (`&mut T`)
- `FnOnce`: the closure captures by value (`T`)

On a variable-by-variable basis, the compiler will capture variables in the least restrictive manner possible.

For instance, consider a parameter annotated as `FnOnce`. This specifies that the closure may capture by `&T`, `&mut T`, or `T`, but the compiler will ultimately choose based on how the captured variables are used in the closure.

This is because if a move is possible, then any type of borrow should also be possible. Note that the reverse is not true. If the parameter is annotated as `Fn`, then capturing variables by `&mut T` or `T` are not allowed.

In the following example, try swapping the usage of `Fn`, `FnMut`, and `FnOnce` to see what happens:

    // A function which takes a closure as an argument and calls it.
    fn apply<F>(f: F) where
        // The closure takes no input and returns nothing.
        F: FnOnce() {
        // ^ TODO: Try changing this to `Fn` or `FnMut`.

        f();
    }

    // A function which takes a closure and returns an `i32`.
    fn apply_to_3<F>(f: F) -> i32 where
        // The closure takes an `i32` and returns an `i32`.
        F: Fn(i32) -> i32 {

        f(3)
    }

    fn main() {
        use std::mem;

        let greeting = "hello";
        // A non-copy type.
        // `to_owned` creates owned data from borrowed one
        let mut farewell = "goodbye".to_owned();

        // Capture 2 variables: `greeting` by reference and
        // `farewell` by value.
        let diary = || {
            // `greeting` is by reference: requires `Fn`.
            println!("I said {}.", greeting);

            // Mutation forces `farewell` to be captured by
            // mutable reference. Now requires `FnMut`.
            farewell.push_str("!!!");
            println!("Then I screamed {}.", farewell);
            println!("Now I can sleep. zzzzz");

            // Manually calling drop forces `farewell` to
            // be captured by value. Now requires `FnOnce`.
            mem::drop(farewell);
        };

        // Call the function which applies the closure.
        apply(diary);

        // `double` satisfies `apply_to_3`'s trait bound
        let double = |x| 2 * x;

        println!("3 doubled: {}", apply_to_3(double));
    }

### See also:

`std::mem::drop`, `Fn`, `FnMut`, and `FnOnce`