# 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`, and they're determined by what the
closure does with captured value. In order of decreasing restriction,
they are:

* `Fn`: the closure uses the captured value by reference (`&T`)
* `FnMut`: the closure uses the captured value by mutable reference (`&mut T`)
* `FnOnce`: the closure uses the captured value 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. However, `&T` is allowed.

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

```rust,editable
// A function which takes a closure as an argument and calls it.
// <F> denotes that F is a "Generic type parameter"
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`][drop], [`Fn`][fn], [`FnMut`][fnmut], [Generics][generics], [where][where] and [`FnOnce`][fnonce]

[drop]: https://doc.rust-lang.org/std/mem/fn.drop.html
[fn]: https://doc.rust-lang.org/std/ops/trait.Fn.html
[fnmut]: https://doc.rust-lang.org/std/ops/trait.FnMut.html
[fnonce]: https://doc.rust-lang.org/std/ops/trait.FnOnce.html
[generics]: ../../generics.md
[where]: ../../generics/where.md
