rust/src/libcore/ops/function.rs

// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

/// A version of the call operator that takes an immutable receiver.
///
/// # Examples
///
/// Closures automatically implement this trait, which allows them to be
/// invoked. Note, however, that `Fn` takes an immutable reference to any
/// captured variables. To take a mutable capture, implement [`FnMut`], and to
/// consume the capture, implement [`FnOnce`].
///
/// [`FnMut`]: trait.FnMut.html
/// [`FnOnce`]: trait.FnOnce.html
///
/// ```
/// let square = |x| x * x;
/// assert_eq!(square(5), 25);
/// ```
///
/// Closures can also be passed to higher-level functions through a `Fn`
/// parameter (or a `FnMut` or `FnOnce` parameter, which are supertraits of
/// `Fn`).
///
/// ```
/// fn call_with_one<F>(func: F) -> usize
///     where F: Fn(usize) -> usize {
///     func(1)
/// }
///
/// let double = |x| x * 2;
/// assert_eq!(call_with_one(double), 2);
/// ```
#[lang = "fn"]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_paren_sugar]
#[fundamental] // so that regex can rely that `&str: !FnMut`
pub trait Fn<Args> : FnMut<Args> {
    /// This is called when the call operator is used.
    #[unstable(feature = "fn_traits", issue = "29625")]
    extern "rust-call" fn call(&self, args: Args) -> Self::Output;
}

/// A version of the call operator that takes a mutable receiver.
///
/// # Examples
///
/// Closures that mutably capture variables automatically implement this trait,
/// which allows them to be invoked.
///
/// ```
/// let mut x = 5;
/// {
///     let mut square_x = || x *= x;
///     square_x();
/// }
/// assert_eq!(x, 25);
/// ```
///
/// Closures can also be passed to higher-level functions through a `FnMut`
/// parameter (or a `FnOnce` parameter, which is a supertrait of `FnMut`).
///
/// ```
/// fn do_twice<F>(mut func: F)
///     where F: FnMut()
/// {
///     func();
///     func();
/// }
///
/// let mut x: usize = 1;
/// {
///     let add_two_to_x = || x += 2;
///     do_twice(add_two_to_x);
/// }
///
/// assert_eq!(x, 5);
/// ```
#[lang = "fn_mut"]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_paren_sugar]
#[fundamental] // so that regex can rely that `&str: !FnMut`
pub trait FnMut<Args> : FnOnce<Args> {
    /// This is called when the call operator is used.
    #[unstable(feature = "fn_traits", issue = "29625")]
    extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output;
}

/// A version of the call operator that takes a by-value receiver.
///
/// # Examples
///
/// By-value closures automatically implement this trait, which allows them to
/// be invoked.
///
/// ```
/// let x = 5;
/// let square_x = move || x * x;
/// assert_eq!(square_x(), 25);
/// ```
///
/// By-value Closures can also be passed to higher-level functions through a
/// `FnOnce` parameter.
///
/// ```
/// fn consume_with_relish<F>(func: F)
///     where F: FnOnce() -> String
/// {
///     // `func` consumes its captured variables, so it cannot be run more
///     // than once
///     println!("Consumed: {}", func());
///
///     println!("Delicious!");
///
///     // Attempting to invoke `func()` again will throw a `use of moved
///     // value` error for `func`
/// }
///
/// let x = String::from("x");
/// let consume_and_return_x = move || x;
/// consume_with_relish(consume_and_return_x);
///
/// // `consume_and_return_x` can no longer be invoked at this point
/// ```
#[lang = "fn_once"]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_paren_sugar]
#[fundamental] // so that regex can rely that `&str: !FnMut`
pub trait FnOnce<Args> {
    /// The returned type after the call operator is used.
    #[stable(feature = "fn_once_output", since = "1.12.0")]
    type Output;

    /// This is called when the call operator is used.
    #[unstable(feature = "fn_traits", issue = "29625")]
    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
}

mod impls {
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a,A,F:?Sized> Fn<A> for &'a F
        where F : Fn<A>
    {
        extern "rust-call" fn call(&self, args: A) -> F::Output {
            (**self).call(args)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a,A,F:?Sized> FnMut<A> for &'a F
        where F : Fn<A>
    {
        extern "rust-call" fn call_mut(&mut self, args: A) -> F::Output {
            (**self).call(args)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a,A,F:?Sized> FnOnce<A> for &'a F
        where F : Fn<A>
    {
        type Output = F::Output;

        extern "rust-call" fn call_once(self, args: A) -> F::Output {
            (*self).call(args)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a,A,F:?Sized> FnMut<A> for &'a mut F
        where F : FnMut<A>
    {
        extern "rust-call" fn call_mut(&mut self, args: A) -> F::Output {
            (*self).call_mut(args)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a,A,F:?Sized> FnOnce<A> for &'a mut F
        where F : FnMut<A>
    {
        type Output = F::Output;
        extern "rust-call" fn call_once(mut self, args: A) -> F::Output {
            (*self).call_mut(args)
        }
    }
}