finally.rs

// Copyright 2013 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.

//! The Finally trait provides a method, `finally` on
//! stack closures that emulates Java-style try/finally blocks.
//!
//! Using the `finally` method is sometimes convenient, but the type rules
//! prohibit any shared, mutable state between the "try" case and the
//! "finally" case. For advanced cases, the `try_finally` function can
//! also be used. See that function for more details.
//!
//! # Examples
//!
//! ```
//! # #![feature(unboxed_closures)]
//!
//! use std::finally::Finally;
//!
//! (|| {
//!     // ...
//! }).finally(|| {
//!     // this code is always run
//! })
//! ```

#![unstable(feature = "core")]
#![deprecated(since = "1.0.0",
              reason = "It is unclear if this module is more robust than implementing \
                        Drop on a custom type, and this module is being removed with no \
                        replacement. Use a custom Drop implementation to regain existing \
                        functionality.")]
#![allow(deprecated)]

use ops::{Drop, FnMut, FnOnce};

/// A trait for executing a destructor unconditionally after a block of code,
/// regardless of whether the blocked fails.
pub trait Finally<T> {
    /// Executes this object, unconditionally running `dtor` after this block of
    /// code has run.
    fn finally<F>(&mut self, dtor: F) -> T where F: FnMut();
}

impl<T, F> Finally<T> for F where F: FnMut() -> T {
    fn finally<G>(&mut self, mut dtor: G) -> T where G: FnMut() {
        try_finally(&mut (), self, |_, f| (*f)(), |_| dtor())
    }
}

/// The most general form of the `finally` functions. The function
/// `try_fn` will be invoked first; whether or not it panics, the
/// function `finally_fn` will be invoked next. The two parameters
/// `mutate` and `drop` are used to thread state through the two
/// closures. `mutate` is used for any shared, mutable state that both
/// closures require access to; `drop` is used for any state that the
/// `try_fn` requires ownership of.
///
/// **WARNING:** While shared, mutable state between the try and finally
/// function is often necessary, one must be very careful; the `try`
/// function could have panicked at any point, so the values of the shared
/// state may be inconsistent.
///
/// # Examples
///
/// ```
/// use std::finally::try_finally;
///
/// struct State<'a> { buffer: &'a mut [u8], len: usize }
/// # let mut buf = [];
/// let mut state = State { buffer: &mut buf, len: 0 };
/// try_finally(
///     &mut state, (),
///     |state, ()| {
///         // use state.buffer, state.len
///     },
///     |state| {
///         // use state.buffer, state.len to cleanup
///     })
/// ```
pub fn try_finally<T, U, R, F, G>(mutate: &mut T, drop: U, try_fn: F, finally_fn: G) -> R where
    F: FnOnce(&mut T, U) -> R,
    G: FnMut(&mut T),
{
    let f = Finallyalizer {
        mutate: mutate,
        dtor: finally_fn,
    };
    try_fn(&mut *f.mutate, drop)
}

struct Finallyalizer<'a, A:'a, F> where F: FnMut(&mut A) {
    mutate: &'a mut A,
    dtor: F,
}

#[unsafe_destructor]
impl<'a, A, F> Drop for Finallyalizer<'a, A, F> where F: FnMut(&mut A) {
    #[inline]
    fn drop(&mut self) {
        (self.dtor)(self.mutate);
    }
}