48 lines
1.4 KiB
Rust
48 lines
1.4 KiB
Rust
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// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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#![feature(core,unboxed_closures)]
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use std::marker::CovariantType;
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// Test that we are able to infer a suitable kind for a "recursive"
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// closure. As far as I can tell, coding up a recursive closure
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// requires the good ol' [Y Combinator].
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//
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// [Y Combinator]: http://en.wikipedia.org/wiki/Fixed-point_combinator#Y_combinator
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struct YCombinator<F,A,R> {
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func: F,
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marker: CovariantType<(A,R)>,
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}
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impl<F,A,R> YCombinator<F,A,R> {
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fn new(f: F) -> YCombinator<F,A,R> {
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YCombinator { func: f, marker: CovariantType }
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}
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}
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impl<A,R,F : Fn(&Fn(A) -> R, A) -> R> Fn<(A,)> for YCombinator<F,A,R> {
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type Output = R;
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extern "rust-call" fn call(&self, (arg,): (A,)) -> R {
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(self.func)(self, arg)
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}
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}
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fn main() {
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let factorial = |recur: &Fn(u32) -> u32, arg: u32| -> u32 {
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if arg == 0 {1} else {arg * recur(arg-1)}
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};
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let factorial: YCombinator<_,u32,u32> = YCombinator::new(factorial);
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let r = factorial(10);
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assert_eq!(3628800, r);
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}
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