46 lines
1.3 KiB
Rust
46 lines
1.3 KiB
Rust
//@ run-pass
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#![feature(fn_traits, unboxed_closures)]
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use std::marker::PhantomData;
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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]: https://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: PhantomData<(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: PhantomData }
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}
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}
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impl<A,R,F : Fn(&dyn Fn(A) -> R, A) -> R> Fn<(A,)> for YCombinator<F,A,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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impl<A,R,F : Fn(&dyn Fn(A) -> R, A) -> R> FnMut<(A,)> for YCombinator<F,A,R> {
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extern "rust-call" fn call_mut(&mut self, args: (A,)) -> R { self.call(args) }
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}
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impl<A,R,F : Fn(&dyn Fn(A) -> R, A) -> R> FnOnce<(A,)> for YCombinator<F,A,R> {
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type Output = R;
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extern "rust-call" fn call_once(self, args: (A,)) -> R { self.call(args) }
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}
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fn main() {
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let factorial = |recur: &dyn 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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