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// run-pass
#![feature(fn_traits, unboxed_closures)]
use std::marker::PhantomData;
// Test that we are able to infer a suitable kind for a "recursive"
// closure. As far as I can tell, coding up a recursive closure
// requires the good ol' [Y Combinator].
//
// [Y Combinator]: https://en.wikipedia.org/wiki/Fixed-point_combinator#Y_combinator
struct YCombinator<F,A,R> {
func: F,
marker: PhantomData<(A,R)>,
}
impl<F,A,R> YCombinator<F,A,R> {
fn new(f: F) -> YCombinator<F,A,R> {
YCombinator { func: f, marker: PhantomData }
}
}
impl<A,R,F : Fn(&dyn Fn(A) -> R, A) -> R> Fn<(A,)> for YCombinator<F,A,R> {
extern "rust-call" fn call(&self, (arg,): (A,)) -> R {
(self.func)(self, arg)
}
}
impl<A,R,F : Fn(&dyn Fn(A) -> R, A) -> R> FnMut<(A,)> for YCombinator<F,A,R> {
extern "rust-call" fn call_mut(&mut self, args: (A,)) -> R { self.call(args) }
}
impl<A,R,F : Fn(&dyn Fn(A) -> R, A) -> R> FnOnce<(A,)> for YCombinator<F,A,R> {
type Output = R;
extern "rust-call" fn call_once(self, args: (A,)) -> R { self.call(args) }
}
fn main() {
let factorial = |recur: &dyn Fn(u32) -> u32, arg: u32| -> u32 {
if arg == 0 {1} else {arg * recur(arg-1)}
};
let factorial: YCombinator<_,u32,u32> = YCombinator::new(factorial);
let r = factorial(10);
assert_eq!(3628800, r);
}
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