| // run |
| |
| // Copyright 2025 The Go Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style |
| // license that can be found in the LICENSE file. |
| |
| package main |
| |
| import "fmt" |
| |
| // Y is the Y-combinator based on https://dreamsongs.com/Files/WhyOfY.pdf |
| func Y[Endo ~func(RecFct) RecFct, RecFct ~func(T) R, T, R any](f Endo) RecFct { |
| |
| type internal[RecFct ~func(T) R, T, R any] func(internal[RecFct, T, R]) RecFct |
| |
| g := func(h internal[RecFct, T, R]) RecFct { |
| return func(t T) R { |
| return f(h(h))(t) |
| } |
| } |
| return g(g) |
| } |
| |
| func main() { |
| |
| fct := Y(func(r func(int) int) func(int) int { |
| return func(n int) int { |
| if n <= 0 { |
| return 1 |
| } |
| return n * r(n-1) |
| } |
| }) |
| |
| want := 3628800 |
| if got := fct(10); got != want { |
| msg := fmt.Sprintf("unexpected result, got: %d, want: %d", got, want) |
| panic(msg) |
| } |
| } |
