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// 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)
}
}
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