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