Source file test/fixedbugs/issue72063.go

     1  // run
     2  
     3  // Copyright 2025 The Go Authors. All rights reserved.
     4  // Use of this source code is governed by a BSD-style
     5  // license that can be found in the LICENSE file.
     6  
     7  package main
     8  
     9  import "fmt"
    10  
    11  // Y is the Y-combinator based on https://dreamsongs.com/Files/WhyOfY.pdf
    12  func Y[Endo ~func(RecFct) RecFct, RecFct ~func(T) R, T, R any](f Endo) RecFct {
    13  
    14  	type internal[RecFct ~func(T) R, T, R any] func(internal[RecFct, T, R]) RecFct
    15  
    16  	g := func(h internal[RecFct, T, R]) RecFct {
    17  		return func(t T) R {
    18  			return f(h(h))(t)
    19  		}
    20  	}
    21  	return g(g)
    22  }
    23  
    24  func main() {
    25  
    26  	fct := Y(func(r func(int) int) func(int) int {
    27  		return func(n int) int {
    28  			if n <= 0 {
    29  				return 1
    30  			}
    31  			return n * r(n-1)
    32  		}
    33  	})
    34  
    35  	want := 3628800
    36  	if got := fct(10); got != want {
    37  		msg := fmt.Sprintf("unexpected result, got: %d, want: %d", got, want)
    38  		panic(msg)
    39  	}
    40  }
    41  

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