aviary.go

package main

import (
	"fmt"
)

// Apply
func A[A, B any](f func(A) B) func(A) B {
	return func(a A) B {
		return f(a)
	}
}

func Arrow[A, B, C any](f func(A) B) func(func(B) C) func(A) C {
	return func(g func(B) C) func(A) C {
		return func(a A) C {
			return g(f(a))
		}
	}
}

// Compose
func B[A, B, C any](f func(B) C) func(func(A) B) func(A) C {
	return func(g func(A) B) func(A) C {
		return func(a A) C {
			return f(g(a))
		}
	}
}

// Constant
func K[A any](a A) func(A) A {
	return func(_ A) A {
		return a
	}
}

// Fix
func Y[A any](f func(func(A) A) func(A) A) func(A) A {
	type YY func(YY) func(A) A
	g := func(h YY) func(A) A {
		return func(x A) A {
			return (f(h(h))(x))
		}
	}
	return g(g)
}

func main() {
	a := func(x int) int {
		return x + 1
	}
	var b int = 1
	fmt.Println(A(a)(b))

	x := func(x int) int {
		return x + 2
	}
	fmt.Println(Arrow[int, int, int](x)(a)(b))
	fmt.Println(B[int, int, int](x)(a)(b))
	
	fact := Y(func(f func(int) int) func(int) int {
		return func(x int) int {
			if x <= 0 {
				return 1
			} else {
				return x * f(x-1)
			}
		}
	})
	fmt.Println(fact(20)) 
}