pipe.go

package main

import (
	"fmt"
	"strconv"
)

//
//
func filter[A any](f func(A) bool, a A) A {
	none := *new(A)
	if !f(a) {
		return none
	}

	return a
}

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

//
//
func filterF[A any](f func(A) bool) func(A) A {
	return func(a A) A { return filter(f, a) }
}

func fmapF[A, B any](f func(A) B) func(A) B {
	return func(a A) B { return fmap(f, a) }
}

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

//
//
func dotJ[A, B, C any](f func(A) B, g func(B) C, fs ...func(C) C) func(A) C {
	return func(a A) C {
		x := g(f(a))
		for _, y := range fs {
			x = y(x)
		}
		return x
	}
}

//
//
type Map[A, B, C any] func(A) B

func (f Map[A, B, C]) fmap(g func(B) C) func(A) C {
	return func(a A) C {
		return g(f(a))
	}
}

//
//
func lt10(x int) bool { return x < 10 }

func like(x string) string { return fmt.Sprintf("+(%v)", x) }

func main() {
	x := 1

	//
	y := fmap(like, fmap(like, fmap(strconv.Itoa, filter(lt10, x))))

	fmt.Printf("==> %s\n", y)

	//
	f := filterF(lt10)
	g := fmapF(strconv.Itoa)
	h := fmapF(like)

	fmt.Printf("==> %s\n", h(h(g(f(x)))))

	//
	w := dot(dot(dot(f, g), h), h)

	fmt.Printf("==> %s\n", w(x))

	//
	p := dotJ(f, g, h, h)

	fmt.Printf("==> %s\n", p(x))

	//
	z := Map[int, string, string](
		Map[int, string, string](
			Map[int, int, string](
				f,
			).fmap(g),
		).fmap(h),
	).fmap(h)

	fmt.Printf("==> %s\n", z(x))
}