functional.go

package main

import (
	"fmt"
	"strings"
)

func main() {
	strs := StringSlice{"Hello", "foo", "bar"}
	ints := IntSlice{1, 2, 3, 4}

	// Example use Map with custom and stdlib functions
	excl := func(s string) string { return s + "!" }
	fmt.Println(strs, "->", strs.Map(excl))
	fmt.Println(strs, "->", strs.Map(strings.Title))

	// Example using Map->Grep chain
	hasOh := func(s string) bool { return strings.ContainsAny(s, "Oo") }
	fmt.Println(strs, "->", strs.Map(strings.ToLower).Grep(hasOh))

	// Same examples for ints

	double := func(i int) int { return i * 2 }
	fmt.Println(ints, "->", ints.Map(double))

	tripple := func(i int) int { return i * 3 }
	even := func(i int) bool { return i&0x1 == 0 }
	fmt.Println(ints, "->", ints.Map(tripple).Grep(even))

	// Type mismatch eror case
	fmt.Println(ints, "->", ints.Map(tripple).Grep(even).Grep(hasOh))
}

////////////////////////////////
//// -- Type definitions -- ////
////////////////////////////////

type Mapper interface {
	Map(transformer interface{}) Functional
}

type Grepper interface {
	Grep(predicate interface{}) Functional
}

type Functional interface {
	Mapper
	Grepper
}

type StringSlice []string

func (slice StringSlice) Map(f interface{}) Functional {
	res := StringSlice(make([]string, 0, len(slice)))
	fn := f.(func(string) string)
	for _, v := range slice {
		res = append(res, fn(v))
	}
	return res
}

func (slice StringSlice) Grep(p interface{}) Functional {
	res := StringSlice(make([]string, 0, len(slice)))
	fn := p.(func(string) bool)
	for _, v := range slice {
		if fn(v) {
			res = append(res, v)
		}
	}
	return res
}

type IntSlice []int

func (slice IntSlice) Map(f interface{}) Functional {
	res := IntSlice(make([]int, 0, len(slice)))
	fn := f.(func(int) int)
	for _, v := range slice {
		res = append(res, fn(v))
	}
	return res
}

func (slice IntSlice) Grep(p interface{}) Functional {
	res := IntSlice(make([]int, 0, len(slice)))
	fn := p.(func(int) bool)
	for _, v := range slice {
		if fn(v) {
			res = append(res, v)
		}
	}
	return res
}

Live example - https://play.golang.org/p/AR1u7EXkno
Output:

[Hello foo bar] -> [Hello! foo! bar!]
[Hello foo bar] -> [Hello Foo Bar]
[Hello foo bar] -> [hello foo]
[1 2 3 4] -> [2 4 6 8]
[1 2 3 4] -> [6 12]
panic: interface conversion: interface {} is func(string) bool, not func(int) bool

goroutine 1 [running]:
panic(0x104da0, 0x10432360)
    /usr/local/go/src/runtime/panic.go:500 +0x720
main.IntSlice.Grep(0x1040e2d0, 0x2, 0x4, 0x100ca0, 0x139664, 0x1040e130, 0x0, 0x0)
    /tmp/sandbox693237750/main.go:86 +0x260
main.(*IntSlice).Grep(0x1040e2e0, 0x100ca0, 0x139664, 0x100aa0, 0x0, 0x0)
    <autogenerated>:6 +0xe0
main.main()
    /tmp/sandbox693237750/main.go:31 +0xbe0