Playing with Maybe in Go

main.go

package main

import (
	"fmt"
	"strings"
)

type Any interface{} // The lack of generics in Go is the pitfall 

type Morphism func(Any) Any

type Option interface {
	Map(Morphism) Option
	Alt(Option) Option
	IsSome() bool
	IsNone() bool
	Get() Any
	Chain(func(Any) Option) Option
}

func FromNullable(x Any) Option {
	if x == nil {
		return None{}
	} else {
		return Some{x}
	}
}

type Some struct {
	Value Any
}

func (x Some) Map(f Morphism) Option {
	return Some{f(x.Value)}
}

func (x Some) Chain(f func(Any) Option) Option {
	return f(x.Get())
}

func (x Some) Alt(_ Option) Option {
	return x
}

func (x Some) Get() Any {
	return x.Value
}

func (x Some) IsSome() bool { return true }
func (x Some) IsNone() bool { return false }

type None struct {
}

func (x None) Map(f Morphism) Option {
	return x
}

func (x None) Chain(_ func(Any) Option) Option {
	return x
}

func (x None) Alt(o Option) Option {
	return o
}

func (x None) Get() Any {
	return nil
}

func (x None) IsSome() bool { return false }
func (x None) IsNone() bool { return true }

func upper(x Any) Any {
  // The abomination that is the next line is 💩
  // Without generics you can't have nice things
	return Any(strings.ToUpper(x.(string)))
}

func constNil(x Any) Any {
	return nil
}

func main() {
	x := FromNullable("hello")
	y := x.Map(upper)
	z := y.Map(constNil).Chain(FromNullable)

	fmt.Printf("isNone=%t, isSome=%t, val=%v\n", x.IsNone(), x.IsSome(), x.Get())
	fmt.Printf("isNone=%t, isSome=%t, val=%v\n", y.IsNone(), y.IsSome(), y.Get())
	fmt.Printf("isNone=%t, isSome=%t, val=%v\n", z.IsNone(), z.IsSome(), z.Get())

}