An incomplete experiment of Option (Maybe) monad in Go language.

option.go

package main

type OptionHandlers[T any, R any] struct {
	OnSome func(value T) R
	OnNone func() R
}
type Option[T any] func(handlers OptionHandlers[T, any]) any

func Some[T any](value T) Option[T] {
	return func(handlers OptionHandlers[T, any]) any {
		return handlers.OnSome(value)
	}
}

func None[T any]() Option[T] {
	return func(handlers OptionHandlers[T, any]) any {
		return handlers.OnNone()
	}
}

func (o Option[T]) Unwrap() T {
	return (o)(OptionHandlers[T, any]{
		func(value T) any {
			return value
		},
		func() any {
			panic("unwrapped None")
		},
	}).(T)
}

func (o Option[T]) UnwrapOr(def T) T {
	return (o)(OptionHandlers[T, any]{
		func(value T) any {
			return value
		},
		func() any {
			return def
		},
	}).(T)
}

func (o Option[T]) Map(mapper func(value T) T) Option[T] {
	return (o)(OptionHandlers[T, any]{
		func(value T) any {
			return Some(mapper(value))
		},
		func() any {
			return None[T]()
		},
	}).(Option[T])
}

func main() {
	opt := Some("foo")
	opt2 := opt.Map(func(s string) string {
		return s + "!"
	}).UnwrapOr("")
	println(opt2)
}