Option type in Go as a struct with a boolean flag

option_struct.go

package main

import (
	"fmt"
)

// begin Option package

type Option[T any] struct {
	hasValue bool
	value    T
}

func None[T any]() Option[T] {
	return Option[T]{
		hasValue: false,
	}
}

func Some[T any](t T) Option[T] {
	return Option[T]{
		hasValue: true,
		value:    t,
	}
}

func (opt Option[T]) Destructure(noneCase func(), someCase func(T)) {
	if opt.hasValue {
		someCase(opt.value)
	} else {
		noneCase()
	}
}

// end option package

func printValueOrDefault(optStr Option[string]) {
	defaultStr := "default"

	optStr.Destructure(func() {
		fmt.Println(defaultStr)
	}, func(str string) {
		fmt.Println(str)
	})
}

func main() {
	noneStr := None[string]()
	someStr := Some("hello!")

	printValueOrDefault(noneStr)
	printValueOrDefault(someStr)
}

Methods can't have type parameters, even when the receiver is a struct type instead of an interface.

func (opt Option[T]) Map[U any](mapper func(T) U) Option[U] {
}

gives a compiler error method must have no type parameters.

Map() as standalone function:

func Map[T any, U any](opt Option[T], mapper func(T) U) Option[U] {
	if !opt.hasValue {
		return None[U]()
	}

	mappedVal := mapper(opt.value)
	return Some(mappedVal)
}