package main

import "fmt"

func main1() {
	name := Map(getUser(100), getName).OrElse("-")
	fmt.Println(name)

	value := Map(Map(getUser(200), getCard), getValue).OrElse(0)
	fmt.Println(value)
}

func getUser(id int) Maybe[User] {
	if id == 100 {
		return None[User]()
	}
	user := User{
		Name: "Test",
		Card: &Card{Value: 100},
	}
	return Some[User](user)
}

func getCard(user User) Maybe[Card] {
	if user.Card != nil {
		return Some[Card](*user.Card)
	}
	return None[Card]()
}

func getValue(card Card) Maybe[int] {
	return Some[int](card.Value)
}

func getName(user User) Maybe[string] {
	return Some[string](user.Name)
}

type User struct {
	Name string
	Card *Card
}

type Card struct {
	Value int
}

type Maybe[T any] struct {
	value   T
	present bool
}

func Some[T any](value T) Maybe[T] {
	return Maybe[T]{value: value, present: true}
}

func None[T any]() Maybe[T] {
	return Maybe[T]{present: false}
}

func Map[T, R any](m Maybe[T], fn func(T) Maybe[R]) Maybe[R] {
	if m.present {
		return fn(m.value)
	}
	return None[R]()
}

func (m Maybe[T]) IsPresent() bool {
	return m.present
}

func (m Maybe[T]) OrElse(defaultValue T) T {
	if m.present {
		return m.value
	}
	return defaultValue
}