golang1.18

gistfile1.txt

package main

import (
	"fmt"
)

// Based off of the Java Optional<T> type.
type Optional[T any] struct {
	value *T
}

type Consumer[T any] func(T)

type Predicate[T any] func(T) bool

type Supplier[T any] func() T

type Function[T, U any] func(T) U

func Empty[T any]() Optional[T] {
	return Optional[T] { value: nil }
}

func Of[T any](value T) Optional[T] {
	return Optional[T] { value: &value }
}

func (o Optional[T]) Get() T {
	if o.value == nil {
		panic("Empty optional")
	}
	return *o.value
}

func (o Optional[T]) OrElse(value T) T {
	if o.value == nil {
		return value
	}
	return *o.value
}

func (o Optional[T]) OrElseGet(supplier Supplier[T]) T {
	if o.value == nil {
		return supplier()
	}
	return *o.value
}

func (o Optional[T]) IfPresent(consumer Consumer[T]) {
	if o.value != nil {
		consumer(*o.value)
	}
}

func (o Optional[T]) Filter(predicate Predicate[T]) Optional[T] {
	if o.value == nil {
		return o
	}
	if predicate(*o.value) {
		return o
	}
	return Empty[T]()
}

// Apparently you can't add additional types in methods, so I had to implement this as a function.
func Map[T, U any](o Optional[T], f Function[T,U]) Optional[U] {
	if &o.value == nil {
		return Empty[U]()
	}
	return Of(f(*o.value))
}

func main() {
	// Prints 236
	Map(Of(123), func(x int) int { return x * 2 }).IfPresent(func(x int) { fmt.Println(x) })

	// Prints 0
	value := Of(123).Filter(func(x int) bool { return x % 2 == 0 }).OrElse(0)
	fmt.Println(value)

	// Prints 1234
	value = Empty[int]().OrElseGet(func() int { return 1234 })
	fmt.Println(value)
}