option.go

package optional

import (
	"bytes"
	"encoding/json"
	"errors"
	"fmt"
)

var (
	// ErrNoneValueTaken represents the error that is raised when None value is taken.
	ErrNoneValueTaken = errors.New("none value taken")
)

// Option is a data type that must be Some (i.e. having a value) or None (i.e. doesn't have a value).
// This type implements database/sql/driver.Valuer and database/sql.Scanner.
type Option[T any] []T

const (
	value = iota
)

// Some is a function to make an Option type value with the actual value.
func Some[T any](v T) Option[T] {
	return Option[T]{
		value: v,
	}
}

// None is a function to make an Option type value that doesn't have a value.
func None[T any]() Option[T] {
	return nil
}

// FromNillable is a function to make an Option type value with the nillable value with value de-referencing.
// If the given value is not nil, this returns Some[T] value. On the other hand, if the value is nil, this returns None[T].
// This function does "dereference" for the value on packing that into Option value. If this value is not preferable, please consider using PtrFromNillable() instead.
func FromNillable[T any](v *T) Option[T] {
	if v == nil {
		return None[T]()
	}
	return Some[T](*v)
}

// PtrFromNillable is a function to make an Option type value with the nillable value without value de-referencing.
// If the given value is not nil, this returns Some[*T] value. On the other hand, if the value is nil, this returns None[*T].
// This function doesn't "dereference" the value on packing that into the Option value; in other words, this puts the as-is pointer value into the Option envelope.
// This behavior contrasts with the FromNillable() function's one.
func PtrFromNillable[T any](v *T) Option[*T] {
	if v == nil {
		return None[*T]()
	}
	return Some[*T](v)
}

// IsNone returns whether the Option *doesn't* have a value or not.
func (o Option[T]) IsNone() bool {
	return o == nil
}

// IsSome returns whether the Option has a value or not.
func (o Option[T]) IsSome() bool {
	return o != nil
}

// Unwrap returns the value regardless of Some/None status.
// If the Option value is Some, this method returns the actual value.
// On the other hand, if the Option value is None, this method returns the *default* value according to the type.
func (o Option[T]) Unwrap() T {
	if o.IsNone() {
		var defaultValue T
		return defaultValue
	}
	return o[value]
}

// UnwrapAsPtr returns the contained value in receiver Option as a pointer.
// This is similar to `Unwrap()` method but the difference is this method returns a pointer value instead of the actual value.
// If the receiver Option value is None, this method returns nil.
func (o Option[T]) UnwrapAsPtr() *T {
	if o.IsNone() {
		return nil
	}
	return &o[value]
}

// Take takes the contained value in Option.
// If Option value is Some, this returns the value that is contained in Option.
// On the other hand, this returns an ErrNoneValueTaken as the second return value.
func (o Option[T]) Take() (T, error) {
	if o.IsNone() {
		var defaultValue T
		return defaultValue, ErrNoneValueTaken
	}
	return o[value], nil
}

// TakeOr returns the actual value if the Option has a value.
// On the other hand, this returns fallbackValue.
func (o Option[T]) TakeOr(fallbackValue T) T {
	if o.IsNone() {
		return fallbackValue
	}
	return o[value]
}

// TakeOrElse returns the actual value if the Option has a value.
// On the other hand, this executes fallbackFunc and returns the result value of that function.
func (o Option[T]) TakeOrElse(fallbackFunc func() T) T {
	if o.IsNone() {
		return fallbackFunc()
	}
	return o[value]
}

// Or returns the Option value according to the actual value existence.
// If the receiver's Option value is Some, this function pass-through that to return. Otherwise, this value returns the `fallbackOptionValue`.
func (o Option[T]) Or(fallbackOptionValue Option[T]) Option[T] {
	if o.IsNone() {
		return fallbackOptionValue
	}
	return o
}

// OrElse returns the Option value according to the actual value existence.
// If the receiver's Option value is Some, this function pass-through that to return. Otherwise, this executes `fallbackOptionFunc` and returns the result value of that function.
func (o Option[T]) OrElse(fallbackOptionFunc func() Option[T]) Option[T] {
	if o.IsNone() {
		return fallbackOptionFunc()
	}
	return o
}

// Filter returns self if the Option has a value and the value matches the condition of the predicate function.
// In other cases (i.e. it doesn't match with the predicate or the Option is None), this returns None value.
func (o Option[T]) Filter(predicate func(v T) bool) Option[T] {
	if o.IsNone() || !predicate(o[value]) {
		return None[T]()
	}
	return o
}

// IfSome calls given function with the value of Option if the receiver value is Some.
func (o Option[T]) IfSome(f func(v T)) {
	if o.IsNone() {
		return
	}
	f(o[value])
}

// IfSomeWithError calls given function with the value of Option if the receiver value is Some.
// This method propagates the error of given function, and if the receiver value is None, this returns nil error.
func (o Option[T]) IfSomeWithError(f func(v T) error) error {
	if o.IsNone() {
		return nil
	}
	return f(o[value])
}

// IfNone calls given function if the receiver value is None.
func (o Option[T]) IfNone(f func()) {
	if o.IsSome() {
		return
	}
	f()
}

// IfNoneWithError calls given function if the receiver value is None.
// This method propagates the error of given function, and if the receiver value is Some, this returns nil error.
func (o Option[T]) IfNoneWithError(f func() error) error {
	if o.IsSome() {
		return nil
	}
	return f()
}

func (o Option[T]) String() string {
	if o.IsNone() {
		return "None[]"
	}

	v := o.Unwrap()
	if stringer, ok := interface{}(v).(fmt.Stringer); ok {
		return fmt.Sprintf("Some[%s]", stringer)
	}
	return fmt.Sprintf("Some[%v]", v)
}

// Map converts given Option value to another Option value according to the mapper function.
// If given Option value is None, this also returns None.
func Map[T, U any](option Option[T], mapper func(v T) U) Option[U] {
	if option.IsNone() {
		return None[U]()
	}

	return Some(mapper(option[value]))
}

// MapOr converts given Option value to another *actual* value according to the mapper function.
// If given Option value is None, this returns fallbackValue.
func MapOr[T, U any](option Option[T], fallbackValue U, mapper func(v T) U) U {
	if option.IsNone() {
		return fallbackValue
	}
	return mapper(option[value])
}

// MapWithError converts given Option value to another Option value according to the mapper function that has the ability to return the value with an error.
// If given Option value is None, this returns (None, nil). Else if the mapper returns an error then this returns (None, error).
// Unless of them, i.e. given Option value is Some and the mapper doesn't return the error, this returns (Some[U], nil).
func MapWithError[T, U any](option Option[T], mapper func(v T) (U, error)) (Option[U], error) {
	if option.IsNone() {
		return None[U](), nil
	}

	u, err := mapper(option[value])
	if err != nil {
		return None[U](), err
	}
	return Some(u), nil
}

// MapOrWithError converts given Option value to another *actual* value according to the mapper function that has the ability to return the value with an error.
// If given Option value is None, this returns (fallbackValue, nil). Else if the mapper returns an error then returns (_, error).
// Unless of them, i.e. given Option value is Some and the mapper doesn't return the error, this returns (U, nil).
func MapOrWithError[T, U any](option Option[T], fallbackValue U, mapper func(v T) (U, error)) (U, error) {
	if option.IsNone() {
		return fallbackValue, nil
	}
	return mapper(option[value])
}

// FlatMap converts give Option value to another Option value according to the mapper function.
// The difference from the Map is the mapper function returns an Option value instead of the bare value.
// If given Option value is None, this also returns None.
func FlatMap[T, U any](option Option[T], mapper func(v T) Option[U]) Option[U] {
	if option.IsNone() {
		return None[U]()
	}

	return mapper(option[value])
}

// FlatMapOr converts given Option value to another *actual* value according to the mapper function.
// The difference from the MapOr is the mapper function returns an Option value instead of the bare value.
// If given Option value is None or mapper function returns None, this returns fallbackValue.
func FlatMapOr[T, U any](option Option[T], fallbackValue U, mapper func(v T) Option[U]) U {
	if option.IsNone() {
		return fallbackValue
	}

	return (mapper(option[value])).TakeOr(fallbackValue)
}

// FlatMapWithError converts given Option value to another Option value according to the mapper function that has the ability to return the value with an error.
// The difference from the MapWithError is the mapper function returns an Option value instead of the bare value.
// If given Option value is None, this returns (None, nil). Else if the mapper returns an error then this returns (None, error).
// Unless of them, i.e. given Option value is Some and the mapper doesn't return the error, this returns (Some[U], nil).
func FlatMapWithError[T, U any](option Option[T], mapper func(v T) (Option[U], error)) (Option[U], error) {
	if option.IsNone() {
		return None[U](), nil
	}

	mapped, err := mapper(option[value])
	if err != nil {
		return None[U](), err
	}
	return mapped, nil
}

// FlatMapOrWithError converts given Option value to another *actual* value according to the mapper function that has the ability to return the value with an error.
// The difference from the MapOrWithError is the mapper function returns an Option value instead of the bare value.
// If given Option value is None, this returns (fallbackValue, nil). Else if the mapper returns an error then returns ($zero_value_of_type, error).
// Unless of them, i.e. given Option value is Some and the mapper doesn't return the error, this returns (U, nil).
func FlatMapOrWithError[T, U any](option Option[T], fallbackValue U, mapper func(v T) (Option[U], error)) (U, error) {
	if option.IsNone() {
		return fallbackValue, nil
	}

	maybe, err := mapper(option[value])
	if err != nil {
		var zeroValue U
		return zeroValue, err
	}

	return maybe.TakeOr(fallbackValue), nil
}

// Pair is a data type that represents a tuple that has two elements.
type Pair[T, U any] struct {
	Value1 T
	Value2 U
}

// Zip zips two Options into a Pair that has each Option's value.
// If either one of the Options is None, this also returns None.
func Zip[T, U any](opt1 Option[T], opt2 Option[U]) Option[Pair[T, U]] {
	if opt1.IsSome() && opt2.IsSome() {
		return Some(Pair[T, U]{
			Value1: opt1[value],
			Value2: opt2[value],
		})
	}

	return None[Pair[T, U]]()
}

// ZipWith zips two Options into a typed value according to the zipper function.
// If either one of the Options is None, this also returns None.
func ZipWith[T, U, V any](opt1 Option[T], opt2 Option[U], zipper func(opt1 T, opt2 U) V) Option[V] {
	if opt1.IsSome() && opt2.IsSome() {
		return Some(zipper(opt1[value], opt2[value]))
	}
	return None[V]()
}

// Unzip extracts the values from a Pair and pack them into each Option value.
// If the given zipped value is None, this returns None for all return values.
func Unzip[T, U any](zipped Option[Pair[T, U]]) (Option[T], Option[U]) {
	if zipped.IsNone() {
		return None[T](), None[U]()
	}

	pair := zipped[value]
	return Some(pair.Value1), Some(pair.Value2)
}

// UnzipWith extracts the values from the given value according to the unzipper function and pack the into each Option value.
// If the given zipped value is None, this returns None for all return values.
func UnzipWith[T, U, V any](zipped Option[V], unzipper func(zipped V) (T, U)) (Option[T], Option[U]) {
	if zipped.IsNone() {
		return None[T](), None[U]()
	}

	v1, v2 := unzipper(zipped[value])
	return Some(v1), Some(v2)
}

var jsonNull = []byte("null")

func (o Option[T]) MarshalJSON() ([]byte, error) {
	if o.IsNone() {
		return jsonNull, nil
	}

	marshal, err := json.Marshal(o.Unwrap())
	if err != nil {
		return nil, err
	}
	return marshal, nil
}

func (o *Option[T]) UnmarshalJSON(data []byte) error {
	if len(data) <= 0 || bytes.Equal(data, jsonNull) {
		*o = None[T]()
		return nil
	}

	var v T
	err := json.Unmarshal(data, &v)
	if err != nil {
		return err
	}
	*o = Some(v)

	return nil
}