package gonads

import "errors"

var (
	ErrOptionIsNone = errors.New("gonads: Option value contains nothing")
)

// Option is a container that might contain a value.
type Option[T any] struct {
	val *T
}

// IsSome returns true if the option contains a value.
func (o Option[T]) IsSome() bool {
	return o.val != nil
}

// IsNone returns false if the option does not contain a value.
func (o Option[T]) IsNone() bool {
	return !o.IsSome()
}

// Take safely fetches the value from the Option.
func (o Option[T]) Take() (*T, error) {
	if o.IsNone() {
		return nil, ErrOptionIsNone
	}

	return o.val, nil
}

// Set assigns a value to an Option.
func (o *Option[T]) Set(val *T) {
	o.val = val
}

func NewOption[T any](val *T) Option[T] {
	return Option[T]{val: val}
}

// Thunk represents an uncomputed value that is cached for faster use later.
type Thunk[T any] struct {
	o    Option[T]
	doer func() *T
}

// Force either returns the cached thunk value or computes it and caches the result.
func (t Thunk[T]) Force() *T {
	if t.o.IsSome() {
		return t.o.val
	}

	t.o.Set(t.doer())
	return t.o.val
}

func NewThunk[T any](doer func() *T) Thunk[T] {
	return Thunk[T]{
		o:    NewOption[T](nil),
		doer: doer,
	}
}