// Package funky provides a few constructs usually present in pure
// functional programming environments, using Go 1.18 generics.
package funky

// Maybe (sometimes called `Option`)

type Maybe[V any] interface {
	IsNothing() bool
	Value() V
}

type maybe[V any] struct {
	isNothing bool
	value     V
}

func Nothing[V any]() Maybe[V] {
	return &maybe[V]{isNothing: true}
}

func Just[V any](v V) Maybe[V] {
	return &maybe[V]{false, v}
}

func (m *maybe[V]) IsNothing() bool {
	return m.isNothing
}

func (m *maybe[V]) Value() V {
	return m.value
}

// Iterator

type Iterator[V any] interface {
	Maybe[V]
	Next() Maybe[V]
}

type sliceIt[V any] struct {
	idx   int
	slice []V
}

func SliceIterator[V any](sl []V) Iterator[V] {
	return &sliceIt[V]{slice: sl}
}

func (it *sliceIt[V]) Next() Maybe[V] {
	if it.IsNothing() {
		return Nothing[V]()
	}
	v := it.Value()
	it.idx++
	return Just(v)
}

func (it *sliceIt[V]) IsNothing() bool {
	return it.idx >= len(it.slice)
}

func (it *sliceIt[V]) Value() V {
	return it.slice[it.idx]
}

func Slice[V any](it Iterator[V]) []V {
	var out []V
	for m := it.Next(); !m.IsNothing(); m = it.Next() {
		out = append(out, m.Value())
	}
	return out
}