/**
* @inheritdoc
*
* Example from haskell source code:
* ``` haskell
* traverse f = List.foldr cons_f (pure [])
* where cons_f x ys = (:) <$> f x <*> ys
* ```
*/
public function traverse(callable $transformation)
{
return f\foldr(function ($ys, $x) use($transformation) {
$functor = $transformation($x);
return $functor->map(f\append)->ap($ys ? $ys : $functor::of([]));
// https://github.com/widmogrod/php-functional/issues/30
}, false, $this);
}
public function test_foldr()
{
$list = Listt::of([1, 2, 3, 4]);
$result = f\foldr(function ($accumulator, $value) {
return f\concatM($accumulator, Listt::of([$value + 1]));
}, Listt::of([]), $list);
$this->assertEquals($result, Listt::of([5, 4, 3, 2]));
}