<?php

namespace PhatCats\ObjectTypeclass;

use PhatCats\Collection;

use PhatCats\Typeclass\Monoid;

trait ObjectFoldable {

// not sure if it's a good idea to extend Collection . . .

use Collection;

/**

* Left-associative fold of a structure.

*

* @param $init The seed value for the fold. This value is the result in the

* case of an empty structure.

* @param $f The folding function. A function of two arguments: the first

* argument should be of the same type as $init (and the return type);

* the second argument is an element of the `ObjectFoldable` structure.

* @return A value that has the same type as $init.

*/

abstract function foldLeft($init, callable $f);

/**

* Right-associative fold of a structure.

*

* @param $init The seed value for the fold. This value is the result in the

* case of an empty structure.

* @param $f The folding function. A function of two arguments: the first

* argument is an element of the `ObjectFoldable` structure; the second

* should be of the same type as $init (and the return type).

* @return A value that has the same type as $init.

*/

abstract function foldRight($init, callable $f);

/**

* Folds the structure by mapping each element to a Monoid (using $toMonoid)

* and then combining the results using that Monoid.

*

* This method has a default implementation (which uses foldLeft) but can be

* overridden if a given type can provide a more efficient implementation.

*

* @param $monoid an instance of a monoid. Should be the same type as the

* return type of $toMonoid as well as the return type of foldMap.

* Needed for the case of an empty structure.

* @param $toMonoid A function that takes an element of the structure and

* returns a Monoid.

* @return A value that has the same type as the return type of $toMonoid and

* $monoid.

*/

public function foldMap(Monoid $monoid, callable $toMonoid) {

$init = $monoid->identity();

$f = function ($acc, $x) use ($monoid, $toMonoid) {

return $monoid->append($acc, $toMonoid($x));

};

return $this->foldLeft($init, $f);

}

/**

* Folds the structure by combining elements using a monoid. Note that the

* elements must already be a monoid.

*

* This method has a default implementation (which uses foldMap) but can be

* overridden if a given type can provide a more efficient implementation.

*

* @param $monoid an instance of a monoid. Should be the same type as the

* elements. Needed for the case of an empty structure.

* @return a value of the same type as the elements.

*/

public function fold(Monoid $monoid) {

$id = function ($x) { return $x; };

return $this->foldMap($monoid, $id);

}

}