This RFC is still in progress
====== Functional elements to work with aggregations ======
==== Abstract ====
A common pattern in PHP is iterating through a list of objects and executing certain methods of the objects. This is especially common when it comes to 1:n-relations (e.g. one object, n adapters). This proposal is meant to abstract those iterations in convenient functional constructs by adding method call iterators and iterator functions.

==== Current usage patterns ====
=== UC-1: Iterating over an aggregation of objects ===
<code php>
foreach ($list as $element) {
    $element->method();
}
</code>

=== UC-2: Iterating over an aggregation of objects and keeping the results ===
<code php>
$result = array();
foreach ($list as $element) {
    $result[] = $element->method();
}
</code>

=== UC-3: Iterating over an aggregation of objects and conditionally executing a method ===
<code php>
foreach ($list as $element) {
    if ($element->conditionalMethod()) {
        $element->method();
    }
}
</code>

=== UC-4: Iterating over an aggregation of objects and conditionally executing a method while keeping the results ===
<code php>
$result = array();
foreach ($list as $element) {
    if ($element->conditionalMethod()) {
        $result[] = $element->method();
    }
}
</code>



==== Required functionality ====
Extracting the required functionality to leads to the following required additions:
  * [[rfc:functional-elements:call-iterator|class CallIterator]]: a simple iterator which walks over a traversable list (including arrays) and calls a specified method. The CallIterator might optionally gather the results
  * [[rfc::functional-elements:is-traversable|function is_traversable()]]: Returns true if a list is traversable (Iterator/IteratorAggregate/array)
  * [[rfc:functional-elements:iterate|function iterate()]]: Walks over a traversable list and does nothing
  * function iterator_true(): Walks over a traversable list and assumes every element to be bool(true)
  * function iterator_false(): Walks over a traversable list and assumes every element to be bool(false)

==== Modified use cases ====
=== UC-1: Iterating over an aggregation of objects ===
<code php>
iterate(new CallIterator($list, 'method'));
</code>

=== UC-2: Iterating over an aggregation of objects and keeping the results ===
<code php>
$result = iterate(new CallIterator($list, 'method'), true);
</code>