TYPO3.Flow

This is an improvement to the Utility\Arrays::arrayMergeRecursiveOverrule function, leading to enormous flexibility gains for YAML Configuration Handling.

Description of the problem and possible solution:

     * Sometimes, you need to merge numerically indexed PHP arrays; and as this function does
     * the merging by key, this can lead to undesired results, as the following example demonstrates:
     *
     * $array1 = array('a1', 'b1', 'c1', 'd1');
     * $array2 = array('a2', 'b2', 'c2');
     * -> the resulting array is now array('a2', 'b2', 'c2', 'd1'), which is usually
     *    not the desired result. Often, you want to append, prepend or replace the collection.
     *
     * To define the behavior with indexed collections, a *Processing Rule* can be specified
     * in the second array using the special *__processingRule* Array Key. The following
     * Processing Rules are supported:
     *
     * APPEND
     * ------
     * The most common processing rule. Appends $array2 to $array1, renumbering the
     * numerical indices in $array2 and (if necessary) overriding the associative indices in $array1.
     *
     * Example:
     * $array1 = array('a1', 'b1', 'c1', 'd1');
     * $array2 = array('__processingRule' => 'APPEND', 'a2', 'b2', 'c2');
     * --> Result: array('a1', 'b1', 'c1', 'd1', 'a2', 'b2', 'c2');
     *
     * PREPEND
     * -------
     * Prepends $array2 before $array1, renumbering the numerical indices in
     * $array1 and (if necessary) overriding the associative indices in $array2.
     *
     * Example:
     * $array1 = array('a1', 'b1', 'c1', 'd1');
     * $array2 = array('__processingRule' => 'PREPEND', 'a2', 'b2', 'c2');
     * --> Result: array('a2', 'b2', 'c2', 'a1', 'b1', 'c1', 'd1');
     *
     * REPLACE
     * -------
     * Completely replaces $array1 with $array2.
     *
     * Example:
     * $array1 = array('a1', 'b1', 'c1', 'd1');
     * $array2 = array('__processingRule' => 'REPLACE', 'a2', 'b2', 'c2');
     * --> Result: array('a2', 'b2', 'c2');

With this addition, the following becomes possible in YAML configuration files:

// per-package Settings.yaml

  Foo:
    Bar: [t1, t2]

// global Settings.yaml

  Foo:
    Bar:
      __processingRule: PREPEND
      0: o2
      1: o3

// Output:
array('Foo' => array('Bar' => array('o2', 'o3', 't1', 't2')));

As soon as the Symfony YAML parser supports custom callbacks for YAML Tags, the processing rules could be written as:

  Foo:
    Bar: !PREPEND
      0: o2
      1: o3

... which is again even nicer :)