Partial application and function composition in PHP.

partial.php

<?php

/**
 * Decorate a function so it can be partially applied.
 * 
 * If the resulting function is called with fewer than the required
 * number of arguments a new function is returned with the supplied
 * arguments applied.
 * 
 * See examples below.
 * 
 * @param  function $fn the function to be decorated.
 * @return function     the decorated function.
 */
function auto_partial($fn) {
  $outer_args = func_get_args();
  $fn = array_shift($outer_args);
  if(!is_a($fn, 'ReflectionFunction')) $fn = new ReflectionFunction($fn);
  return count($outer_args) >= $fn->getNumberOfRequiredParameters() ?
    $fn->invokeArgs($outer_args) :
    function() use ($fn, $outer_args) {
      return call_user_func_array(
        'auto_partial',
        array_merge(array($fn), $outer_args, func_get_args()));
    };
}

$map = auto_partial(function($fn, $arr) {
  return array_map($fn, $arr);
});

$filter = auto_partial(function($fn, $arr) {
  return array_filter($arr, $fn);
});

$foldl = auto_partial(function($fn, $arr, $init) {
  return array_reduce($arr, $fn, $init);
});

$foldr = auto_partial(function($fn, $arr) use ($foldl) {
  return $foldl($fn, array_reverse($arr));
});

$compose = function() use ($foldr) {
  return $foldr(function($init, $fn) {
    return $fn($init);
  }, func_get_args());
};

$mod = auto_partial(function($div, $num) { return $num % $div; });
$odd_nums_in = $filter($mod(2));
$squares = $map(function($x) { return $x * $x; });
$odd_nums_in_squares = $compose($odd_nums_in, $squares);
print_r($odd_nums_in_squares(range(1, 100)));
// > Array
// > (
// >     [0] => 1
// >     [2] => 9
// >     [4] => 25
// >     [6] => 49
// >     [8] => 81
// >     [10] => 121
// >     [12] => 169
// >     [14] => 225
// >     [16] => 289
// >     [18] => 361
// >     [20] => 441
// >     [22] => 529
// >     [24] => 625
// >     [26] => 729
// >     [28] => 841
// >     [30] => 961
// >     [32] => 1089
// >     [34] => 1225
// >     [36] => 1369
// >     [38] => 1521
// >     [40] => 1681
// >     [42] => 1849
// >     [44] => 2025
// >     [46] => 2209
// >     [48] => 2401
// >     [50] => 2601
// >     [52] => 2809
// >     [54] => 3025
// >     [56] => 3249
// >     [58] => 3481
// >     [60] => 3721
// >     [62] => 3969
// >     [64] => 4225
// >     [66] => 4489
// >     [68] => 4761
// >     [70] => 5041
// >     [72] => 5329
// >     [74] => 5625
// >     [76] => 5929
// >     [78] => 6241
// >     [80] => 6561
// >     [82] => 6889
// >     [84] => 7225
// >     [86] => 7569
// >     [88] => 7921
// >     [90] => 8281
// >     [92] => 8649
// >     [94] => 9025
// >     [96] => 9409
// >     [98] => 9801
// > )