Functional Programming in PHP

functional_programming.php

<?php

// Accumulator gets passed around for reuse - function as a value
$accumulator = function (
  string $accumulated_string, 
  string $mapped_list_element
) {
  return $accumulated_string . $mapped_list_element . "\n";
};

// Notice how array_map, array_filter and array_reduce
// accept functions as parameters - they are higher order functions
$mapped_array = array_reduce(
  array_map(
    function (int $list_element): string {
      return "A list element: " . $list_element;
    }, 
    [1, 2, 3, 4]
  ), 
  $accumulator,
  ""
);
echo "Mapped Array: \n";
echo $mapped_array;

$filtered_array = array_reduce(
  array_filter(
    [1, 2, 3, 4],
    function (int $list_element): bool {
      return $list_element > 2;
    }
  ),
  $accumulator,
  ""
);
echo "Filtered Array: \n";
echo $filtered_array;

// Closures "enclose" over their surrounding state
// The $closure_incrementer function here returns a function
// making it a higher order function.
echo "Closure Incrementer: \n";
$closure_incrementer = function () {
  $internal_variable = 0;
  return function () use (&$internal_variable) {
    return $internal_variable += 1;
  };
};
$instance = $closure_incrementer();
echo $instance() . " is equal to 1\n";
echo $instance() . " is equal to 2\n";