Lazy evaluation

Intent

A function/method call results in a promise for future computation

Motivation

We want to be able to postpone the effects of a computation, for example delay printing from a function to webserver output until whole web-page is assembled

Implementation

Instead of evaluating a function, we deliver a promise - a callable with no arguments, which when forced is executed

Examples

//for simplicity handle only functions with one argument 
class promise { 
  private $args = null;
  private $func = null;
  function __construct($func, $args) { 
    $this->func = $func; 
    $this->args = $args;
  } 
  function evaluate() { 
    call_user_func($this->func,$this->args);
  } 
} 

function delay( $func, $arg ) {
  return array(new promise($func, $arg), 'evaluate'); 
}

//example use
...
$chunks[] = delay('a_printer',$a_variable);
...
foreach($chunks as $chunk ) {
  if(is_callable($chunk)) {
    $chunk(); //force - this does it's own printing in this example
  } else {
    print $chunk;
  }
}