set_builder.js

/**
 * SetBuilder(expression[, context = this]) -> *
 * - expression(String): expression to evaluate
 * - scope(Object): context to evaluate expression within
 *
 *    var numbers = [1,2,3,4,5,6,7,8,9];
 *    
 *    var lessThanFive = numbers.findAll(function(n) { return n<5 });
 *    // vs
 *    var lessThanFive = SetBuilder('x | x <- numbers, x<5');
 *
 *    var moreThanFiveSquare = numbers.findAll(function(n) { return n>5 }).map(function(n) { return n*n });
 *    // or
 *    var moreThanFiveSquare = numbers.inject([], function(result, n) { if (n>5) result.push(n); return result });
 *    // vs
 *    var moreThanFiveSquare = SetBuilder('x*x | x <- numbers, x>5');
 *
 */
function SetBuilder(expr, context) {
  var re = /([^|]*)\|([^<]*)<-([^,]*),([^$]*)$/,
      match = expr.match(re),
      id, action, source, condition, results = [];

  if (!match) 
    throw new SyntaxError('Error parsing expression');

  match = match.invoke('strip');

  id = match[2];
  source = match[3];
  action = match[1].replace(id, '_', 'g');
  condition = match[4].replace(id, '_', 'g');

  (context || this)[source].each(function(_) {
    if (eval(condition))
      results.push(eval(action))
  })
  
  return results;
}