"Thunks, Trampolines, and Continuation Passing" in JavaScript

cps.js

/* "Thunks, Trampolines, and Continuation Passing"
 * Python implementation from http://jtauber.com/blog/2008/03/30/
 * JavaScript implementation by Thomas Darr <[email protected]>.
 */

// thunk = lambda name: lambda *args: lambda: name(*args)
var thunk = function thunk(fn) {  
  return function _thunk() {
    var splat = Array.prototype.slice.apply(arguments);
    return function __thunk() { return fn.apply(this, splat); };
  };
};

/* def _trampoline(bouncer):
 *   while callable(bouncer):
 *     bouncer = bouncer()
 *   return bouncer
 * 
 * trampoline = lambda f: lambda *args: _trampoline(f(*args))
 */
var trampoline = function trampoline(fn) {
  return function _trampoline() {
    var splat = Array.prototype.slice.apply(arguments);
    return (function __trampoline(bouncer) {
      while (typeof bouncer === 'function')
        bouncer = bouncer.apply();
      return bouncer;
    })(fn.apply(this, splat));
  };
};

var factorial = function factorial(n) {
  /* _factorial = (lambda n, c=identity: c(1) if n == 0
   *   else thunk(_factorial)(n - 1, lambda result: thunk(c)(n * result)))
   * 
   * factorial = trampoline(_factorial)
   */
  return trampoline(function _factorial(n, continuation) {
    // identity = lambda x: x
    continuation = continuation || function identity(x) { return x; };
   
    if (n < 2) return continuation(1); 
    else return thunk(_factorial)(n - 1, function(result) {
      return thunk(continuation)(n * result);
    });
  })(n);
};

var fibonacci = function fibonacci(n) {
  return trampoline(function _fibonacci(n, continuation) {
    continuation = continuation || function identity(x) { return x; };
    
    if (n < 2) return continuation(1);
    else return thunk(_fibonacci)(n - 1, function(one) {
      return thunk(_fibonacci)(n - 2, function(two) {
        return thunk(continuation)(one + two);
      });
    });
  })(n);
};

var sum = function sum(n) {
  return trampoline(function _sum(n, continuation) {
    continuation = continuation || function identity(x) { return x; };
    
    if (n < 1) return continuation(n);
    else return thunk(_sum)(n - 1, function(result) {
      return thunk(continuation)(n + result);
    });
  })(n);
};

console.log(24 === factorial(5));
console.log(89 === fibonacci(10));
console.log(55 === sum(10));