yin-yang.js

## yin-yang.js
// jonas :: https://github.com/michaelsbradleyjr/jonas
//       :: https://npmjs.org/package/jonas

var jonas = require("jonas"),

    Cont = jonas.Monad.Cont,
    pCont = jonas.Monad.pCont,

    Q = jonas.Q,
    util = require("util");


// see: http://en.wikipedia.org/wiki/Call-with-current-continuation

// for reference, here is the "yin-yang" call/cc puzzle as it's commonly
// implemented in Scheme:

// (let* ((yin
//         ((lambda (cc) (display #\@) cc) (call-with-current-continuation (lambda (c) c))))
//        (yang
//         ((lambda (cc) (display #\*) cc) (call-with-current-continuation (lambda (c) c)))))
//  (yin yang))


// we'll implement it twice in JavaScript, per jonas; the first implementation
// will be syncrhronous and the second one asynchronous


/* -------------------------------------------------------------------------- */


// Synchronous implementation - Cont monad

function λ(c) { return c(c); }

var mReturn = Cont.mReturn,

        mFn = function (t) {
                  return function (cc) {
                      util.print(t);
                      return mReturn(cc);
                  };
              };


Cont.callCC( λ ).mBind( mFn("@") )( Cont.callCC( λ ).mBind( mFn("*") ) );

// => @*@**@***@****@*****@******@*******@********@*********@**********@*****...

// will blow the call stack rather quickly; jonas is currently being revised so
// that synchronous monadic `run` does not involve recursion, i.e. it won't blow
// the stack, stay tuned...

// comment out the last line above in order to run the second implementation
// below


/* -------------------------------------------------------------------------- */


// Asynchronous implementation - pCont monad

mReturn = pCont.mReturn;

    mFn = function (t, delay) {
              return function (cc) {
                  util.print(t);

                  // since we're using pCont, we can introduce a variable delay
                  // in terms of a promise; resolution of the promises is
                  // handled automatically by the "monadic plumbing" of pCont;
                  // if delay is null or the value "s", the computation will
                  // implicitly make use of the JavaScript environment's
                  // setImmediate function per Q's contract re: `then`, owing to
                  // pCont's "inner monad" (pIdentity)

                  return mReturn(
                      ( delay == null ) ||
                      ( delay === "s" ) ? cc
                                        : Q.delay(delay).then(
                                              function () { return cc; }
                                          )
                      );
              };
          };


pCont.callCC( λ ).mBind( mFn("@", 100) )( pCont.callCC( λ ).mBind( mFn("*", 100) ) );

// same output as above, but ~100ms delay between each character printed; won't
// blow the stack since each step in the monadic computation involves a trip
// across the event loop, but memory consumption is unbounded and the process
// will eventually crash