Monad in JS

promise.js

/* 
   jsclass
   http://jsclass.jcoglan.com/
   npm install jsclass
*/
require('jsclass');
JS.require('JS.Deferrable');

Promise = new JS.Class({
                           include: JS.Deferrable,
                           initialize: function(value) {
                               if (value !== undefined) this.succeed(value);
                           }
                       });

// foldl :: (a -> b -> a) -> a -> [b] -> a
function foldl (g, a, bs) {
    var ret = a;
    for (var i = 0, n = bs.length; i < n; i++) {
        ret = g(ret, bs[i]);
    }
    return ret;
};

// unit :: a -> Promise a
function unit(x) {
    return new Promise(x);
};

// wrap :: a -> Promise () -> void
function wrap(x, p) {
    p.succeed(x);
};

// bind :: Promise a -> (a -> Promise b) -> Promise b
function bind(input, f) {
    var output = new Promise();
    input.callback(function(x) {
                       f(x).callback(function(y) {
                                         output.succeed(y);
                                     });
                   });
    return output;
};
        
exports.Promise = Promise;
exports.unit   = unit;
exports.wrap   = wrap;
exports.bind   = bind;
exports.pipe   = foldl.bind(null, bind);

test.js

// invoke via : node test.js 

var fs   = require('fs'),
    url  = require('url'),
    http = require('http'),
    sys  = require('util'),
    p    = require('./promise');

// readFile :: String -> Promise String
var readFile = function(path) {
  var promise = new p.Promise();
  fs.readFile(path, function(err, content) {
      p.wrap(content, promise);
  });
  return promise;
};
    
// printUrl :: String -> Promise String
var printUrl = function(json) {
  var uri = JSON.parse(json).url;
  return p.unit(sys.puts(uri));
};

p.pipe(p.unit(__dirname + '/urls.json'), [readFile, printUrl ]);

urls.json

{"url":"http://github.com/api/v2/json/user/show/jcoglan"}