promise-monad.js

JSCLASS_PATH = 'build/min';
require('./' + JSCLASS_PATH + '/loader');

JS.require('JS.Deferrable');

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

var fs   = require('fs'),
    url  = require('url'),
    http = require('http'),
    sys  = require('sys');

// pipe :: m a -> [a -> m b] -> m b
var pipe = function(x, functions) {
  for (var i = 0, n = functions.length; i < n; i++) {
    x = bind(x, functions[i]);
  }
  return x;
};

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

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

// readFile :: String -> Promise String
var readFile = function(path) {
  var promise = new Promise();
  fs.readFile(path, function(err, content) {
    promise.succeed(content);
  });
  return promise;
};

// getUrl :: String -> Promise URI
var getUrl = function(json) {
  var uri = url.parse(JSON.parse(json).url);
  return new Promise(uri);
};

// httpGet :: URI -> Promise Response
var httpGet = function(uri) {
  var client  = http.createClient(80, uri.hostname),
      request = client.request('GET', uri.pathname, {'Host': uri.hostname}),
      promise = new Promise();
  
  request.addListener('response', function(response) {
    promise.succeed(response);
  });
  request.end();
  return promise;
};

// responseBody :: Response -> Promise String
var responseBody = function(response) {
  var promise = new Promise(),
      body    = '';
  
  response.addListener('data', function(c) { body += c });
  response.addListener('end', function() {
    promise.succeed(body);
  });
  return promise;
};

// print :: String -> Promise null
var print = function(string) {
  return new Promise(sys.puts(string));
};

pipe(unit(__dirname + '/urls.json'),
        [ readFile,
          getUrl,
          httpGet,
          responseBody,
          print         ]);

urls.json

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