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| // In Haskell, a type that implements the Monad type class has the following functions: | |
| // | |
| // return :: a -> m a | |
| // bind :: m a -> (a -> m b) -> m b | |
| // | |
| // The Monad type classes could have also been implemented as: | |
| // | |
| // return :: a -> m a | |
| // fmap :: (a -> b) -> m a -> m b | |
| // join :: m (m a) -> m a | |
| // | |
| // Given `fmap` and `join`, `bind` could be implemented as: | |
| // | |
| // bind = join . fmap | |
| // | |
| // In the JavaScript example below: | |
| // | |
| // return a => Promise(a); | |
| // fmap func promise => promise.map(func); | |
| // join promise => promise.join(); | |
| // | |
| // This code still needs a bit of clean up. | |
| function Promise(x) { | |
| var callbacks = []; | |
| var addCallback = function(cb) { | |
| callbacks.push(cb); | |
| } | |
| var resolve = function(x) { | |
| if (this.value !== undefined) { | |
| return; | |
| } | |
| this.value = x; | |
| callbacks.forEach(function(cb) { | |
| cb(x); | |
| }); | |
| } | |
| var map = function(f) { | |
| var deferred = new Promise(); | |
| if (this.value === undefined) { | |
| addCallback(function(value) { | |
| deferred.resolve(f(value)); | |
| }); | |
| } else { | |
| deferred.resolve(f(this.value)); | |
| } | |
| return deferred; | |
| } | |
| var join = function() { | |
| var deferred = new Promise(); | |
| var innerPromise = this.value; | |
| var resolved = innerPromise !== undefined; | |
| var innerResolved = resolved && innerPromise.value !== undefined; | |
| if (resolved && innerResolved) { | |
| // Resolve the deferred in both the inner and outer promises are resolved | |
| deferred.resolve(innerPromise.value); | |
| } else if (resolved && !innerResolved) { | |
| // Add a callback since the inner promise is not resolved | |
| innerPromise.addCallback(function(value) { | |
| deferred.resolve(value); | |
| }) | |
| } else { | |
| // Add a callback since the outer promise is not resolved | |
| addCallback(function(innerPromise) { | |
| if(innerPromise.value !== undefined) { | |
| deferred.resolve(innerPromise.value); | |
| } else { | |
| innerPromise.addCallback(function(value) { | |
| deferred.resolve(value); | |
| }); | |
| } | |
| }); | |
| } | |
| return deferred; | |
| } | |
| var bind = function(f) { | |
| return map(f).join(); | |
| } | |
| return { | |
| bind: bind, | |
| join: join, | |
| map: map, | |
| resolve: resolve, | |
| value: x, | |
| addCallback: addCallback | |
| }; | |
| } | |
| // map | |
| var a = Promise(10); | |
| var b = a.map(function(x) { return x + 1; }); | |
| // Chained maps | |
| var c = Promise(); | |
| var d = c.map(function (x) { return x + 1; }); | |
| var e = d.map(function (x) { return x + 10; }); | |
| c.resolve(31); | |
| // join | |
| var f = Promise(Promise(10)); | |
| var g = f.join(); | |
| // bind | |
| var h = Promise(); | |
| var i = h.bind(function(value) { return Promise(value + 1) }); | |
| h.resolve(10); | |
| // Nested binds | |
| var j = Promise(); | |
| var k = Promise(); | |
| var l = j.bind(function(v1) { | |
| return k.bind(function(v2) { | |
| return Promise(v1 + v2); | |
| }); | |
| }); | |
| j.resolve(10); | |
| k.resolve(32); |
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