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callbackless-js

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callbackless.js - abstract away callbacks

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callbackless.js - abstract away callbacks
=======

JavaScript is asynchronous in nature, but too many callbacks put the code in a mess. Over the years, the community has invented a bunch of libraries trying to make it possible to write sync-style code to express async logic. However, it seems to me that until today none of them is doing the thing right. The Promise abstraction in some of the libraries looks awkward, it doesn't make much sense than the async APIs, we still smell callbacks. And some other libraries involve a compilation and ``eval``, which is unnecessarily heavyweight and complex for 99% of the use cases.

Callbackless.js is the effort to abstract away callbacks in a much better way. It's in plain ECMAScript 5, has no dependency on any advanced features of ECMAScript 6 (e.g. Generator), and there's no compilation and ``eval`` involved. It's extremely simple and elegant, but stands on the solid ground of functional abstractions.

The core of callbackless.js is the abstraction of Promise Monad (forget about the Promise you have known in other libraries, they're totally different). A monad is a computational context which returns values of type T. A promise monad is a computational context which will be returning a value of type T in the future (maybe asynchronously). Any operation on a plain type ``T``, e.g. ``toUpperCase :: String -> String``, there's a corresponding one on ``Promise<T>``, e.g. ``toUpperCase$ :: Promise<String> -> Promise<String>``. That's isomorphism. Because the computational context and the value are 2 orthogonal aspects, you don't need to reimplement the existing functions on ``T`` for ``Promise<T>`` from scratch, callbackless.js helps you simply lift them to the promise context, see the example below (*promises and functions on promises are named as xxx$*):

```javascript
// we have a function of type Int -> Int -> Int
var squareSum = function (x, y) { return x * x + y * y; };

// lift it to a function of type Promise<Int> -> Promise<Int> -> Promise<Int>,
// then it's all set to work on promises
var squareSum$ = liftA(squareSum);

// apply it to Promise<Int> the same way as you apply squareSum to Int
var result$ = squareSum$(p1$, p2$); // p1$, p2$ and result$ are promises of Int
```

It turns out that callbackless.js is not only a successful application of Functor and Monad in JavaScript, but also a great tutorial to advanced functional programming for JavaScript programmers. I have the faith that every JavaScript programmer would be able to understand the "scary monsters" with this library.

Now, let's get a sense of how the code looks like with a few more practical test cases. Firstly, you'll need to import some APIs:

**Import**

```javascript
// import the core APIs
var cbs = require('../callbackless.js');
var unit = cbs.unit;
var fmap = cbs.fmap;
var liftA = cbs.liftA;
var flatMap = cbs.flatMap;
var continue$ = cbs.continue$;

// import the file APIs
var cbs_fs = require('../callbackless-fs.js');
var readFile = cbs_fs.readFile;
var readFile$ = cbs_fs.readFile$;

// import the string APIs
var cbs_str = require('../callbackless-str.js');
var toUpperCase$ = cbs_str.toUpperCase$;
var concat$ = cbs_str.concat$;

// import the testing APIs
var cbs_testing = require('../callbackless-testing.js');
var assertEquals$ = cbs_testing.assertEquals$;
```

**Test Case 1** - reads 2 text files asynchronously, converts the contents of the first file to upper case, then concatenates it with the contents of the second file. At runtime, there're 2 async file reading operations, but you see no callbacks.

```javascript
function testFilePromise_functor() {
  // readFile :: String -> Promise<String>
  // readFile returns a promise of the contents of the file
  var data1$ = readFile('data/data_1.txt');
  var data2$ = readFile('data/data_2.txt');

// toUpperCase$ :: Promise<String> -> Promise<String>
  // at this point, the value of data1 may not be available, but we don't care,
  // because establishing the relationship between 2 promises doesn't require
  // to know their actual value.
  var upperCaseData1$ = toUpperCase$(data1$);
  
  // concat$ :: Promise<String> -> Promise<String> -> Promise<String>
  // concatenates 2 promises of string returns another promise of string
  var data1AndData2$ = concat$(upperCaseData1$, data2$); 
  
  // unit :: String -> Promise<String>
  // wrapps the directly available data into the computation context of promise
  // then it could work with the lifted functions
  var expectedData1AndData2$ = unit("HELLO CALLBACKLESS.JS!cool!");
  
  // assertEquals$ :: Promise<String> -> Promise<String> -> Promise<Boolean>
  // asserts the values of the 2 promises are equal. The underlying assert will
  // be invoked when both of the 2 promises are finished.
  assertEquals$(expectedData1AndData2$, data1AndData2$);
}
```

**Test Case 2** - there're 3 text files, the contents of the previous file is the path of the next file. This test starts from the path of the first file, then follows the path one by one until the third file. At runtime, there're 3 async file reading operations, but you see no callbacks.

```javascript
function testFilePromise_monad() {
  var path1 = 'data/file_1.txt';

// readFile :: String -> Promise<String>
  // readFile returns a promise of the contents of the file
  var path2$ = readFile('data/file_1.txt');
  
  // readFile$ :: Promise<String> -> Promise<String>
  // readFile$ is the flatMapped form of readFile, which accepts a promise type
  // path parameter.
  var path3$ = readFile$(path2$);

var data3$ = readFile$(path3$);
  var expectedData3$ = unit('Hey, I am here!')
  assertEquals$(expectedData3$, data3$);
}
```

See [tests](https://github.com/weidagang/callbackless-js/blob/master/tests/test-callbackless-fs.js) for more details.

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