functools

functools is a JavaScript library for functional programming.

Inspired by: Common Lisp, Clojure and Python.

SYNOPSIS

Function Composition:

var compose = require("functools").compose;

compose(select, update, prettify, display)("body .messages");

Async Function Compositon:

function findFiles(path, callback){  ... }
function readContents(files, callback){ ... }
function upload(files, callback){}

compose.async(findFiles, readContents, upload)('~/messages', function(error, uploadResult){
  ...
});

Async Juxtaposition:

function turkish(word, callback){ /* some magic here */ }
function french(word, callback){ /* some magic here */ }
function polish(word, callback){ /* some magic here */ }

juxt.async({ 'tr': turkish, 'fr': french, 'pl': polish })("hello", function(error,  results){
  assert.equal(results.tr, "merhaba");
  assert.equal(results.fr, "bonjour");
  assert.equal(results.pl, "cześć");
});

Currying:

var fn = require("functools");

var pickEvens = fn.curry(fn.filter)(function(num){ return num%2==0 });

pickEvens([3,1,4]) // returns [4]
pickEvens([1,5,9,2,6,5]) // returns [2,6]

INSTALL

$ npm install functools

or

$ wget https://raw.github.com/azer/functools/master/lib/functools.js

API

compose(functions ...)(value)

Combine functions in a new one, passing the result of each function to next one, from left to right.

function cube(x){ return x*x*x };

compose(Math.sqrt,cube)(4); // returns 8

compose.async(functions ...)(value,callback)

Asynchronous, continuation passing based version of compose function. Requires specified functions to call a callback function, passing an error object (if there is one) and the result to be carried.

function receiveMessage(message, callback){ ... callback(); }
function findRelatedUser(message, callback){ ... callback(null, user, message); }
function transmitMessage(user, message){ ... callback(); }

var messageTransmission = compose.async(receiveMessage, findRelatedUser, transmitMessage);

messageTransmission({ msg:"Hello !", 'user': 3 }, function(error, result){
  ...
})

curry(function, args ...)

Transform multiple-argument function into a chain of functions that return each other until all arguments are gathered.

function sum(x,y){ return x+y; }

var add3 = curry(sum, 3);

add3(14); // returns 17
add3(20); // returns 23

partial(function,initial arguments,*context *)

Return a new function which will call function with the gathered arguments.

function testPartial(){
  var args = reduce(function(x,y){ x+", "+y },arguments);

  console.log("this:",this);
  console.log("args:",args);
}

partial(testPartial, [3,14], 3.14159)(1,5,9);

The example code above will output:

this: 3.14159
args: 3,14,1,5,9

each(function,iterable)

Call function once for element in iterable.

each(function(el,ind,list){ console.assert( el == list[ind] ); }, [3,1,4]);

map(function,iterable)

Invoke function once for each element of iterable. Creates a new iterable containing the values returned by the function.

function square(n){ 
  return n*n;
}

map(square,[3,1,4,1,5,9]); // returns [9,1,16,1,25,81]

Objects can be passed as well;

var dict = { 'en':'hello', 'tr': 'merhaba', 'fr':'bonjour' };

function capitalize(){
  return string.charAt(0).toUpperCase() + string.slice(1);
}

map(capitalize, dict); // returns { 'en':'Hello', 'tr':'Merhaba', 'fr':'Bonjour' }

map.async(function,iterable, callback)

Apply async function to every item of iterable, receiving a callback function which takes error (if there is) and replacement parameters.

function readFile(id, callback){ ... callback(undefined, data); }

map.async(readFile, ['./foo/bar', './foo/qux', './corge'], function(error, files){
  if(error) throw error;

  console.log(files[0]); // will put the content of ./foo/bar
});

filter(function,iterable)

Construct a new array from those elements of iterable for which function returns true.

filter(function(el,ind,list){ return el%2==0 },[3,1,4]); // returns [4]

filter.async(function,iterable, callback)

Call async function once for each element in iterable, receiving a boolean parameter, and construct a new array of all the values for which function produces true

var users = [ 3, 5, 8, 13, 21 ]; // only user#3 and user#8 have permission in this example

function hasPermission(userId, callback){ ... callback(/* true or false */); }

filter.async(hasPermission, users, function(permittedUsers){
  assert.equal(permittedUsers.length, 4);
});

juxt(functions ...)

Take a set of functions, return a function that is the juxtaposition of those functions. The returned function takes a variable number of arguments and returns a list containing the result of applying each fn to the arguments.

function inc1(n){ return n+1 };
function inc2(n){ return n+2 };
function inc3(n){ return n+3 };

juxt(inc1, inc2, inc3)(314); // returns [315,316,317]

juxt.async(functions ...)

Async implementation of juxt.

function turkish(word, callback){ /* some magic here */ }
function french(word, callback){ /* some magic here */ }
function polish(word, callback){ /* some magic here */ }

juxt.async(turkish, french, polish)("hello", function(error,  results){
  assert.equal(results[0], "merhaba");
  assert.equal(results[1], "bonjour");
  assert.equal(results[2], "cześć");
});

reduce(function,iterable)

Apply function cumulatively to the items of iterable, as to reduce the iterable to a single value

reduce(function(x,y){ return x*y }, [3,1,4]); // returns 12

reduce.async(function,iterable, callback)

Async implementation of reduce.

var users = [2, 3, 5, 8, 13];

function usernames(accum, userId){ ... callback(undefined, accum + ', ' + username); }

reduce.async(usernames, users, function(error, result){
  if(error) throw error;

  console.log(result); // foo, bar, qux ...
});

SEE ALSO

• Functional Programming - Eloquent JavaScript