Yoan Blanc’s weblog

I’m still earring a colleague of mine saying: “all the human problems are solvable with an Object Oriented approach.” I didn’t argue a long time with her. She maybe never had to think about other kind of problems, problems that require to break that paradigm apart.

Google offers an interesting introduction to the Map/Reduce paradigm. Map/Reduce is a way to distribute operations on datas (map) on several computers and then to aggregate the outputs to get the result (reduce).

I made the warm up sheet using Functional JavaScript on SpiderMonkey because my Haskell skills are still bad.

First, set it up, make sure functional.js is in the same directory:

$ js
> var window = this
> load(['functional.js'])
> Functional.install()

Before Functional JavaScript is built for web browser, you have to set window to the context (loading will fail instead). Then here we go with the first exercise: concat([[1,2],[3,4,5],[6]]) → [1,2,3,4,5,6].

function concat(list){
 return foldl("x.concat(y)", [], list)
}

Next one is a group by key, or an object merger: group([{'a':0},{'b':1},{'a':2}]) → {'a':[0,2],'b':[1]}.

function group(list){
 return foldl(function(x,y){
  for(var key in y)
   if (typeof x[key] === "undefined")
    x[key] = [y[key]];
   else
    x[key].push(y[key]);
  return x
 }, {}, list)
}

the for loop is the only way to discover the keys of an object, so don’t tweak Object.prototype. Next one a way to split a list with an arbitrate key: foldk([1,2,3,4], 3) → [[1,2],[3,4]]

function foldk(list, k){
 return foldl(function(x,y){
  x[y>=k?1:0].push(y);
  return x
 }, [[],[]], list)
}

And the next one, more trivial about composing two functions.

function compose(f, g) {
 return function() {
  return f(g.apply(this, arguments))
 }
}

NB: there are maybe better solution. I would be thankful to ear them from you.

If all this looks pointless, some people are already thinking about using JSON for Map/Reduce (JSON is JavaScript) which is all about building applications like CouchDB that works this way.

Last but not least, two posts on Raganwald about doing the same things in Ruby (1.9) that Functional Programming in JavaScript able you to. update: find the source code here.