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Composition is a fancy name for passing the result of one function as an argument into the next function. If we have some functions like these:

add2 = function(number) {
  return number + 2;

subtract1 = function(number) {
  return number - 1;
We could do this:

addSubtract = function(n) {
  var addedValue = add2(n);
  return subtract1(addedValue);
Or We could do this:

addSubtract = compose(subtract1, add2);
Both would yield 4:

addSubtract(3);  // 4
compose makes a brand new function. The data flows from right to left applying each function along the way. Right to left makes sense when you think about the parenthesis going to the right of the newly constructed function. Here’s another few examples

dasherize = compose(join('-'), split(/\s+/g);
dasherize("Big Al's Trucks"); // "Big-Al's-Trucks"

doFunkyThingsWithName = compose('.toUpperCase()', reverse, '.name')
doFunkyThingsWithName({name: "George"}); // "EGROEG"
The combination possibilities are endless. We can compose groups of functions that go together into little programs. Then we can even compose those programs! Checkout how easy it is to play around:

var loggingFun = compose(log, subtract1, log, add2);
// 5
// 4
// 4

var memoizedFun = memoize(loggingFun);
// 6
// 5
// 5

// 5

var newProgram = compose(otherProgram, memoizedFun);

parallel_compose(thirdProgram, newProgram);

Why is this so amazing?

No data

You may notice I don’t need to name any intermediate results. This is a big deal. This assists in separating your data from your functions since you simply don’t see the data. I think a lot of people are turned off from functional programming by the thought of juggling arguments (data) around all the time instead of tucking them away in a nice object. Here, there is no data at all, rather functions plugged together to work on anything. This gives you megagenericicity which is a word i just made up. Starting from the original data call, we just start composing away.

UsersController = (function(){
  index = db.all('users', compose(renderView, manipulateUserData));

  return {index: index}

Build up from smaller functions

Is it a surprise to find out most of what you write is the same stuff over and over again? Instead of writing out the same function calls with different names or the same assignment statements to different variables, you can skip all of that and just drop the function name in. The code drops in size, becomes more generic, and you distill the code to just the important behavior. If you’re worried about names missing since names give you an idea of what we’re operating on, don’t sweat it - just assign a specific name to each function:

// no one says it has to be like this:
getArgNames = compose(last, match(/\((.*)\)/), first, split('\n'));

// it can easily be like this:
firstLine = compose(first, split('\n'));
contentInsideParenthesis = compose(last, match(/\((.*)\)/));
getArgNames = compose(contentInsideParenthesis, firstLine);
Smaller functions that do one thing means more reusable parts. Taking advantage of the divide and conquer strategy, we can build very complex systems with easy to grok simple functions. Turns out, lots of the smaller functions you break out can be put in to utility libraries and moved out of your application code. This technique promotes reusability (common theme here…) and makes your app simpler.

Declarative Programming

In declarative programming, we tell the computer what to do and let it accomplish the task however it sees fit. Think of SQL. You express a query as a single expression and let the computer do it’s job. The SQL folks can make the compiler better and better and you don’t have to change a single line. Imperative, order of execution style programming has a funny way of hindering that freedom. Like doing our own garbage collection in code, when we hard code these low level steps, we steal responsibility away from the compiler/interpreter and place it in our application. Also, one expression can be manipulated much easier than several lines of separate steps: you simply copy and paste little chunks of code around. Want logging? Add an extra function in the chain. etc. When we program like this, our application code likely won’t need to change when we optimize. We can fix bugs in our underlying functions, but the statement of what needs to happend doesn’t change. Ladies and gentlemen, we’ve risen to a higher level.

Free Dsl

Don’t actually do anything! Just say it and let the other stuff do the work. Brilliant.
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