Curried Functions

So, currying, what's that all about?

Currying is a technique of transforming a function that takes multiple arguments into a function that takes one argument. In short, it's a way of defaulting parameters of a function call.

In a language like Java, you could accomplish the same thing by delegating method calls. For example:

int add(int a, int b) {
   return a + b;
}

int add_one(int b) {
   return add(1, b)
}

However, since functions aren't first class citizens in Java you're limited in what you can do. In a functional language, the possibilities are a more open. You can make use of currying to reduce the size of your code base and reduce the amount of boilerplate code you'd otherwise have to write.

An example of this is a set of accessor methods where the code is boilerplate. Modern IDEs will generate these for you, but wouldn't it be nice if you didn't have to see the code for them all the time? What happens if some of your setters need to manipulate the value being set? I suppose you could find all of the setters and update them manually...or if you're working in a functional language, you could just leverage the features of the language to make your job easier.

Take a look at the following code:

function set(attribute_name, value) {
   this[attribute_name] = value;
}

function get(attribute_name) {
   return this[attribute_name];
}

function attr_reader(attribute_name) {
   return function() { 
      return get.call(this, attribute_name);
   }
}

function attr_writer(attribute_name) {
   return function(value) {
      set.call(this, attribute_name, value)
   };
}

function MyClass() {}
MyClass.prototype.setName = attr_writer('name');
MyClass.prototype.getName = attr_reader('name');

var o = new MyClass()
o.setName('Bryan');
o.getName();  // Bryan

In this example, we have two methods: set and get which are the most simplistic and generic setter and getter functions one could write. We've also added a currying function for each of them. We then call through the currying functions to create a curried call to getter and setters on the prototype of the constructor for MyClass.

While this example is not the best possible code for currying getters and setters I believe it illustrates the concept of currying.

If you can tell me why attr_reader and atter_writer return the functions curried using their call method you get bonus points. If you can't, I'll explain later.

Closures and Javascript

Ah Closures, one of the must misunderstood and accidentally used features of javascript. But why is this the case? Well, because it's so easy to accidentally use them few understand the mechanics behind them. In this post I hope I can clear up the confusion and explain how they work.

In computer science, a closure is a function that is evaluated in an environment containing one or more bound variables.

Or, in normal people terms, a closure is a function that can access variables from the same scope that it was declared in. You can think of it as a natural extension of the standard scoping rules where functions declared in the global scope can access global variables.

var my_global = "hello, world";

function printGlobal() {
   print(my_global);
}

In the above code the function printGlobal can access a global variable. As an extension of the same principle, in the following code the function printLocal can access a local variable in its declaring scope.

function main() {
   var my_local = "hello, world";

   function printLocal() {
      print(my_local); 
   }
}

So that's all well and good, but why on earth would you ever want to use it? All your life you've been told that global variables are evil (which they are), so surely closures must also be evil. Well, if used incorrectly of course they're evil, but with great power comes great responsibility.

Here's quick example of a function that returns a function to increment a value by another fixed value.

function incr(start, delta) {
   return function() {
      start = start + delta;
      return  start;
   }
}

var i = incr(10, 1);
i(); // 11
i(); // 12
i(); // 13

Pretty freakin' wild isn't it? But how does that work? When a call is made to the incr function start and delta are bound to its scope. The function it returns simply holds onto these variables. Since it has a reference to the variables (not just their values, but a place to store them) they can be freely modified.

The next question you might be asking is, "So that's all well and good, but what if I call incr twice and hold on to both of the returned functions? Won't the second call clobber the first?". The answer is: NO! Each call to the method creates a new local scope just for that invocation. Exactly the same as how two calls to the same function at the same time won't interfere with each other.

But by far, the most classic use of closures is to enable Currying, and I'm not talking about the delicious spice blend. No no, I'm talking about higher-order functions. Functions that work with functions. Functions that return functions that call other functions. Functions that I'll explain in another post.