Manu Ganji

My start in Functional Programming

I had been hearing about Functional Programming for a lot of time. This Paul Graham’s essay was a good one and piqued my interest. And then I saw this Pete Hunt’s talk on React.js at a Meteor dev shop.

I could instantly connect that the reason for majority of the bugs is state dependent design. A bug is simply your program entering a state that you didn’t predict it to enter. The state is the value of all the variables in your program at any one moment. If you took a series of snapshots of your program’s execution, you’d see a progress from one state to the next with each processor cycle. It’s a great talk and I recommend you to watch it.

The ‘Function’ in Functional Programming comes from the Mathematics.

A function is a relation between a set of inputs and a set of permissible outputs with the property that each input is related to exactly one output.

The general focus in a functional program is on the what not the how. The code tries to tell what is the output of a function and generally speaks less about how to get to that output. So, if you were looping over a list, you would generally use a map function over writing your own for loop.

My favorite feature of FP is Pure functions. It means that functions should not have any side effect and their behaviour should be completely determined by their inputs. Pause and think about that for a moment. How much predictability that brings to your programs!

My second favorite feature is actually using the fact that functions are first class objects in these languages. You can pass around a function just like any other variable. So you can make a function compose another function for use in that scope.

In Python, the functools, itertools and operator modules provide some great methods to get your program closer to this philosophy.

For example functools.lru_cache helps to memoize a function, functools.partial takes a function and few of it’s arguments and returns a function that takes the remaining arguments. For example:

def write_to_disk(directory, page, json_object):
    if not os.path.exists(directory):
    filename = directory + "/" + str(page) + ".json"
    with open(filename, "w") as f:

write_page_to_disk = functools.partial(write_to_disk, "my_directory", 1), itertools.filter, etc will help to avoid ugly looping structures like for and while.

operator.itemgetter, operator.add, etc also help to avoid some redundant code. Remember that time when you had to get the [a] of every [b] of every [c]? You can write that more elegantly:

get_a = operator.itemgetter("a")
get_b = operator.itemgetter("b")
get_c = operator.itemgetter("c")

a = get_a(get_b(get_c(input_obj)))

I tried not to duplicate anything that has been written already by other people. Please share your feedback in the comments.