Functional thinking and seemingly non-efficent code

Ever since I started to learn coding, I’ve been using imperative programming languages, in which your focus as programmer is commonly described as “writing code that exactly defines how to perform a certain task”.

These languages rely on loops, conditionals, and state changes or mutations.

But I guess every programmer who has spent some time learning new things, has found some interest in functional languages, such as Haskell, Clojure, Elm, Scala, etc…

I first tried to learn Haskell, and solved some programming problems with it. I got somewhat familiar with pattern matching, tail recursion, pure functions, and immutability. But I got back to what I was used to, namely JavaScript, Python, and C.

After almost two years, I got the hots for functional programming again. The tipping point to give it a try, was finding out that one of the companies I’d like to work in was using Clojure.

Migrating to Clojure

To start exploring it, I decided to migrate a personal project originally written in JavaScript. I read Brave Clojure along the way to ease the process.

The two main Clojure features that made the migration interesting were:

The LISP syntax is something that just slows you down a little bit for the first few hours. It feels weird to get a bunch of parentheses thrown at your face, but you quickly get used to it, and may even start to like it.

The real struggle is the immutability thing.

Originally, I didn’t think it would matter enough.

Oh, yeah, just make this function return a new object with the updated values instead of mutating them, pff.

But oh boy if I was wrong.

I mean, yeah, at the end you can describe it like that. But to actually write code in that way was harder than expected.

Functional programming enforces immutability, which left me without so many of the things I just assumed were shared among all languages.

For-loops, for example, are now extinct, since you need a “flag variable” that mutates every loop. Now you need to rely entirely in recursion.

Reassigning values to variables is discouraged. (Yes, discouraged, since it’s not forbidden. And in fact, it’s one of the things that allows Clojure to have such a powerful REPL

Functional thinking

For you to easily write functional code, you need to develop what is referred as functional thinking — I read Neal Ford’s Functional Thinking with the expectation that it’d make it easier for me to do it, but I later realised that it was better to just practise it, allowing myself to slow down every once in a while and reflect on how dumb and non-idiomatic my code was.

I’m not suggesting it’s a useless read. In fact, if I’d have no prior experience with functional programming, I think it’d be a nice primer.

Core concepts

The first part of developing this mindset consists of getting familiar with the core concepts of functional programming.

You also need to know how to get stuff done with map, filter, and reduce. If you’re new to these, I’d recommend the second chapter of Functional Thinking : Shift.

Then you practise and check your own code to find flaws in your approach.

I don’t feel like I could give any more advice than that. What follows is what I felt was my biggest issue trying to write idiomatic Clojure.

Apparent non-efficiency

One of the most important aspects of adopting the mindset was un-learning some things about efficiency.

Every other function I wrote made me felt like I was doing such an inefficient, inelegant, dumb thing, when actually was the idiomatic way to do it.


When I was first learning about recursion, it looked like a panacea that could (and should) be used instead of every loop I needed to do.

Then I learned that it wasn’t.

Most of the times you’ll be better off by using loops instead of recursion. Adding a new stack frame to the call stack for every call is just too costly.

In Clojure (and most functional languages) you are encouraged to use recursion, which made me feel I was in some way writing poor code.

But the thing is, these languages should be optimized for recursive calls. At least for tail calls (those at the end of the function), avoiding a new stack frame for every call.

Ideally, tail recursion should be as efficient as an equivalent loop.

Data structures

Another thing was data structures. Specifically, “modifications”.

Since they’re not modifications, but “a modified copy of the original data structure”, they require you to copy the structure.

And deep copying entire data structures in most languages is costly. You’ll need to keep both the original and the copied structure in memory. And you’ll need to copy n values, thus it’d have O(n) time complexity.

However, Clojure implements persistent vectors , which makes copying to have a time complexity of O(log32 n) 1.

Is it faster to modify a persistent vector than mutating a raw array? No.

Is it faster to modify a persistent vector than copying and modifying a raw array, trying to mimick immutability? Yes.

Conclusion (?

I’m certainly used to imperative languages. And the things I know about memory management and complexity, were explained using C code.

That makes some common patterns in the functional paradigm seem inefficient.

And sometimes they are, but most of those times, it’s part of the paradigm.

Enforcing mutability needs different data structures, which have their own time and space complexities associated with them.

At the end, you’re accepting a trade-off.

Using the Clojure data structures is comfortable. One of the core ideas of functional programming is that having few data structures with many operations available to them is better than having many structures with few operations for each one.

It is better to have 100 functions operate on one data structure than 10 functions on 10 data structures. —Alan Perlis

You may not reach the benchmark scores of strongly typed languages with raw data structures in Clojure, but you won’t have Clojure’s data structures flexibility and convenience in those languages, either.

To me, it’s good enough to know that my code is:

If I ever forget what I’ve learned in the last week, I’ll just re-read this:

Don’t worry for the performance issues you feel you’re causing by having to deal with immutability or recursion depth.

There has been some discussion in Hacker News around Jean Pierre’s article on Clojure Persistent Vectors.

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