Functional Programming Is Hard,
That's Why It's Good


Odds are, you don’t use a functional programming language every day. You probably aren’t getting paid to write code in Scala, Haskell, Erlang, F#, or a Lisp Dialect. The vast majority of people in the industry use OO languages like Python, Ruby, Java or C#–and they’re happy with them. Sure, they might occasionally use a “functional feature” like “blocks” now and then, but they aren’t writing functional code.

And yet, for years we’ve been told that functional languages are awesome. I still remember how confused I was when I first read ESR’s famous essay about learning Lisp. Most people are probably more familiar with Paul Graham’s “Beating The Averages” which makes the case that:

But with Lisp our development cycle was so fast that we could sometimes duplicate a new feature within a day or two of a competitor announcing it in a press release. By the time journalists covering the press release got round to calling us, we would have the new feature too.

A common thread among people proselytizing functional programming is that learning this new, functional language is “good for you”; almost like someone prescribing 30m in the gym a day will “make you fit,” but it also implies difficulty and dedication. Haskell, Ocaml, and Scala are different from Lisp in that they have a certain notoriety for being very hard to learn. Polite people call this “being broad & deep”. Less polite people call it “mental masturbation” or “academic wankery” or just plain “unnecessary.” I submit that this difficulty is a familiar situation, and it’s a strong indicator that learning one of these languages will make you more productive and competent at writing software.

Your First Time Wasn’t Gentle

I learned to code when I was 7, fiddling on my grandfather’s computer on long, boring suburban summers. I learned BASIC, made a ball dance on screen. I learned Pascal and wrote a program to play back music on the PC speaker. I learned C around age 10, but hit a pretty big wall there that I dodged around until high school: pointers. Even ignoring those damnable pointers, I worked and read and studied and practiced and failed a whole lot. I deleted my grandfather’s hard drive twice (once by accident), and ended up reinstalling my OS more than a few times. I failed, over and over.

Maybe your story is similar, maybe it’s wildly different. But I think nearly everyone who learned to program can relate to the difficulty of the process. And then, after learning the basics, we again encounter well-known conceptual thresholds like “pointers.” Many computer science professors describe pointers as a kind of filter in their curricula. If you are going to be a good programmer, you must be able to understand pointers. And the people who easily learn them are few and far in between. Most people, myself included, needed practice and examples to understand what pointers were and why they were important.

This nearly universal struggle isn’t a coincidence. Pointers are a very powerful and fundamental abstraction. Learning them makes you a better programmer by helping you think more symbolically. Even if you work in a language that doesn’t offer you pointers, pointer-like structures and concepts abound in the wild.


Once you learn a few languages, they all start to look the same. Someone who knows Python probably won’t have too many problems learning Ruby, someone who knows Java has a leg up on learning C#. Sure, there are hangups. A Rubyist learning Python might have a few surprises learning for comprehensions, and the Java user might have some problems wrapping their heads around C# delegates. Still, if you squint, they all sort of look like one another. And I can assure you, if you don’t know this yourself, that once you learn A Lisp you start to see the similarities in all the variants of Lisp.

That said, most people are totally unprepared when they first meet up with Haskell or Ocaml. Heck, in Haskell even the semicolons are different. This isn’t just an issue of syntax; Haskell and MLs are actually based on fundamentally different abstractions and a whole new language of patterns. You build apps differently, you structure them differently, and you extend them differently.

Lots of these new ideas are incredibly powerful. Monads are at least as fundamental and powerful an idea as pointers (and you probably use them implicitly without realizing what they’re called). So unlike learning C# after learning Java, aspirants to functional languages have to go back much further and learn much more fundamental concepts to continue. It’s like learning pointers all over again. And, just like back when we were first setting out to learn programming, these big concepts can be frustratingly illusive and vague until you’ve worked (and failed) with them.

Take Your Medicine, Find Your Pharmacist

Despite this downside, I submit that learning these functional languages is good for you, professionally. I suspect some readers are rolling their eyes at this point, being unable to imagine a world where monoids or monads are useful to them in Java or C#. To me, it’s no surprise that people balk at the prospect of learning FP; they are trying to learn new abstractions on the same fundamental level as pointers and recursion. That requires patience and dedication on a level that most professionals reserve only for actually completing clear business goals. Very few people feel comfortable failing-let alone failing over and over- once they’re out of their formative years. We’re all supposed to be professionals now, right?

To further complicate matters, a lot of language and algorithm research takes place in functional languages (particularly Haskell). It’s very easy to get lost in an unfamiliar field of category theory, half-finished abstractions, and failed ideas. Without a clear guide (like a good book written by a pragmatic author), an already-difficult task becomes even more daunting.

This combination leads to an unsurprising result: many people are reluctant to put the time and effort into learning FP. Justifying this reluctance is easy, “Couldn’t I be spending this time making something as opposed to learning something?” But this line of thinking means you never stray towards any new technologies (or only very familiar ones). In an industry that changes as rapidly as software engineering does, I think it’s not a realistic value judgment.

Saying is Believing

The most obvious benefit from learning a functional language is that you will learn the pattern language for basic functional concepts. This gives you the power to consider and mentally manipulate surprisingly big concepts in your head very succinctly. This isn’t a magic property of FP; languages and patterns emerge to describe certain classes of problems succinctly. It’s just relevant because FP’s conceptual sweet spots has been becoming increasingly relevant in a world where parallelism and meta-programming are important.

For example, consider a simplified, local version of the famous Google MapReduce paradigm. Describing this paradigm succinctly in a functional way with well-known functional patterns is surprisingly brief:

mapReducer dataset partitioner mapper reducer =
  let partitions = partitioner dataset in
    reduce reducer (map mapper partitions)

Changing this to support parallelism and distributed concurrency is trivial (for local parallelism, many libraries support “pmap” and “preduce”–which exploit simple properties of functional languages–as drop in replacements). The concept of maps, partitions, generators, streams, reductions, folds, and function chaining are all very simple patterns that are part of the shared language of functional programming, so anyone with a passing familiarity with Lisp, Haskell, OCaml, or even sorta-functional languages like Python and Ruby will be able to understand the gist of this.

Consider for a moment what it would take to succinctly describe this framework in an OO language using only common OO patterns. At bare minimum you’d be asked to define the spec for what a mapper and reducer are. If you’re curious, try laying out a minimal spec for “OO” MapReduce in your ideal OO language. I found it to be quite verbose. With a Java-like language, one might say:

 interface Mapper<A,B> {
   B map(A input);

 interface Reducer<X,Y> {
   Y reduce(X a, X b);

 abstract class MapReduce<X,Y,Z> {
   private Mapper<X,Y> mapper;
   private Reducer<Y,Z> reducer;

   public MapReduce(Mapper<X,Y> map, Reducer<Y,Z> reduce) {
     // ...

   public run(SeqenceType<X> data) {
     // ...

Without even going through the loop logic, our inability to access the common nouns and verbs of the functional paradigm makes the MapReduce technique very expensive to define. This definition is almost comically naive, but it helps show how much you can relate with functional concepts. Another great example is how Scala could take the already great Java Fork/Join library and easily integrate its features into Scala’s natural syntax.

One Person’s Work is Another Person’s Pleasure

And so, I encourage everyone who wants to be a better programmer: consider learning a functional language. Haskell and OCaml are both great choices, and F# and Erlang are pretty good as well. It won’t be easy, but that is probably a good sign. Try and identify the difficult concepts you encounter and see if other people are leveraging them; frequently you can break through a mental roadblock by finding out what the intent of an unfamiliar abstraction really is.

While you’re learning, do be careful not to take it too seriously. Like anything that requires time and effort, there is a danger of becoming over-invested in FP. Falling into this cognitive trap will ruin your investment. It’s easy to forget how many models of computation there are out there, and even easier to forget how much beautiful software has been written with any of them.

It’s a narrow path to walk down, but on the other side, you emerge with more core concepts and models to leverage in your everyday programming. You will almost certainly become more comfortable with denser code, and will certainly gain new insights into how to be a better software engineer.


A few of the nice people who proofread this essay asked me the same question once they were done: “This sounds great, Dave, but which language should I learn?” This is, of course, the toughest question they could ask me.

I think if you’re already a competent programmer then there isn’t a “right” answer other than: “Whichever one meets your needs.” If you need to work on the JVM, pick Scala or Clojure. If you want to write big distributed software systems quickly, pick Erlang. If you want amazing general workhorse languages with terrific compilers, pick Haskell or OCaml. If you want a prototyping medium with more potential than Ruby or Python, go for Scheme.

Remember, the name of the game here is practical skills and self-improvement. If you can spare the time, try stepping out of your comfort zone and challenging yourself.

Since I already knew Lisp and Erlang and have done professional work with OCaml, I decided to tackle Haskell, which is a whole other world unto itself. The only way I found that language penetrable was with the helpful guides of Learn You A Haskell and Real World Haskell. These books are well-written, helpful, and freely available online. Should you choose to try your hand at Haskell, these books can be your road map.