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August 15, 2023

Thinking about functional programming

schemestandard mlhaskell

Someone on Discord asked about how to learn functional programming.

The question and my initial tweet on the subject prompted an interesting discussion with Shriram Krishnamurthi and other folks.

So here's a slightly more thought out exploration.

And just for backstory sake: I spent a few years a while ago programming in Standard ML and I wrote a chunk of a Scheme implementation. I'm not an expert, but I have a bit of background.

Hey, this is a free opinion.

Concepts from functional programming

When people talk about functional programming, I think of a few key choices you can make while programming:

  • Immutability by default
  • (Tail) recursion by default
  • First-class functions (and the suite of tools that go along with it. e.g. map, reduce/fold)

And if you have experience as a programmer, you either get the basic gist of these tenets or you can easily read about the basics.

That is, while most programmers I've met understood the basics, most programmers I've met were not particularly comfortable or fluent expressing programs with these ideas.

For myself, the only way I got comfortable expressing code with these ideas was lots of practice (as I mentioned above). And yet, even after I did a bunch of programming in Standard ML and Scheme, I really didn't see a particular benefit to practicing in a language other than one with which I wa already generally comfortable.

You have to learn a lot of other random things when you pick up Scheme or Standard ML that aren't just: practice immutability by default, recursion, and first-class functions.

So I think it's kind of misguided when person A asks how to learn functional programming and person B responds that they should learn Haskell or OCaml or whatever. I see this happen pretty often online.

Beyond any "language for functional programming" as a recommendation in general, Haskell is a particularly egregious suggestion to make in my opinion because not only are you trying to practice functional programming tenets but you're also dealing with a complex type system and lazy evaluation.

Instead, practice immutability, recursion, map/reduce in whatever language you like.

Programming languages

If you want to study programming languages, that's awesome. However, functional programming doesn't really have any direct connection to studying programming languages.

Languages are all over the place. Scheme, Standard ML, and Haskell are worlds apart, even within the functional programming family.

And modern languages have mostly adapted the aspects of functional programming that used to be unique 20 years ago.

Moreover, there are many other worthwhile families of languages to learn about:

  • Imperative/C-like (ok, you probably already know these)
  • Stack-based (JVM, x86 assembly sort of, Forth)
  • Array-oriented (APL, J)
  • Declarative (CSS, SQL, TLA+, Prolog)
  • Data (HTML, JSON, YAML)
  • Proof assistants (Isabelle/HOL, Coq)

The list isn't exhaustive, and the variations within families can be massive. But the point is that functional programming doesn't mean crazy programming languages or crazy programming ideas. Functional programming is a subset of crazy programming languages and crazy programming ideas.

If you want to learn about crazy programming languages and crazy programming ideas, you should! Go for it!

Introduction to Computer Science

SICP is famous as the (former) introductory textbook for computer science at MIT, and for its use of Scheme and the Metacircular Evaluator.

I don't have any experience teaching beginners how to program so I don't have thoughts on if this made sense. That's for folks like Shriram to think about.

However, I'm a half-decent programmer and I can't make it through this book. If you liked the book or want to read it, that's great! But I don't recommend it to anyone.

And many introductory Computer Science textbooks just don't make much sense to give to experienced programmers. For an experienced programmer, they can be quite slow!

Most of the folks I see asking about how to learn functional programming are experienced programmers.

Do whatever you feel like doing

I don't mean to overanalyze things, or get you overanalyzing things. If you want to learn functional programming by writing Haskell, that's awesome, you should go for it.

Wanting to do something is basically the best motivation there is.

The only reason I write this sort of post is so that folks who think that using Haskell or Standard ML or Scheme or reading SICP is the only way to learn functional programming see those ideas aren't necessarily true.

Write a Scheme!

Finally, for folks with time and motivation wanting to seriously work out their functional programming muscles, writing a Scheme implementation with a decent chunk of the standard library can be an immensely enjoyable project.

You'll learn a lot about languages and compilers and algorithms and data structures. It's leetcode with meaning.