Published on December 6, 2019, last updated December 11, 2019

This is a comparison between the two languages by someone who has written code professionally in both. That said, I’m much more familiar with Haskell than with OCaml, so if you think there is a mistake in the text, let me know.

## Background and users

Haskell is a niche, rarely used language. The situation is changing, but make no mistake, it is very far away from the mainstream languages such as Python, JavaScript, or Java. The users of Haskell are banks, payment systems, companies related to the crypto-currency business, big corporations, and others. It is usually used because there are engineers who like Haskell and who have enough influence to promote it for solving actual business problems. Sometimes there are CEOs who believe that Haskell will give them an edge (visionaries).

OCaml is a product of the French academic circles, although its use is expanding and it is now not limited to the academy. It is no coincidence that French research projects like Coq are written in OCaml. Many companies that focus on OCaml ecosystem and tooling are located in France. I know of at least OCamlPRO and Tarides. Who is the consumer of those tooling efforts? The crypto boom gave money to the crypto-currency Tezos which is being developed by several players including a French company called Nomadic Labs. Many people there come from the French academy. Another big player in the OCaml world is Jane Street which is a trading firm in the US. Facebook and Bloomberg also use OCaml, as well as a few other companies.

## Libraries and ecosystem

If Haskell is a niche language, then OCaml is a super-niche language. The OCaml community is much smaller. Where Haskell is doing more-or-less fine with libraries, OCaml has significantly less to propose.

There are some nice libraries in OCaml, but in many areas the situation is not perfect. For example, you would expect a modern Unicode-aware string type in your language of choice. While Haskell has Text, OCaml mostly uses its built-in string type, which is simply an array of bytes. There is the uutf library, but it is essentially just byte-by-byte encoder/decoder. Regardless if its technical merits, the story about Unicode in OCaml is incomplete.

When discussing the issue with OCaml programmers, I was told to read how the library works, or to introduce implicit invariants (I’ll validate all my strings on the boundaries of my system), or to define Unicode-aware string type on per-project basis. It is important to understand that all of this is not good enough:

• To use a Unicode-aware string type one should not need to read a wall of text, unless you need to hack on lower level or to work on the Unicode library itself.

• To use a Unicode-aware string type one should not need to re-define it every time.

• The fact that we have a valid Unicode text must be reflected on the type level and the same type must be used by most libraries in the ecosystem. If I have a value of this type, the string inside must be correct Unicode text. It is a basic way to leverage the type system.

• There should be a vocabulary of operations for that Unicode type.

Another annoying fact is that for OCaml there is no place where to lookup library documentation online. You have to build it locally.

In general, I’d compare the library situation to that of Common Lisp—individual projects may be high quality, but the integration between libraries and coverage are lacking.

## Tooling

Haskell’s tooling is far from perfect. Most people just use Emacs, Vim, or another popular text editor. There are a few ways to get on-the-fly error reporting as you edit your program. My preferred choice is ghcid, which is just a terminal application that I run in a different window. There are solutions which can make the errors appear in your editor, but ghcid is simple and works for any project with any setup as long as you can start a REPL, which means that I never have a problem with it. Granted, it is a bit hardcore for users who are used to normal IDEs. There are two package managers: Cabal and Stack. Both are usable. The older one, Cabal, improved a lot in the recent years. One can also build Haskell code with Nix or Bazel, and many people do.

For OCaml, you’ll also need to use a text editor instead of a specialized IDE. Emacs with Merlin is probably your best bet. I noticed that Merlin doesn’t always predict correct types for expressions, for example it frequently confuses bytes and string. Still it’s better than nothing. The package manager is opam and the most popular build system is dune. The combo works if you are a hacker and you know how to use it. Otherwise it can be rough. To get a package set which is known to be good and is under your control, you’d need to setup your own opam server. You do not need to do it with Haskell. Nix support for OCaml doesn’t look as mature as for Haskell, so I wouldn’t use it unless I’m working for a big enough company that could invest in Nix support for OCaml.

Overall, Haskell is doing better with respect to tooling. With Stack you can build a project with a single command without knowing much about the infrastructure. A full-featured IDE is still but a dream for the users of both languages.

## Purity

Now to the features of the languages themselves. Haskell is pure and lazy, that’s a big game changer. Purity is imposed by the type system and it is one of the main features that gives Haskell its unique feel. Once you approach language design this bravely, doing away with the old ways of writing programs, you get a very different language in the end. I think we discovered a lot by building our programs around purity.

OCaml is impure and strict. Many people will praise strictness because of more predictable performance, but I’ll save the “lazy vs strict” discussion for another post. Because OCaml is impure, you can do effects anywhere you want. One consequence of this is that monads, although not unknown of in the OCaml world, are less popular and the support for monads is way worse. There is no do-notation and no proven ways to combine monadic layers, athough it looks like it’s about to change. You’ll probably also have to have as many bind operators as you have monads in your program—there is no ad-hoc polymorphism like in Haskell with type classes.

It is no coincidence that OCaml people are more practical. They won’t come after you with a baseball bat in the middle of the night because you used a partial function. While searching for the truth is such a big thing in Haskell where every self-respecting programmer will write his/her own effect system based on the latest and coolest papers, OCaml folks just stay cool and write their amazing unikernel operating systems which work just fine.

## Error messages

Haskell is well-known for its long and incomprehensible compile errors. I can’t say that OCaml errors are more readable. I think there are fewer things that may go wrong, but once they do, it can be very hard to figure out what is going on. The fact that OCaml has polymorphic variants and so can “grow” types according to the code you write doesn’t help. I’ve seen very long, barely readable declarations of the compiler’s agony. For example, if type of a module in your .ml file is different from its .mli declaration you’ll have to find the differences yourself looking at full module signatures of both. This all could be presented just a bit nicer to the user.

## Named and optional arguments

I’ve come to the conclusion that optional and named arguments that OCaml provides are not a good idea. Granted, this is just my opinion. Here is why:

• Named arguments. There are two problems that named arguments solve in OCaml:

• When you have several arguments of the same type and you do not want to mix them up. In that case naming the arguments helps, but it is not the best fix. The problem is that inside of a function which takes named arguments you may need to pass those arguments down to other functions. Nothing prevents you from mixing things up here, because all the arguments still have the same type. The solution is simple: the types of different arguments should be different. This what Haskell achieves through newtypes. Once you have different types for username and password you cannot mix them up and you’ll be forced to pass them in the right order—no mistake is possible.

Note that consistent naming with labeled arguments in OCaml makes it very unlikely that you’ll mix up two arguments of the same type. Yet, it is only true as long as you pass the arguments unchanged and follow a consistent naming scheme. There is a difference between correct because it is easier this way and correct because it is the only way it compiles.

• When you have many arguments and you don’t want to remember their exact order. The problem here is that you do not really need to pass many arguments to a function. Perhaps 5 or 6 is the maximum. If you have more (and OCaml programs can easily have 13 or more), think how to organize your API better. Maybe you could use a reader monad or something else to share context. Maybe you need to group several things in a record which represents something in your domain. Or perhaps your function is doing too much at the same time and you need to decompose it. Common Lisp also has named arguments, and this is why, in part, standard functions in Common Lisp tend to do a lot more than in Haskell where each function does just one thing.

• Optional arguments can be replaced by explicit Maybe or option wrappers. This is a bit verbose, but there is not much to gain from that little extra brevity (as pointed out above, we do not want too many arguments). On the other hand, optional arguments do not play nicely with currying. The last argument of a function cannot be optional, this is why some OCaml functions take a dummy unit () as their last argument, which is not nice.

## Module system

The OCaml module system is refreshing and interesting, but it has its own flaws. For example, we have to endlessly repeat the type definitions and signatures in .ml and .mli files. There are some tricks to avoid duplication but they can be a real pain sometimes. I mention this because even though I can understand that it is nice to have your API in a single file, I found the workflow a lot less ergonomic than in Haskell. You end up in a web of modules and endless repetitions of signatures. Many times when I changed or moved a function I’d have to read through lengthy module signatures to find where to perform the adjustments to make it compile again. You do it by looking at the entire signature of a module—often several hundreds of lines.

One could say that .mli files can be generated automatically (which I do not recommend, it’s just a possible objection), but then why make it part of the source code that is supposed to be written by the programmer?

## Conclusion

It might appear that in my opinion Haskell is better at everything, but it is not true. The main advantage of OCaml is that its compiler is way simpler and produces code with more predictable performance. Some of the consequences are:

• It is easier to port it to new platforms. OCaml can compile to portable byte code and to machine code as well as to JavaScript. The story with running OCaml in browser is way better that for Haskell.

• OCaml is a better fit for things like unikernel operating systems, see Mirage OS.

Haskell is an alien language, or the most widely used language of the truly weird ones. It is ahead of its time. OCaml feels somewhat more conventional, like something caught in the middle of the mutation from conventional imperative language to something Haskell-like. In the end, both languages are great and have rather unique and interesting features.