7 OCaml Gotchas

I’ve been writing OCaml for about 1 year (check my previous post 8 months of OCaml after 8 years of Haskell).

I enjoy OCaml. But as any other programming language, OCaml has its quirks. That’s fine, you can enjoy imperfect things too. But it could be useful to learn about potential surprising behaviours.

In this blog post, I’m highlighting 7 OCaml gotchas. Some of them might be obvious to experienced OCamlers. But I hope everyone can learn something new or at least enjoy reading!

Let’s start.

1. Structural vs Physical equality

Being bitten by different types of equalities in a language like JavaScript, you exhale with relief when you learn that in OCaml you can easily compare numbers with == and it doesn’t allow you to compare values of different types 😮‍💨

utop # 255 == 255 ;;
- : bool = true

utop # 0 == false ;;
Error: This expression has type bool but an expression was expected of type int

However, quite soon, with horror, you realise that == doesn’t work with strings!

utop # "OCaml" == "OCaml" ;;
- : bool = false

Or with lists:

utop # [1; 2; 3] == [1; 2; 3] ;;
- : bool = false

Or with pairs

utop # (true, 1) == (true, 1) ;;
- : bool = false

Or with optionals:

utop # Some 10 == Some 10 ;;
- : bool = false

Or literally with anything else!

Well, the thing is, OCaml has two equalities:

• =: structural, actually compares values
• ==: physical, compares pointers to values

As well as two inequalities, <> and !=. To figure out which one is physical and which is structural is left as an exercise for the reader.

So, to actually check values for equality, use =:

utop # "I Love OCaml" = "I Love OCaml" ;;
- : bool = true

And for the love of god, configure your linter to warn on usages of ==. How many bugs has it caused…

2. Nested match-with

Consider the following types:

type reason =
  | Waiting
  | Validating

type status =
  | Pending of reason
  | Cancelled
  | Done

Let’s write a function to pattern match on a value of type status and convert the value to a string. We can use the match-with syntax in OCaml for this.

However, the following code doesn’t compile!

let show_status status =
  match status with
  | Pending reason ->
    match reason with
    | Waiting -> "Pending: Waiting"
    | Validating -> "Pending: Validating"
  | Cancelled -> "Cancelled"
  | Done -> "Done"

The compilation error is:

File "lib/example.ml", line 16, characters 4-13:
16 |   | Cancelled -> "Cancelled"
         ^^^^^^^^^
Error: This variant pattern is expected to have type reason
       There is no constructor Cancelled within type reason

The explanation is that OCaml is not a layout-sensitive language 🙅

When matching on reason, the compiler thinks that the | Cancelled -> ... case is the next pattern, hence the error.

I know three fixes:

1. Put () around the nested match explicitly

let show_status status =
  match status with
  | Pending reason ->
    (match reason with
    | Waiting -> "Pending: Waiting"
    | Validating -> "Pending: Validating")
  | Cancelled -> "Cancelled"
  | Done -> "Done"

2. Move the only nested match-with to the end:

let show_status status =
  match status with
  | Cancelled -> "Cancelled"
  | Done -> "Done"
  | Pending reason ->
    match reason with
    | Waiting -> "Pending: Waiting"
    | Validating -> "Pending: Validating"

3. Extract nested match-with into a separate function

let show_reason reason =
  match reason with
  | Waiting -> "Pending: Waiting"
  | Validating -> "Pending: Validating"

let show_status status =
  match status with
  | Pending reason -> show_reason reason
  | Cancelled -> "Cancelled"
  | Done -> "Done"

3. Labelled and Optional Arguments

OCaml has labelled (aka named) and optional arguments.

However, if your function uses both labelled and optional arguments without positional arguments, you get a compiler error!

The following code implements a function that generates all numbers between the given two with an optional step:

let range ?(step = 1) ~from ~until =
  let rec loop i =
    if i > until
      then []
      else i :: loop (i + step)
  in
  loop from

Unfortunately, it doesn’t compile!

File "lib/example.ml", line 21, characters 12-20:
21 | let range ?(step = 1) ~from ~until =
                 ^^^^^^^^
Error (warning 16 [unerasable-optional-argument]):
this optional argument cannot be erased.

The explanation is that you can specify both labelled and optional arguments in any order (you can mix and match):

range ~step:2 ~from:10 ~until:20  (* this is valid *)
range ~from:10 ~until:20 ~step:2  (* also valid! *)

So when you call the range function like this:

range ~from:10 ~until:20

OCaml doesn’t know whether you want to apply the default value of step or whether you want to have a partially applied range with only the default argument missing!

One of the solutions in this case is to add a positional argument of type unit at the end of the function, like this:

let range ?(step = 1) ~from ~until () =
  let rec loop i =
    if i > until
      then []
      else i :: loop (i + step)
  in
  loop from

Alternatively, if it makes sense, you can convert one or more labelled arguments to positional to avoid adding an extra unit.

4. Type inference doesn’t work well: Part 1

OCaml has type inference and it works even if you define your own custom types. Usually, it works pretty well.

Like in the example below, when we have a record type but we don’t write explicit type annotations, OCaml is smart enough to figure out the types:

type book =
  { author: string;
    title: string;
    words: int;
  }

let is_novel book =
  book.words >= 50000

The OCaml compiler can easily infer the type of is_novel as

val is_novel : book -> bool

However, if you move the type definition into a separate module, OCaml gives up immediately:

(* --- book.ml --- *)
type book =
  { author: string;
    title: string;
    words: int;
  }

(* --- example.ml --- *)
let is_novel book =
  book.words >= 50000

The error message is:

File "lib/example.ml", line 31, characters 7-12:
31 |   book.words >= 50000
            ^^^^^
Error: Unbound record field words

On one hand, it makes sense. Trying to guess the correct type across all possible modules and dependencies can decrease the compilation speed and introduce surprising behaviour.

However, this can be quite annoying when dealing with lots of types.

One solution is to specify the type explicitly in the inline type signature:

let is_novel (book : Book.book) =
  book.words >= 50000

Alternatively, you can use the local open syntax:

let is_novel book =
  Book.(book.words) >= 50000

5. Type Inference doesn’t work well: Part 2

You want to write a function that creates a list by replicating the same element n times.

The implementation is straightforward:

let replicate n x = List.init n (fun _ -> x)

This function works and OCaml correctly infers the polymorphic type of replicate:

val replicate : int -> 'a -> 'a list

Now, let’s say we replicate numbers five times specifically a lot, and we want to create a helper function by partially applying replicate to 5 (honestly, it’s easier to write the code than to explain it in English):

let replicate_5 = replicate 5

This function is partially applied only to the number, so you’d still expect it to be polymorphic, right? Oh, boy…

Unfortunately, if you use replicate_5 two times with different types, the OCaml compiler is not happy:

let two_lists =
  let five_bools = replicate_5 true in
  let five_ints = replicate_5 21 in
  (five_bools, five_ints)

The error message is:

File "lib/example.ml", line 42, characters 30-32:
42 |   let five_ints = replicate_5 21 in
                                   ^^
Error: This expression has type int but an expression was expected of type
         bool

You won’t believe what is the fix the problem.

The fix is to avoid partial application for polymorphic functions:

let replicate_5 x = replicate 5 x

Unfortunately, I know why it’s done this way. OCaml has valid reasons for this behaviour, believe me (you can read on Weak polymorphism). Still, it makes me a bit annoyed.

6. Implicit variable quantification

I want to write a function that takes an argument and returns it without changes. Again, the implementation is pretty simple:

let id x = x

This function doesn’t do anything specific, and OCaml correctly infers the polymorphic type:

val id : 'a -> 'a

I can write this function slightly differently by using an anonymous function:

let id = fun x -> x

And, if I want, I can even specify the inline type signature for the entire function

let id : 'a -> 'a = fun x -> x

The example may look artificial, but sometimes I don’t want to bother with creating a separate .mli file, and I want to have type signatures written explicitly

What I can also do, is completely ignore the type signature and write any nonsense in my implementation:

let id : 'a -> 'a = fun _ -> 123

And the compiler error will be.. Or, wait, there’s no error this time. OCaml is perfectly fine with this code 🥲

Turns out, if I really want to enforce the fact that the alpha 'a indeed stands for a polymorphic variable, I need to introduce explicit quantification like this:

let id : 'a . 'a -> 'a = fun x -> x

And with this, I can no longer write nonsense.

7. Right-to-left order of execution

If you want to write a function that takes two actions and runs them sequentially, like this one:

let (>>) action1 action2 = action1; action2

And then you want to use it:

let run_example () =
  print_endline "Hello, " >> print_endline "World"

You’ll be surprised by the actual behaviour:

utop # run_example () ;;
World
Hello,
- : unit = ()

Apparently, OCaml evaluates arguments from right to left, so the second argument is evaluated first.

In fact, the order of evaluation is not even guaranteed.

The only solution is to avoid relying on this behaviour. Make your functions accept arguments of type unit -> ... or Lazy.t, so the functions can control the execution order of their arguments.

Conclusion

That’s all! If you found anything surprising in OCaml, feel free to share!

As I mentioned, every language has some pitfalls. If you don’t see them in your favourite language, you either don’t know it well enough or nobody uses this language anymore.

Human brains are really good at focusing on bad things. However, I wrote this blog post not to say that OCaml is bad but rather to reduce the frustration when experiencing something surprising for the first time 😌

If you liked this blog post, consider supporting my work on GitHub Sponsors, or following me on the Internet:

• GitHub Sponsors: @chshersh
• YouTube: @chshersh
• 𝕏: @chshersh
• BlueSky: @chshersh.com
• Mastodon: functional.cafe@chshersh
• Twitch: @chshersh