View on GitHub


Documenting everything about OCaml


Frequently asked questions


  • What is a “toplevel”?

    A/the “toplevel” is an OCaml term for a program that provides a prompt at which you can enter OCaml expressions and see what happens when they are evaluated. In some languages this is known as a “REPL”, a Read-Eval-Print loop. (It Reads what you enter, Evaluates it, and then Prints the result.) Many people who use an OCaml toplevel use the utop toplevel. It’s more full-featured than the ocaml toplevel that comes with an OCaml compiler. You can also evaluate OCaml expressions in an editor or IDE, for example. See the Code tools page.

  • Why are compound types such as float list and int option written with the “enclosing”, polymorphic type last?

    There’s no real reason, except that this is how it’s always been done in OCaml. It might be a bad idea to change the syntax of types in a language that puts so much emphasis on types. (Standard ML uses the same convention. Since it and OCaml both developed from an earlier ML, it’s likely that the convention predates OCaml.)

    However, the type syntax kind of makes sense if you think about English syntax: A “passenger ship” is a ship that carries passengers. An int list is a list that contains ints. (You could also write “list of integers” in English, but then you need to add the “of”.)

  • Why do OCaml’s constructors not work like Haskell’s, i.e. they can’t be curried?

    Originally, there was thought of doing them that way. The implementation was more complicated though, and OCaml’s module system makes it hard to deal with them. See the full answer here.

  • What does +'a and -'a mean?

    They indicate variance. See this blog post.

  • Why are OCaml’s objects considered slow?

    Objects in OCaml are structural, as opposed to most other static languages (C++, Java, C#), where they’re nominal. What does this mean? Well, nominal objects can only be created via classes. This means they can be compiled extremely efficiently using virtual tables. You can know ahead of time (during compilation) where each method is, which means you can have excellent performance using only a few pointer indirections. OCaml didn’t choose this path. OCaml objects don’t follow any class hierarchy and can be created willy-nilly. As a result, any method can be found on any object, and the only way to find a method is to look it up in a dictionary during runtime. This is similar to what dynamic languages (Python, Javascript, Ruby etc) do, and has similar performance implications. All of this doesn’t matter too much if you’re not working in high performance domains. GUIs, for example, are as slow as humans are. For code that needs to run as fast as possible, however, OCaml objects are a performance bottleneck.

OCaml Gotchas

Every language has its weak spots, or ‘gotchas’. This could be either a result of needing to maintain backwards compatibility, or of the specific design choices of the language. It’s good to know what the ‘gotchas’ are before you head in, so you can look out for them.

  • Polymorphic compare: OCaml’s comparison functions - =, <, > etc. use structural comparison by default. This means that while they work on any type, they don’t understand the meaning of the data, but instead rely on the layout of the data in memory. This is a problem, and most standard library replacements have disabled this feature by making standard polymorphic comparison work only on integers.

    To understand why this is a problem, let’s think about how a Map.t is organized. Map.t uses a tree of variants to store its data. Suppose I store 1, 2, 3 and 4 in map a, in that order. Now suppose I store 4, 3, 2, 1 in map b, in that order. Content-wise, the maps are the same. But do their backing trees look the same? No. The semantic meaning of the maps don’t match their structure, and in fact each logical Map.t can be backed by many possible structures in memory. This is a problem when comparing maps using the default comparisons.

    It’s an even worse problem when you have a higher-order function, such as find. find will work fine on a list of integers, because integers can only have one possible structural representation each. But it will often fail if you pass it a list of maps. Even worse, if you pass it a list of hashtables, which are backed by arrays mixed with buckets of lists, the arrays will match, but once in a while, the lists will not, causing subtle, quite horrible bugs.

    This is why polymorphic comparison should never be used except in places where you know the code will not be changed (which is hard to predict). Even if you know what you’re doing with your code today, someone will come and modify it someday and use your polymorphic comparison on a data structure that doesn’t support it.

  • Semicolons and if statements with let: When a let expression begins in the final branch of an if expression, a semicolon will not terminate the let; instead the let expression will include the code that comes after the semicolon. One might expect that the semicolon ends the if expression, and that the code following it will execute after the entire if expression, but instead that code is part of the if expression. This can introduce subtle bugs, although it is a consequence of the normal (and useful) scoping rule for let. To make the semicolon follow the entire if with the embedded let, wrap the let expression in parentheses or begin/end. For more, see if with semicolons.

  • Weak type variables: What is “the value restriction”? Why do some of my type variables start with underscore? What is the difference between a function with (for example) type 'a -> 'b, and one with type '_weak1 -> '_weak2? Why am I getting a compiler error like these? (It worked when I tried it out in utop!)

    Values do not match:
      val foo : '_weak1 -> '_weak1
    is not included in
      val foo : 'a -> 'a

    This means that your function (or any other value that gets this type) is not really polymorphic, and the compiler doesn’t have enough information to infer its concrete type, thus it represents the unknown parts of the type with so called “weak type variables”, the placeholders the need to be filled in by you. (The compiler will do it itself, if possible, but when you see the weak type in the error produced by the compiler, it means that it wasn’t possible, and your help is needed).

    The Weak Type Variables article describes in detail why they exist in OCaml and how to cope with them.


  • Printf directives: Where can I find a list of printf, sprintf, etc. directives such as %s, %b, %d, %f, etc.?

    See the Printf module documentation in the OCaml Manual.

  • Infix operators: Where can I find documentation on standard infix operators such as @@?

    For general documentation on operators, see the OCaml manual: Operators in the Expressions section, Index of values, or the Stdlib module documentation. For the Stdlib document, you’ll want to search the page for “operator” or for the specific operator you’re interested in.

    For operator precedence and associativity, see the Expressions section of the manual, and scroll down past the BNF syntax specification or search for “Construction or operator”. The Stdlib module also specifies precedence and associativity in the documentation for each operator.

    (Also possibly useful: Built-in operators and functions at OCaml for the Skeptical.)

  • ppx: What are these [%% ...], [@@ ...] expressions that I see in people’s code? What does it mean when there’s a percent sign in the middle of a name, as in let%lwt? What are extension points?

    See A Guide to PreProcessor eXtensions or A Tutorial to OCaml -ppx Language Extensions.

  • +’a, -‘a: Why are some type variables prefaced by “+” or “-“, as in type +'a t?

    These are called “variance annotations”. They’re used to constrain subtyping relations. See +’a and -‘a at the Jane Street Tech Blog.

  • open!: What’s the difference between open My_module and open! My_module?

    Both make the values and types in My_module available without having to write “My_module.” in front of them. open My_module will trigger a warning if My_module overrides existing definitions. open! suppresses this warning. See Overriding in open statements from Chapter 8 Language extensions in the OCaml manual.

    At present open! also suppresses another warning as well. This is a warning that no definition in the opened module is in fact used within open!’s scope. There is some disagreement about whether this behavior is desirable. Those who use open! for this purpose use it to open modules such as Core, Batteries, or Containers that shadow many built-in definitions, using such a module as a way of providing an alternative standard programming environment. See the discussion at this OCaml PR: Fix PR6638: “unused open” warning was incorrectly suppressed by “open!”.

Additional Resources

There are few other interesting OCaml FAQs, that you might want to check out: