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
utoptoplevel. It’s more full-featured than the
ocamltoplevel 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
int optionwritten 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 listis 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.
They indicate variance. See this blog post.
Why are OCaml’s objects considered slow?
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.tuses a tree of variants to store its data. Suppose I store
1, 2, 3 and 4in map
a, in that order. Now suppose I store
4, 3, 2, 1in 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.tcan 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
findwill 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.
let: When a
letexpression begins in the final branch of an
ifexpression, a semicolon will not terminate the
let; instead the
letexpression will include the code that comes after the semicolon. One might expect that the semicolon ends the
ifexpression, and that the code following it will execute after the entire
ifexpression, but instead that code is part of the
ifexpression. 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
ifwith the embedded
let, wrap the
letexpression in parentheses or
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
sprintf, etc. directives such as
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
Both make the values and types in
My_moduleavailable without having to write “My_module.” in front of them.
open My_modulewill trigger a warning if
My_moduleoverrides existing definitions.
open!suppresses this warning. See Overriding in open statements from Chapter 8 Language extensions in the OCaml manual.
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
Containersthat 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!”.
There are few other interesting OCaml FAQs, that you might want to check out: