A modular, clean and powerful extension of the OCaml standard library

A modular, clean and powerful extension of the OCaml standard library.

(Jump to the current API documentation)

Containers is an extension of OCaml's standard library (under BSD license)
focused on data structures, combinators and iterators, without dependencies on
unix, str or num. Every module is independent and is prefixed with 'CC' in the
global namespace. Some modules extend the stdlib (e.g. CCList provides safe
map/fold_right/append, and additional functions on lists).
Alternatively, open Containers will bring enhanced versions of the standard
modules into scope.

Quick Summary

Containers is:

  • A usable, reasonably well-designed library that extends OCaml's standard
    library (in 'src/core/', packaged under containers in ocamlfind. Modules
    are totally independent and are prefixed with CC (for "containers-core"
    or "companion-cube" because I'm a megalomaniac). This part should be
    usable and should work. For instance, CCList contains functions and
    lists including safe versions of map and append. It also
    provides a drop-in replacement to the standard library, in the module
    Containers (intended to be opened, replaces some stdlib modules
    with extended ones).

  • Several small additional libraries that complement it:

    • with additional data structures that don't have an
      equivalent in the standard library;

    • containers.iter with list-like and tree-like iterators;

  • Utilities around the unix library in containers.unix (mainly to spawn
    sub-processes easily and deal with resources safely)

  • A lightweight S-expression printer and streaming parser in containers.sexp

  • A library for threaded programming in containers.thread,
    including a blocking queue, semaphores, an extension of Mutex, and
    thread-pool based futures.

Some of the modules have been moved to their own repository (e.g. sequence (now iter),
gen, qcheck) and are on opam for great fun and profit.

Migration Guide

To 2.0

  • The type system should detect issues related to print renamed into pp easily.
    If you are lucky, a call to sed -i 's/print/pp/g' on the concerned files
    might help rename all the calls

  • many optional arguments have become mandatory, because their default value
    would be a polymorphic "magic" operator such as (=) or (>=).
    Now these have to be specified explicitly, but during the transition
    you can use Pervasives.(=) and Pervasives.(>=) as explicit arguments.

  • if your code contains open Containers, the biggest hurdle you face
    might be that operators have become monomorphic by default.
    We believe this is a useful change that prevents many subtle bugs.
    However, during migration and until you use proper combinators for
    equality (CCEqual), comparison (CCOrd), and hashing (CCHash),
    you might want to add open Pervasives just after the open Containers.
    See the section on monomorphic operators for more details.

Monomorphic operators: why, and how?

Why shadow polymorphic operators by default?

To quote @bluddy in #196:

The main problem with polymorphic comparison is that many data structures will
give one result for structural comparison, and a different result for semantic
comparison. The classic example is comparing maps. If you have a list of maps
and try to use comparison to sort them, you'll get the wrong result: multiple
map structures can represent the same semantic mapping from key to value, and
comparing them in terms of structure is simply wrong. A far more pernicious bug
occurs with hashtables. Identical hashtables will seem to be identical for a
while, as before they've had a key clash, the outer array is likely to be the
same. Once you get a key clash though, you start getting lists inside the
arrays (or maps inside the arrays if you try to make a smarter hashtable) and
that will cause comparison errors ie. identical hashtables will be seen as
different or vice versa.

Every time you use a polymorphic comparison where you're using a data type
where structural comparison != semantic comparison, it's a bug. And ever time
you use polymorphic comparison where the type of data being compared may vary
(e.g. it's an int now, but it may be a map later), you're planting a bug for
the future.

See also:



Sometimes polymorphic operators still make sense!

If you just want to use polymorphic operators, it's fine! You can access them
easily by using Pervasives.(=), Pervasives.max, etc.

When migrating a module, you can add open Pervasives on top of it to restore
the default behavior. It is, however, recommended to export an equal function
(and compare, and hash) for all the public types, even if their internal
definition is just the corresponding polymorphic operator.
This way, other modules can refer to Foo.equal and will not have to be
updated the day Foo.equal is no longer just polymorphic equality.
Another bonus is that Hashtbl.Make(Foo) or Map.Make(Foo) will just work™.

Further discussions

See issues

Debugging with ocamldebug

To print values with types defined in containers in the bytecode debugger,
you first have to load the appropriate bytecode archives. After starting a
session, e.g. ocamldebug your_program.bc,

# load_printer containers_monomorphic.cma
# load_printer containers.cma

For these archives to be found, you may have to run the program first. Now
printing functions that have the appropriate type Format.formatter -> 'a -> unit can be installed. For example,

# install_printer Containers.Int.pp

However, printer combinators are not easily handled by ocamldebug. For
instance # install_printer Containers.(List.pp Int.pp) will not work out of
the box. You can make this work by writing a short module which defines
ready-made combined printing functions, and loading that in ocamldebug. For

module M = struct
	let pp_int_list = Containers.(List.pp Int.pp)

loaded via # load_printer m.cmo and installed as # install_printer M.pp_int_list.

Change Log

See this file.

Finding help


You might start with the tutorial to get a picture of how to use the library.

You can either build and install the library (see build), or just copy
files to your own project. The last solution has the benefits that you
don't have additional dependencies nor build complications (and it may enable
more inlining). Since modules have a friendly license and are mostly
independent, both options are easy.

In a toplevel, using ocamlfind:

# #use "topfind";;
# #require "containers";;
# CCList.flat_map;;
- : ('a -> 'b list) -> 'a list -> 'b list = <fun>
# open Containers;;  (* optional *)
# List.flat_map ;;
- : ('a -> 'b list) -> 'a list -> 'b list = <fun>

If you have comments, requests, or bugfixes, please share them! :-)


This code is free, under the BSD license.


See the documentation
and the tutorial below for a gentle introduction.


In general, see for the API documentation.

Some examples can be found there,
per-version doc there.


You will need OCaml >= 4.02.0.

Via opam

The preferred way to install is through opam.

$ opam install containers

From Sources

You need dune (formerly jbuilder).

$ make

To build and run tests (requires oUnit and qtest):

$ opam install oUnit qtest
$ make test

To build the small benchmarking suite (requires benchmark):

$ opam install benchmark batteries
$ make bench
$ ./benchs/


PRs on github are very welcome (patches by email too, if you prefer so).

First-Time Contributors

Assuming your are in a clone of the repository:

  1. Some dependencies are required, you'll need
    opam install benchmark qcheck qtest iter.

  2. run make devel to enable everything (including tests).

  3. make your changes, commit, push, and open a PR.

  4. use make test without moderation! It must pass before a PR
    is merged. There are around 1150 tests right now, and new
    features should come with their own tests.

If you feel like writing new tests, that is totally worth a PR
(and my gratefulness).

General Guidelines

A few guidelines to follow the philosophy of containers:

  • no dependencies between basic modules (even just for signatures);

  • add @since tags for new functions;

  • add tests if possible (using qtest).
    There are numerous inline tests already,
    to see what it looks like search for comments starting with (*$
    in source files.

For Total Beginners

Thanks for wanting to contribute!
To contribute a change, here are the steps (roughly):

  1. click "fork" on on the top right of the page. This will create a copy of the repository on your own github account.

  2. click the big green "clone or download" button, with "SSH". Copy the URL (which should look like<your username>/ocaml-containers.git) into a terminal to enter the command:

    $ git clone<your username>/ocaml-containers.git
  3. then, cd into the newly created directory.

  4. make the changes you want. See <#first-time-contributors> for
    more details about what to do in particular.

  5. use git add and git commit to commit these changes.

  6. git push origin master to push the new change(s) onto your
    copy of the repository

  7. on github, open a "pull request" (PR). Et voilà !


This tutorial contains a few examples to illustrate the features and
usage of containers.

We assume containers is installed and that
the library is loaded, e.g. with:

# #require "containers";;
# Format.set_margin 50;; (* for readability here *)
- : unit = ()


We will start with a few list helpers, then look at other parts of
the library, including printers, maps, etc.

# (|>) ;;  (* quick reminder of this awesome standard operator *)
- : 'a -> ('a -> 'b) -> 'b = <fun>

# open CCList.Infix;;

# let l = 1 -- 100;;
val l : int list =
  [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21;
   22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39;
   40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57;
   58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75;
   76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93;
   94; 95; 96; 97; 98; 99; 100]

# l
  |> CCList.filter_map
     (fun x-> if x mod 3=0 then Some (float x) else None)
  |> CCList.take 5 ;;
- : float list = [3.; 6.; 9.; 12.; 15.]

# let l2 = l |> CCList.take_while (fun x -> x<10) ;;
val l2 : int list = [1; 2; 3; 4; 5; 6; 7; 8; 9]
(* an extension of Map.Make, compatible with Map.Make(CCInt) *)
module IntMap = CCMap.Make(CCInt)
# (* conversions using the "iter" type, fast iterators that are
   pervasively used in containers. Combinators can be found
   in the opam library "sequence". *)
  let map : string IntMap.t =
    |> (fun x -> x, string_of_int x)
    |> CCList.to_seq
    |> IntMap.of_seq;;
val map : string IntMap.t = <abstr>

# CCList.to_seq ;; (* check the type *)
- : 'a list -> 'a CCList.sequence = <fun>
# IntMap.of_seq ;;
- : (int * 'a) CCMap.sequence -> 'a IntMap.t = <fun>

# (* we can print, too *)
  Format.printf "@[<2>map =@ @[<hov>%a@]@]@."
    (IntMap.pp CCFormat.string_quoted)
map =
  1->"1", 2->"2", 3->"3", 4->"4", 5->"5", 
  6->"6", 7->"7", 8->"8", 9->"9"
- : unit = ()

# (* options are good *)
  IntMap.get 3 map |> (fun s->s ^ s);;
- : string option = Some "33"

New types: CCVector, CCHeap, CCResult

Containers also contains (!) a few datatypes that are not from the standard
library but that are useful in a lot of situations:

  • CCVector:
    A resizable array, with a mutability parameter. A value of type
    ('a, CCVector.t is an immutable vector of values of type 'a,
    whereas a ('a, CCVector.t is a mutable vector that
    can be modified. This way, vectors can be used in a quite functional
    way, using operations such as map or flat_map, or in a more
    imperative way.

  • CCHeap:
    A priority queue (currently, leftist heaps) functorized over
    a module sig val t val leq : t -> t -> bool that provides a type t
    and a partial order leq on t.

  • CCResult
    An error type for making error handling more explicit (an error monad,
    really, if you're not afraid of the "M"-word).
    Subsumes and replaces the old CCError.
    It uses the new result type from the standard library (or from
    the retrocompatibility package on opam) and provides
    many combinators for dealing with result.

Now for a few examples:

# (* create a new empty vector. It is mutable, for otherwise it would
   not be very useful. *)
- : unit -> ('a, CCVector.t = <fun>

# (* init, similar to Array.init, can be used to produce a
   vector that is mutable OR immutable (see the 'mut parameter?) *)
  CCVector.init ;;
- : int -> (int -> 'a) -> ('a, 'mut) CCVector.t = <fun>
# (* use the infix (--) operator for creating a range. Notice
   that v is a vector of integer but its mutability is not
   decided yet. *)
  let v = CCVector.(1 -- 10);;
val v : (int, '_a) CCVector.t = <abstr>
# Format.printf "v = @[%a@]@." (CCVector.pp CCInt.pp) v;;
v = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
- : unit = ()
# CCVector.push v 42;;
- : unit = ()

# v;; (* now v is a mutable vector *)
- : (int, CCVector.t = <abstr>

# (* functional combinators! *)
  let v2 : _ CCVector.ro_vector = v
  |> (fun x-> x+1)
  |> CCVector.filter (fun x-> x mod 2=0)
  |> CCVector.rev ;;
val v2 : int CCVector.ro_vector = <abstr>

# Format.printf "v2 = @[%a@]@." (CCVector.pp CCInt.pp) v2;;
v2 = 10, 8, 6, 4, 2
- : unit = ()
(* let's transfer to a heap *)
module IntHeap = CCHeap.Make(struct type t = int let leq = (<=) end);;
# let h = v2 |> CCVector.to_seq |> IntHeap.of_seq ;;
val h : IntHeap.t = <abstr>

# (* We can print the content of h
  (printing is not necessarily in order, though) *)
  Format.printf "h = [@[%a@]]@." (IntHeap.pp CCInt.pp) h;;
h = [2,4,6,8,10]
- : unit = ()

# (* we can remove the first element, which also returns a new heap
   that does not contain it — CCHeap is a functional data structure *)
  IntHeap.take h;;
- : (IntHeap.t * int) option = Some (<abstr>, 2)

# let h', x = IntHeap.take_exn h ;;
val h' : IntHeap.t = <abstr>
val x : int = 2

# IntHeap.to_list h' ;; (* see, 2 is removed *)
- : int list = [4; 6; 8; 10]

IO helpers

The core library contains a module called CCIO that provides useful
functions for reading and writing files. It provides functions that
make resource handling easy, following
the pattern with_resource : resource -> (access -> 'a) -> 'a where
the type access is a temporary handle to the resource (e.g.,
imagine resource is a file name and access a file descriptor).
Calling with_resource r f will access r, give the result to f,
compute the result of f and, whether f succeeds or raises an
error, it will free the resource.

Consider for instance:

# CCIO.with_out "./foobar"
    (fun out_channel ->
      CCIO.write_lines_l out_channel ["hello"; "world"]);;
- : unit = ()

This just opened the file '/tmp/foobar', creating it if it didn't exist,
and wrote two lines in it. We did not have to close the file descriptor
because with_out took care of it. By the way, the type signatures are:

val with_out :
  ?mode:int -> ?flags:open_flag list ->
  string -> (out_channel -> 'a) -> 'a

val write_lines_l : out_channel -> string list -> unit

So we see the pattern for with_out (which opens a function in write
mode and gives its functional argument the corresponding file descriptor).

NOTE: you should never let the resource escape the
scope of the with_resource call, because it will not be valid outside.
OCaml's type system doesn't make it easy to forbid that so we rely
on convention here (it would be possible, but cumbersome, using
a record with an explicitly quantified function type).

Now we can read the file again:

# let lines : string list = CCIO.with_in "./foobar" CCIO.read_lines_l ;;
val lines : string list = ["hello"; "world"]

There are some other functions in CCIO that return generators
instead of lists. The type of generators in containers
is type 'a gen = unit -> 'a option (combinators can be
found in the opam library called "gen"). A generator is to be called
to obtain successive values, until it returns None (which means it
has been exhausted). In particular, python users might recognize
the function

# CCIO.File.walk ;;
- : string -> walk_item gen = <fun>;;

where type walk_item = [ ``Dir | ``File ] * string is a path
paired with a flag distinguishing files from directories.

To go further:

There is also a sub-library called, with lots of
more specialized data-structures.
The documentation contains the API for all the modules; they also provide
interface to iter and, as the rest of containers, minimize
dependencies over other modules. To use you need to link it,
either in your build system or by #require;;

A quick example based on purely functional double-ended queues:

# #require "";;
# #install_printer CCFQueue.pp;;  (* better printing of queues! *)

# let q = CCFQueue.of_list [2;3;4] ;;
val q : int CCFQueue.t = queue {2; 3; 4}

# let q2 = q |> CCFQueue.cons 1 |> CCFQueue.cons 0 ;;
val q2 : int CCFQueue.t = queue {0; 1; 2; 3; 4}

# (* remove first element *)
  CCFQueue.take_front q2;;
- : (int * int CCFQueue.t) option = Some (0, queue {1; 2; 3; 4})

# (* q was not changed *)
  CCFQueue.take_front q;;
- : (int * int CCFQueue.t) option = Some (2, queue {3; 4})

# (* take works on both ends of the queue *)
  CCFQueue.take_back_l 2 q2;;
- : int CCFQueue.t * int list = (queue {0; 1; 2}, [3; 4])

Common Type Definitions

Some structural types are used throughout the library:

  • gen: 'a gen = unit -> 'a option is an iterator type. Many combinators
    are defined in the opam library gen

  • sequence: 'a sequence = (unit -> 'a) -> unit is also an iterator type.
    It is easier to define on data structures than gen, but it a bit less
    powerful. The opam library iter
    can be used to consume and produce values of this type. It was renamed
    from 'a sequence to 'a iter to distinguish it better from Core.Sequence
    and the standard seq.

  • error: 'a or_error = ('a, string) result = Error of string | Ok of 'a
    using the standard result type, supported in CCResult.

  • klist: 'a klist = unit -> [Nil | Cons of 'a * 'a klist] is a lazy list
    without memoization, used as a persistent iterator. The reference
    module is CCKList (in containers.iter).

  • printer: 'a printer = Format.formatter -> 'a -> unit is a pretty-printer
    to be used with the standard module Format. In particular, in many cases,
    "foo: %a" Foo.print foo will type-check.

Extended Documentation

See the extended documentation for more examples.

HOWTO (for contributors)

Make a release

Beforehand, check grep deprecated -r src to see whether some functions
can be removed.

  • make all

  • update version in containers.opam

  • make update_next_tag (to update @since comments; be careful not to change symlinks)

  • check status of modules ({b status: foo}) and update if required;
    removed deprecated functions, etc.

  • make unlabel to see if labelled interfaces are up to date (requires compiler-libs)

  • update (see its end to find the right git command)

  • commit the changes

  • make test doc

  • export VERSION=<tag here>; git tag -f $VERSION; git push origin :$VERSION; git push origin $VERSION

  • new opam package: opam publish<tag>.tar.gz

  • re-generate doc: make doc and put it into gh-pages

List Authors

git log --format='%aN' | sort -u
12 Nov 2019
with-test & < "4.11"
>= "4.02.0" & < "5.0"
with-test & >= "0.9" & < "0.14"
< "1.5"
Reverse Dependencies