package containers

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


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OCaml-containers 📦

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), and a small S-expression printer and parser that can be functorized over the representation of values.

  • Some sub-libraries with a specific focus each:

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

    • A bencode codec in containers.bencode. This is a tiny json-like serialization format that is extremely simple. It comes from bittorrent files.

    • A CBOR codec in containers.cbor. This is a compact binary serialization format.

    • The Strongly Connected Component algorithm, functorized, in containers.scc

  • A separate library containers-data with additional data structures that don't have an equivalent in the standard library, typically not as thoroughly maintained. This is now in its own package since 3.0.

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.

Containers-thread has been removed in favor of Moonpool.

Migration Guide

To 3.0

The changelog's breaking section contains a list of the breaking changes in this release.

  1. The biggest change is that some sub-libraries have been either turned into their own packages (containers-data), deleted (containers.iter),or merged elsewhere (containers.sexp). This means that if use these libraries you will have to edit your dune/_oasis/opam files.

  • if you use containers.sexp (i.e. the CCSexp module), it now lives in containers itself.

  • if you used anything in, you need to depend on the containers-data package now.

  1. Another large change is the removal (at last!) of functions deprecated in 2.8, related to the spread of Seq.t as the standard iterator type. Functions like CCVector.of_seq now operate on this standard Seq.t type, and old-time iteration based on iter is now named of_iter, to_iter, etc.

Here you need to change your code, possibly using search and replace. Thankfully, the typechecker should guide you.

  1. Array_slice and String.Sub have been removed to simplify the code and String more lightweight. There is no replacement at the moment. Please tell us if you need this to be turned into a sub-library.

  2. Renaming of some functions into more explicit/clear names. Examples:

  • CCVector.shrink is now CCVector.truncate

  • CCVector.remove is now CCVector.remove_unordered, to be contrasted with the new CCVector.remove_and_shift.

  • CCPair.map_fst and map_snd now transform a tuple into another tuple by modify the first (resp. second) element.

  1. All the collection pretty-printers now take their separator/start/stop optional arguments as unit printer (i.e. Format.formatter -> unit -> unit functions) rather than strings. This gives the caller better control over the formatting of lists, arrays, queues, tables, etc.

  2. Removal of many deprecated functions.

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 properly.

  • 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 Stdlib.(=) and Stdlib.(>=) 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 Stdlib 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 every 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 Stdlib.(=), Stdlib.max, etc.

When migrating a module, you can add open Stdlib 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 #196, #197

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 instance

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";;
# #require "containers-data";;
# 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 >= 4.03.0.

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 qcheck-core, gen, iter):

$ opam install qcheck-core
$ 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).

how to contribute (click to unfold)

List of authors

The list of contributors can be seen on github.

Alternatively, git authors from git-extras can be invoked from within the repo to list authors based on the git commits.

First-Time Contributors

Assuming your are in a clone of the repository:

  1. Some dependencies are required, you'll need opam install benchmark qcheck-core iter gen mdx uutf yojson.

  2. run make all 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 (see tests/ dir) 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.

an introduction to containers (click to unfold)

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>
# 10 |> succ;;
- : int = 11

# 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]

# (* transform a list, dropping some elements *)
  |> 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 "iter". *)
  let map : string IntMap.t =
    |> (fun x -> x, string_of_int x)
    |> CCList.to_iter
    |> IntMap.of_iter;;
val map : string IntMap.t = <abstr>

# CCList.to_iter (* check the type *);;
- : 'a list -> 'a CCList.iter = <fun>
# IntMap.of_iter ;;
- : (int * 'a) CCMap.iter -> '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, CCSexp, CCByte_buffer

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.

  • CCSexp and CCCanonical_sexp: functorized printer and parser for S-expressions, respectively as actual S-expressions (like sexplib) and as canonical binary-safe S-expressions (like csexp)

  • CCByte_buffer: a better version of the standard Buffer.t which cannot be extended and prevents access to its internal byte array. This type is designed for (blocking) IOs and to produce complex strings incrementally in an efficient way.

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_iter |> IntHeap.of_iter ;;
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 '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: containers-data

There is also a library called containers-data, 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 containers-data you need to link it, either in your build system or by #require containers-data;;

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

# #require "containers-data";;
# #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

  • iter: 'a iter = (unit -> 'a) -> unit is also an iterator type, formerly named sequence. 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.

  • 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.

  • 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

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