package res

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RES is a library containing resizable arrays, strings, and bitvectors.

Published: 26 Oct 2018


RES - Automatically Resizing Contiguous Memory for OCaml

What is RES?

This OCaml-library consists of a set of modules which implement automatically resizing (= reallocating) data structures that consume a contiguous part of memory. This allows appending and removing of elements to/from arrays (both boxed and unboxed), strings (buffers), bit strings and weak arrays while still maintaining fast constant-time access to elements.

There are also functors that allow the generation of similar modules which use different reallocation strategies.


  • Fast constant-time access to indexed elements (e.g. in arrays and strings) is often a prerequisite for short execution times of programs.

    Still, operations like adding and/or removing elements to/from the end of such data structures are often needed. Unfortunately, having both properties at the same time sometimes requires reallocating this contiguous part of memory.

    This module does not eliminate this problem, but hides the process of reallocation from the user, i.e. it happens automatically.

    Thus, the user is liberated from this bug-attracting (e.g. index errors) task.

  • This library allows the user to parameterize allocation strategies at runtime. This is an important feature, because it is impossible for any allocation algorithm to perform optimally without having knowledge about the user program.

    For example, the programmer might know that a consecutive series of operations will alternately add and remove large batches of elements. In such a case it would be wise to keep a high reserve of available slots in the data structure, because otherwise it will resize very often during this procedure which requires a significant amount of time.

    By raising a corresponding threshold in appropriate places at runtime, programmers can fine-tune the behavior of e.g. their buffers for optimal performance and set this parameter back later to save memory.

  • Because optimal reallocation strategies may be quite complex, it was also a design goal to have users supply their own ones (if required).

    By using functors users can parameterize these data structures with their own reallocation strategies, giving them even more control over how and when reallocations are triggered.

  • Users may want to add support for additional low-level implementations that require reallocations. In this case, too, it is fairly easy to create new modules by using functors.

  • The library implements a large interface of functions, all of which are completely independent of the reallocation strategy and the low-level implementation.

    All the interfaces of the corresponding low-level implementations of data structures (e.g. array, string) are fully supported and have been extended with further functionality. There is even a new buffer module which can be used in every context of the standard one.

  • OCaml makes a distinction between unboxed and boxed arrays. If the type of an array is float, the representation will be unboxed in cases in which the array is not used in a polymorphic context (native code only).

    To benefit from these much faster representations there are specialized versions of automatically resizing arrays in the distribution.


The API is fully documented and can be built as HTML using make doc. It is also available online.

The preparameterized modules (default strategy) and the functors for mapping strategy-implementations to this kind of modules are contained and documented in file lib/res.mli.

For examples of how to use the functors to implement new strategies and/or low-level representations, take a look at the implementation in lib/

Their function interface, however, is documented in files lib/ (for parameterized "low-level" types like e.g. normal arrays) and in lib/ (for non-parameterized "low-level" types like e.g. float arrays, strings (buffers), etc.).


It should be noted that it is possible to use the standard notation for accessing elements (e.g. ar.(42)) with resizable arrays (and even with Buffer, Bits, etc...). This requires a short explanation of how OCaml treats such syntactic sugar:

All that OCaml does is that it replaces such syntax with an appropriate Array.get or Array.set. This may be any module that happens to be bound to this name in the current scope. The same principle is true for the String-module and the .[]-operator.

Thus, the following works:

module Array = Res.Bits
module String = Res.Buffer

let () =
  let ar = Array.empty () in
  Array.add_one ar true;
  print_endline (string_of_bool ar.(0));
  let str = String.empty () in
  String.add_one str 'x';
  print_char str.[0];
  print_newline ()

Do not forget that it is even possible to bind modules locally. Example:

let () =
  let module Array = Res.Array in
  Printf.printf "%d\n" (Array.init 10 (fun x -> x * x)).(7)

If you want to change one of your files to make use of resizable arrays instead of standard ones without much trouble, please read the following:

You may want to "save" the standard Array-module and its type for later access:

module StdArray = Array
type 'a std_array = 'a array

Make the resizable implementation (includes the index operators!) available:

open Res

Or more explicitly:

module Array = Res.Array

Or if you want to use a specific Array-implementation:

module Array = Res.Bits

Then set the type:

type 'a array = 'a Array.t

If you create standard arrays with the built-in syntax, change lines like:

let ar = [| 1; 2; 3; 4 |] in


let ar = Array.of_array [| 1; 2; 3; 4 |] in

This should allow all of your sources to compile out-of-the-box with the additional functionality. In places where you still need the standard implementation you should have no problems to use the rebound module and type to do so.

This trick works similarly for the old and the new Buffer-module. You might also want to replace the String-module in this fashion. The latter one, however, supports a number of functions like e.g. escape, which are not available then.

Contact Information and Contributing

Please submit bugs reports, feature requests, contributions and similar to the GitHub issue tracker.

Up-to-date information is available at:

Dependencies (2)

  1. dune >= "1.4.0"
  2. ocaml >= "4.04"

Dev Dependencies


Used by (5)

  1. aifad
  2. capnp
  3. core < "109.58.00"
  4. core_extended < "108.00.02" | >= "109.19.00" & < "109.58.00"
  5. core_kernel < "109.58.00"