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Documenting everything about OCaml


Standard Libraries

OCaml comes with its own Standard Library. Most OCamlers, however, agree that the built-in standard library is problematic: it’s not comprehensive enough, and it uses outmoded patterns, such as using exceptions rather than error types. The community has had trouble expanding the library due to namespacing issues.

  • Note, however, that since OCaml 4.07, the namespace issue has mostly been fixed, and the standard library is being expanded to cover more missing use-cases. Additionally, many functions that threw exceptions now have no-exception counterparts.

In the absence of a comprehensive standard library, several competitors developed. Due to OCaml’s support for modularity, each can be swapped out for the other wholesale.

  • The standard library offers a selection of functional data structures (List, Map) and imperative data structures (Array, Queue, Stack). Many of the functions throw an exception rather than returning an option type. The library also includes the Unix module, which, despite its name, has many functions that work on Windows. The Unix module is a catch-all for many IO-processing functions. Also included is the Format module for pretty printing and the Str module for regular expression matching, among other useful modules.
  • Base: Jane Street Capital’s internally developed, battle-tested standard library implementation. It’s also the library covered by the 2nd edition of Real World OCaml. The library doesn’t try to extend the basic standard library, but instead chooses its own design. To the users, the most obvious difference compared to the other standard library options will be the choice of having labels in function arguments, similar to the ListLabels, ArrayLabels, etc modules in the standard library. Additionally, Base does not use polymorphic comparison anywhere in its API.
  • Containers is a lightweight and modern-style standard library which extends the standard library with additional functionality while applying more modern design concepts. Code written using the stdlib should continue working after adding open Containers, and benefit from richer List, Option, Seq, etc. modules. A few new modules are introduced for critical functionalities, such as Vector, Heap, IO, and Sexp.
  • Core is Jane Street’s expanded standard library, sitting on top of Base. Core is more comprehensive than Base.
  • Batteries Included is a mature, full-featured standard library, taking the approach of extending the standard library with additional functionality (like Containers) rather than replacing it wholesale (unlike Base). It includes many data structures and algorithms. Note that Containers was created to follow a different design than Batteries.
  • BOS - Basic OS Interaction is a higher-level OS interaction layer containing many elements, including file manipulation, command line argument parsing, etc. It supplements and replaces functionality present in the standard library.
  • For a very detailed technical comparison of the standard libraries, see here.


Some (unfortunately) not-very-compatible modern alternatives are recommended:

  • Containers + stdlib + BOS: Containers is a modern extension of the stdlib, and extends it in powerful ways. For high-level OS interaction, BOS is a great fit.
  • Base (+ Core): The Jane Street standard libraries eschew the conventions of the stdlib, and present an alternative, full-feaured combination that works particularly well with other Jane Street libraries. Core is a little heavier than Base, and can be avoided unless one specifically requires functionality that is only provided in Core.