Base is a standard library for OCaml. It provides a standard set of general purpose modules that are well-tested, performant, and fully-portable across any environment that can run OCaml code. Unlike other standard library projects, Base is meant to be used as a wholesale replacement of the standard library distributed with the OCaml compiler. In particular it makes different choices and doesn’t re-export features that are not fully portable such as I/O, which are left to other libraries.
You also might want to browse the API Documentation.
Install Base via OPAM:
$ opam install base
Using the OCaml standard library with Base
Base is intended as a full stdlib replacement. As a result, after an
open Base, all the modules, values, types, … coming from the OCaml
standard library that one normally gets in the default environment are
In order to access these values, one must use the
which re-exports them all through the toplevel name
The recommended way to build code using Base is as follows:
$ ocamlc -open Base
Differences between Base and the OCaml standard library
Programmers who are used to the OCaml standard library should read through this section to understand major differences between the two libraries that one should be aware of when switching to Base.
The comparison operators exposed by the OCaml standard library are polymorphic:
val compare : 'a -> 'a -> int val ( <= ) : 'a -> 'a -> bool ...
What they implement is structural comparison of the runtime representation of values. Since these are often error-prone, i.e. they don’t correspond to what the user expects, they are not exposed directly by Base.
To use polymorphic comparison with Base, one should use the
Polymorphic_compare module. The default comparison operators exposed
by Base are the integer ones, just like the default arithmetic
operators are the integer ones.
The recommended way to compare arbitrary complex data structures is to
use the specific
compare functions. For instance:
List.compare String.compare x y
The ppx_compare rewriter offers an alternative way to write this:
[%compare: string list] x y
Base and ppx code generators
Base uses a few ppx code generators to implement:
- reliable and customizable comparison of OCaml values
- reliable and customizable hash of OCaml values
- conversions between OCaml values and s-expression
However, it doesn’t need these code generators to build. What it does instead is use ppx as a code verification tool during development. It works in a very similar fashion to expectation tests.
Whenever you see this in the code source:
type t = ... [@@deriving_inline sexp_of] let sexp_of_t = ... [@@@end]
the code between the
[@@deriving_inline] and the
generated code. The generated code is currently quite big and hard to
read, however we are working on making it look like human-written
You can put the following elisp code in your
~/.emacs file to hide
(defun deriving-inline-forward-sexp (&optional arg) (search-forward-regexp "\\[@@@end\\]") nil nil arg) (defun setup-hide-deriving-inline () (inline) (hs-minor-mode t) (let ((hs-hide-comments-when-hiding-all nil)) (hs-hide-all))) (require 'hideshow) (add-to-list 'hs-special-modes-alist '(tuareg-mode "\\[@@deriving_inline[^]]*\\]" "\\[@@@end\\]" nil deriving-inline-forward-sexp nil)) (add-hook 'tuareg-mode-hook 'setup-hide-deriving-inline)
Things are not yet setup in the git repository to make it convenient to change types and update the generated code, but they will be setup soon.
OCaml Version Support
Base will maintain compatibility with the latest OCaml release, and the three prior minor version releases. Because of this, there will be a lag of four minor versions before features introduced in the Stdlib will reach Base.
Base coding rules
There are a few coding rules across the code base that are enforced by lint tools.
These rules are:
- Opening the
Camlmodule is not allowed. Inside Base, the OCaml stdlib is shadowed and accessible through the
Camlmodule. We forbid opening
Camlso that we know exactly where things come from.
Caml.Foomodules cannot be aliased, one must use
Caml.Fooexplicitly. This is to avoid having to remember a list of aliases at the beginning of each file.
- For some modules that are both in the OCaml stdlib and Base, such as
String, we define a module
String0for common functions that cannot be defined directly in
Base.Stringto avoid creating a circular dependency. Except for
Stringitself, other modules are not allowed to use
Caml.Stringand must use either
- Indentation is exactly the one of
- A few other coding style rules enforced by ppx_js_style.
The Base specific coding rules are checked by
ppx_base_lint, in the
lint subfolder. The indentation rules are checked by a wrapper around
ocp-indent and the coding style rules are checked by
These checks are currently not run by
dune, but it will soon get a
-dev flag to run them automatically.