package base

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The full API is browsable here.

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.

Note that an API for OCaml's channel-based I/O can be found in the Stdio library.

Relationship to Core

  • Base: Minimal stdlib replacement. Portable and lightweight and intended to be highly stable.
  • Core: Extension of Base. More fully featured, with more code and dependencies, and APIs that evolve more quickly. Portable, and works on Javascript.

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, etc., coming from the OCaml standard library that one normally gets in the default environment are deprecated.

In order to access these values, one must use the Caml library, which re-exports them all through the toplevel name Caml: Caml.String, Caml.print_string, ...

The new modules and values made available by Base are documented here.

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.

Comparison operators

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: 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; and
  • conversions between OCaml values and s-expression.

However, it doesn't need these code generators to build. Instead, it uses ppx as a code verification tool during development. It works in a very similar fashion to expect tests.

Whenever you see this in the code source:

type t = ... [@@deriving_inline sexp_of]
let sexp_of_t = ...

the code between the [@@deriving_inline] and the [@@@end] is generated code. The generated code is currently quite big and hard to read, however we are working on making it look like human-written code.

You can put the following elisp code in your ~/.emacs file to hide these blocks:

(defun deriving-inline-forward-sexp (&optional arg)
  (search-forward-regexp "\\[@@@end\\]") nil nil arg)

(defun setup-hide-deriving-inline ()
  (hs-minor-mode t)
  (let ((hs-hide-comments-when-hiding-all nil))

(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 set up in the git repository to make it convenient to change types and update the generated code, but they will be set up soon.

Base coding rules

There are a few coding rules across the code base that are enforced by lint tools.

These rules are:

  • Opening the Caml module is not allowed. Inside Base, the OCaml stdlib is shadowed and accessible through the Caml module. We forbid opening Caml so that we know exactly where things come from.
  • Caml.Foo modules cannot be aliased, one must use Caml.Foo explicitly. 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 String0 for common functions that cannot be defined directly in Base.String to avoid creating a circular dependency. Except for String itself, other modules are not allowed to use Caml.String and must use either String or String0 instead.
  • Indentation is exactly the one of ocp-indent.
  • 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 ppx_js_style.

These checks are currently not run by dune, but it will soon get a -dev flag to run them automatically.


Base is still under active development and there are several missing feature that are yet to be added. Consult the roadmap to see what is happening.


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