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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

Base has no runtime dependencies and is fast to build. Its sole build dependencies are dune, which itself requires nothing more than the compiler, and sexplib0.

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 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, …

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 Poly 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
  • 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 = ...

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 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 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.

Sexp (de-)serializers

Most types in Base have sexp_of_t and t_of_sexp functions for converting between values of that type and their sexp representations.

One pair of functions deserves special attention: String.sexp_of_t and String.t_of_sexp. These functions have the same types as Sexp.of_string and Sexp.to_string but very different behavior.

String.sexp_of_t and String.t_of_sexp are used to encode and decode strings “embedded” in a sexp representation. On the other hand, Sexp.of_string and Sexp.to_string are used to encode and decode the textual form of s-expressions.

The following example demonstrates the two pairs of functions in action:

open! Base
open! Stdio

(* Embed a string in a sexp *)

let example_sexp : Sexp.t = List.sexp_of_t String.sexp_of_t [ "hello"; "world" ]

let () =
  assert (Sexp.equal example_sexp (Sexp.List [ Sexp.Atom "hello"; Sexp.Atom "world" ]))

let () =
  assert (
      [ "hello"; "world" ]
      (List.t_of_sexp String.t_of_sexp example_sexp))

(* Embed a sexp in text (string) *)

let write_sexp_to_file sexp =
  Out_channel.write_all "/tmp/file" ~data:(Sexp.to_string example_sexp)

(* /tmp/file now contains:

     (hello world)
   v} *)

let () =
  assert (Sexp.equal example_sexp (Sexp.of_string (In_channel.read_all "/tmp/file")))