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Standard library for OCaml

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  • Base

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]].

** Installation

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:

+begin_src ocaml

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:

+begin_src ocaml x y


The [[][ppxcompare]] rewriter offers an alternative way to write this:

+begin_src ocaml

[%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:

+begin_src ocaml

type t = ... [@@derivinginline sexpof] let sexpoft = ... [@@@end]


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:

+begin_src scheme

(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 =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 [[][ppxjsstyle]].

The Base specific coding rules are checked by =ppxbaselint=, in the =lint= subfolder. The indentation rules are checked by a wrapper around =ocp-indent= and the coding style rules are checked by =ppxjsstyle=.

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 ~sexpoft~ and ~tofsexp~ functions for converting between values of that type and their sexp representations.

One pair of functions deserves special attention: ~String.sexpoft~ and ~String.tofsexp~. These functions have the same types as ~Sexp.ofstring~ and ~Sexp.tostring~ but very different behavior.

~String.sexpoft~ and ~String.tofsexp~ are used to encode and decode strings "embedded" in a sexp representation. On the other hand, ~Sexp.ofstring~ and ~Sexp.tostring~ are used to encode and decode the textual form of s-expressions.

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

+begin_src ocaml

open! Base open! Stdio

(* Embed a string in a sexp *)

let examplesexp : Sexp.t = List.sexpoft String.sexpof_t [ "hello"; "world" ]

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

let () = assert ( List.equal String.equal "hello"; "world" ) ;;

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

let writesexptofile sexp = Outchannel.writeall "/tmp/file" ~data:(Sexp.tostring example_sexp) ;;

(* /tmp/file now contains:

   (hello world)
 v} *)

let () = assert (Sexp.equal examplesexp (Sexp.ofstring (Inchannel.readall "/tmp/file"))) ;;


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