Last week, we published an alpha version of a new OCaml documentation generator, codoc 0.2.0. In the 2014 OCaml workshop presentation (abstract, slides, video), we mentioned the 'module wall' for documentation and this attempts to fix it. To try it out, simply follow the directions in the README on that repository, or browse some samples of the current, default output of the tool. Please do bear in mind codoc and its constituent libraries are still under heavy development and are not feature complete.

codoc's aim is to provide a widely useful set of tools for generating OCaml documentation. In particular, we are striving to:

  1. Cover all of OCaml's language features
  2. Provide accurate name resolution and linking
  3. Support cross-linking between different packages
  4. Expose interfaces to the components we've used to build codoc
  5. Provide a magic-free command-line interface to the tool itself
  6. Reduce external dependencies and default integration with other tools

We haven't yet achieved all of these at all levels of our tool stack but are getting close. codoc 0.2.0 is usable today (if a little rough in some areas like default CSS). This post outlines the architecture of the new system to make it easier to understand the design decisions that went into it.

The five stages of documentation

There are five stages in generating documentation from OCaml source code. Here we describe how each was handled in the past (using OCamldoc), the present (using our current prototype), and the future (using the final version of the tools we are developing).

Associating comments with definitions

The first stage is to associate the various documentation comments in an .ml or .mli file with the definitions that they correspond to.


Associating comments with definitions is handled by the OCamldoc tool, which does this in two steps. First it parses the file using the regular OCaml parser or camlp4, just as in normal compilation. It uses the syntax tree from the first step and then re-parses the file looking for comments. This second parse is guided by the location information in the syntax tree; for example if there is a definition which ends on line 5 then OCamldoc will look for comments to attach to that definition starting at line 6.

The rules used for attaching comments are quite intricate and whitespace dependent. This makes it difficult to parse the file and attach comments using a single parser. In particular, it would be difficult to do so in a way that doesn't cause a lot of problems for camlp4 extensions. This is why OCamldoc does the process in two steps.

A disadvantage of this two-step approach is that it assumes that the input to any preprocessor is something which could reasonably be read by the compiler/tool creating documentation, which may not always be the case.


Our current prototype associates comments with definitions within the compiler itself. This relies on a patch to the OCaml compiler (pull request #51 on GitHub). Comment association is activated by the -doc command-line flag. It uses (a rewritten version of) the same two-step algorithm currently used by OCamldoc. The comments are then attached to the appropriate node in the syntax tree as an attribute. These attributes are passed through the type-checker and appear in .cmt/.cmti files, where they can be read by other tools.


We intend to move away from the two-step approach taken by OCamldoc. To do this we will need to simplify the rules for associating comments with definitions. One suggestion was to use the same rules as attributes, however that seems to be overly restrictive. So the approach we hope to take is to keep quite close to what OCamldoc currently supports, but disallow some of the more ambiguous cases. For example,

val x : int
(** Is this for x or y? *)
val y : float

may well not be supported in our final version. We will take care to understand the impact of such design decisions and we hope to arrive at a robust solution for the future. By avoiding the two-step approach, it should be safe to always turn on comment association rather than requiring a -doc command-line flag.

Parsing the contents of comments

Once you have associated documentation comments with definitions, you must parse the contents of these comments.


OCamldoc parses the contents of comments.


In our current prototype, the contents of comments are parsed in the compiler, so that the documentation attributes available in .cmt/.cmti files contain a structured representation of the documentation.


We intend to separate parsing the contents of documentation comments from the compiler. This means that the documentation will be stored as strings within the .cmt/.cmti files and parsed by external tools. This will allow the documentation language (and its parser) to evolve faster than the distribution cycle of the compiler.

Representing compilation units with types and documentation

The typed syntax tree stored in .cmt/.cmti files is not a convenient representation for generating documentation from, so the next stage is to convert the syntax tree and comments into some suitable intermediate form. In particular, this allows .cmt files and .cmti files to be treated uniformly.


OCamldoc generates an intermediate form from a syntax tree, a typed syntax tree, and the comments that it found and parsed in the earlier stages. The need for both an untyped and typed syntax tree is a historical artefact that is no longer necessary.


Our current prototype creates an intermediate form in the doc-ock library. This form can be currently be created from .cmti files or .cmi files. .cmi files do not contain enough information for complete documentation, but you can use them to produce partial documentation if the .cmti files are not available to you.

This intermediate form can be serialised to XML using doc-ock-xml.


In the final version, doc-ock will also support reading .cmt files.

Resolving references

Once you have a representation for documentation, you need to resolve all the paths and references so that links can point to the correct locations. For example,

(* This type is used by {!Foo} *) type t = Bar.t

The path Bar.t and the reference Foo must be resolved so that the documentation can include links to the corresponding definitions.

If you are generating documentation for a large collection of packages, there may be more than one module called Foo. So it is important to be able to work out which one of these Foos the reference is referring to.

Unlike most languages, resolving paths can be very difficult in OCaml due to the powerful module system. For example, consider the following code:

module Dep1 : sig
 module type S = sig
   class c : object
     method m : int
 module X : sig
   module Y : S

module Dep2 :
 functor (Arg : sig module type S module X : sig module Y : S end end) ->
     module A : sig
       module Y : Arg.S
     module B = A.Y

type dep1 = Dep2(Dep1).B.c;;

Here it looks like, Dep2(Dep1).B.c would be defined by a type definition c within the submodule B of the functor Dep2. However, Dep2.B's type is actually dependent on the type of Dep2's Arg parameter, so the actual definition is the class definition within the module type S of the Dep1 module.


OCamldoc does resolution using a very simple string based lookup. This is not designed to handle collections of projects, where module names are not unique. It is also not sophisticated enough to handle advanced uses of OCaml's module system (e.g. it fails to resolve the path Dep2(Dep1).B.c in the above example).


In our current prototype, path and reference resolution are performed by the doc-ock library. The implementation amounts to a reimplementation of OCaml's module system that tracks additional information required to produce accurate paths and references (it is also lazy to improve performance). The system uses the digests provided by .cmti/.cmi files to resolve references to other modules, rather than just relying on the module's name.


There are still some paths handled incorrectly by doc-ock-lib, which will be fixed, but mostly the final version will be the same as the current prototype.

Producing output

Finally, you are ready to produce some output from the tools.


OCamldoc supports a variety of output formats, including HTML and LaTeX. It also includes support for plugins called "custom generators" which allow users to add support for additional formats.


codoc only supports HTML and XML output at present, although extra output formats such as JSON should be very easy to add once the interfaces settle down. codoc defines a documentation index XML format for tracking package hierarchies, documentation issues, and hierarchically localized configuration.

codoc also defines a scriptable command-line interface giving users access to its internal documentation phases: extraction, linking, and rendering. The latest instructions on how to use the CLI can be found in the README. We provide an OPAM remote with all the working versions of the new libraries and compiler patches required to drive the new documentation engine.


As previously mentioned, codoc and its constituent libraries doc-ock-lib and doc-ock-xml are still under heavy development and are not yet feature complete. Notably, there are some important outstanding issues:

  1. Class and class type documentation has no generated HTML. (issue codoc#9)
  2. CSS is subpar. (issue codoc#27)
  3. codoc HTML does not understand --package. (issue codoc#42)
  4. opam doc is too invasive (temporary for demonstration purposes; tracked by (issue codoc#48))
  5. Documentation syntax errors are not reported in the correct phase or obviously enough. (issue codoc#58)
  6. Character sets are not handled correctly (issue doc-ock-lib#43)
  7. -pack and cmt extraction are not supported (issue doc-ock-lib#35 and issue doc-ock-lib#3)
  8. Inclusion/substitution is not supported (issue doc-ock-lib#2)

We are very happy to take bug reports and patches at For wider suggestions, comments, complaints and discussions, please join us on the Platform mailing list. We do hope that you'll let us know what you think and help us build a next generation documentation tool which will serve our community admirably.