How to drive odoc

This 'live' document describes how to use odoc to produce the documentation of odoc itself. The aim is to show a short, simple example of how odoc can be used, covering most of the important features. The document built here includes not only the documentation of odoc itself, but it also builds the docs for a subset of odoc's dependent libraries to show how this may be done. For a much more complete and comprehensive use of odoc, see the Voodoo project, the tool that is being used to build the package docs for

First we need to initialise MDX with some libraries and helpful values.

(* Prelude *)
#require "bos";;
#install_printer Fpath.pp;;
open Bos;;
let (>>=) = Result.bind;;
let (>>|=) m f = m >>= fun x -> Ok (f x);;
let get_ok = function | Ok x -> x | Error (`Msg m) -> failwith m

Desired Output

odoc produces output files (html or others) in a structured directory tree, so before running odoc, the structure of the output must be decided. For these docs, we want the following structure:

The odoc model for achieving this is that we have pages (.mld files) that have children which are either further pages (.mld files) or modules (from .cmti files). This parent/child relationship is specified on the command line. Parent pages must be compiled by odoc before their children. Then compiling a page mypage.mld will produce the file page-mypage.odoc.

In the example below, there will be a file odoc.mld that corresponds with the top-level directory odoc/. It will be compiled as follows:

odoc compile odoc.mld --child page-odoc_model --child page-odoc_parser --child deps ...

The file deps.mld which corresponds with the sub-directory odoc/deps/, will be compiled as follows:

odoc compile deps.mld -I . --parent `odoc` --child page-stdlib --child page-astring ...

The file odoc_model.mld will have a child module Odoc_model. It will be compiled as follows:

odoc compile odoc_model.mld -I . --parent `odoc` --child module-Odoc_model

When compiling any .mld file, the parent and all children must be specified. Parents can only be pages from other .mld files, and children may be pages (from .mld files) or modules (from .cmti/.cmt or .cmi files).

The parent page must exist before the child page is created, and it must have had the child specified when it was initially compiled.

Document Generation Phases

Using odoc is a three-phase process:

  1. Compilation: odoc compile

    This takes the output from the compiler in the form of .cmti, .cmt, or .cmi files (in order of preference), translates it into odoc's internal format, and performs some initial expansion and resolution operations. For a given input /path/to/file.cmti it will output the file /path/to/file.odoc unless the -o option is used to override the output file. If there were .cmi dependencies required for OCaml to compile these files, then there will be equivalent .odoc dependencies needed for the odoc compile step. odoc will search for these dependencies in the paths specified with the -I directive on compilation. odoc provides a command to help with this: odoc compile-deps:

    As an example we can run odoc compile-deps on the file ../src/xref2/.odoc_xref2.objs/byte/odoc_xref2__Compile.cmti:

    $ `odoc` compile-deps ../src/xref2/.odoc_xref2.objs/byte/odoc_xref2__Compile.cmti | tail -n 5
    Stdlib__result 2ba42445465981713146b97d5e185dd5
    Stdlib__seq d6a8de25c9eecf5ae9420a9f3f8b2e88
    Stdlib__set 5d365647a10f75c22f2b045a867b4d3e
    Stdlib__uchar ab6f1df93abf9e800a3e0d1543523c96
    Odoc_xref2__Compile e0d620d652a724705f7ed620dfe07be0

    so we can see we will need to run odoc compile against several Stdlib modules before we can compile odoc_xref2__Compile.cmti

  2. Linking: odoc link

    This takes the odoc files produced during the compilation step and performs the final steps of expansion and resolution. It is during this phase that all the references in the documentation comments are resolved. In order for these to be resolved, everything that is referenced must have been compiled already, and their odoc files must be on the include path as specified by the -I arguments to odoc link. In this example, we achieve that by compiling all modules and .mld files before linking anything. The output of the link step is an odocl file, which is in the same path as the original odoc file by default.

    Please note: it's only necessary to link the non-hidden modules (i.e., without a double underscore).

  3. Generation: odoc html-generate

    Once the compile and link phases are complete, the resulting odocl files may be rendered in a variety of formats. In this example we output HTML.

odoc Documentation

In this section odoc is used to generate the documentation of odoc and some of its dependent packages. We can make a few simplifying assumptions here:

  1. Since we're working with one leaf package, we can assume that there can be no module name clashes in the dependencies. As such, we can afford to put all of our .odoc files into one directory and then hard-code the include path to be this directory. When using odoc in a context where there may be module name clashes, it requires more careful partitioning of output directories.
  2. We'll do all of the compiling before any linking.

Let's start with some functions to execute the three phases of odoc.

Compiling a file with odoc requires a few arguments: the file to compile, an optional parent, a list of include paths, a list of children for .mld files, and an output path. Include paths can be just '.', and we can calculate the output file from the input because all of the files are going into the same directory.

Linking a file with odoc requires the input file and a list of include paths. As for compile, we will hard-code the include path.

Generating the HTML requires the input odocl file and an output path. We will hard-code the output path to be html.

In all of these, we'll capture stdout and stderr so we can check it later.

let odoc = Cmd.v "../src/odoc/bin/main.exe"

let compile_output = ref [ "" ]

let link_output = ref [ "" ]

let generate_output = ref [ "" ]

let add_prefixed_output cmd list prefix lines =
  if List.length lines > 0 then
    list :=
      @ Bos.Cmd.to_string cmd :: (fun l -> prefix ^ ": " ^ l) lines

let compile file ?parent ?(ignore_output = false) children =
  let output_file =
    let ext = Fpath.get_ext file in
    let basename = Fpath.basename (Fpath.rem_ext file) in
    match ext with
    | ".mld" -> "page-" ^ basename ^ ".odoc"
    | ".cmt" | ".cmti" | ".cmi" -> basename ^ ".odoc"
    | _ -> failwith ("bad extension: " ^ ext)
  let open Cmd in
  let cmd =
    odoc % "compile" % Fpath.to_string file % "-I" % "." % "-o" % output_file
    |> List.fold_right (fun child cmd -> cmd % "--child" % child) children
  let cmd =
    match parent with
    | Some p -> cmd % "--parent" % ("page-\"" ^ p ^ "\"")
    | None -> cmd
  let lines = OS.Cmd.(run_out ~err:err_run_out cmd |> to_lines) |> get_ok in
  if not ignore_output then
    add_prefixed_output cmd compile_output (Fpath.to_string file) lines

let link ?(ignore_output = false) file =
  let open Cmd in
  let cmd = odoc % "link" % p file % "-I" % "." in
  let lines = OS.Cmd.(run_out ~err:err_run_out cmd |> to_lines) |> get_ok in
  if not ignore_output then
    add_prefixed_output cmd link_output (Fpath.to_string file) lines

let html_generate ?(ignore_output = false) file =
  let open Cmd in
  let cmd =
    odoc % "html-generate" % p file % "-o" % "html" % "--theme-uri" % "odoc"
    % "--support-uri" % "odoc"
  let lines = OS.Cmd.(run_out cmd ~err:err_run_out |> to_lines) |> get_ok in
  if not ignore_output then
    add_prefixed_output cmd generate_output (Fpath.to_string file) lines

let support_files () =
  let open Cmd in
  let cmd = odoc % "support-files" % "-o" % "html/odoc" in
  OS.Cmd.(run_out cmd |> to_lines) |> get_ok

We'll now make some library lists. We have not only external dependency libraries, but odoc itself is also separated into libraries too. These two sets of libraries will be documented in different sections, so we'll keep them in separate lists. Additionally we'll also construct a list containing the extra documentation pages. Finally let's create a list mapping the section to its parent, which matches the hierarchy declared above.

let dep_libraries_core =

let extra_deps =

let dep_libraries =
  match Sys.getenv_opt "ODOC_BENCHMARK" with
  | Some "true" -> dep_libraries_core @ extra_deps
  | _ -> dep_libraries_core

let odoc_libraries =

let all_libraries = dep_libraries @ odoc_libraries

let extra_docs =

let parents =
  let add_parent p l = (fun lib -> (lib, p)) l in
  add_parent "deps" dep_libraries @ add_parent "odoc" odoc_libraries

odoc operates on the compiler outputs. We need to find them for both the files compiled by Dune within this project and those in libraries we compile against. The following uses ocamlfind to locate the library paths for our dependencies:

let ocamlfind = Cmd.v "ocamlfind"

let lib_path lib =
  let cmd = Cmd.(ocamlfind % "query" % lib) in
  OS.Cmd.(run_out cmd |> to_lines >>|= List.hd)

let lib_paths =
    (fun lib acc ->
      acc >>= fun acc ->
      lib_path lib >>|= fun l -> (lib, l) :: acc)
    dep_libraries (Ok [])
  |> get_ok

We need a function to find odoc inputs given a search path. odoc operates on .cmti, .cmt or .cmi files, in order of preference, and the following function finds all matching files given a search path. Then it returns an Fpath.Set.t that contains the Fpath.t values representing the absolute file path, without its extension.

let find_units p =
  OS.Dir.fold_contents ~dotfiles:true
    (fun p acc ->
      if List.exists (fun ext -> Fpath.has_ext ext p) [ "cmt"; "cmti"; "cmi" ]
      then p :: acc
      else acc)
    [] (Fpath.v p)
  >>|= fun paths ->
  let l = Fpath.rem_ext paths in
  let l =
      (fun f ->
        not @@ Astring.String.is_infix ~affix:"ocamldoc" (Fpath.to_string f))
  List.fold_right Fpath.Set.add l Fpath.Set.empty

Since the units returned by this function have their extension stripped, we need function to find the best file to use with this basename.

let best_file base = (fun ext -> Fpath.add_ext ext base) [ "cmti"; "cmt"; "cmi" ]
  |> List.find (fun f -> Bos.OS.File.exists f |> get_ok)

Many of the units will be 'hidden' -- that is, their name will be mangled by Dune in order to namespace them. This is achieved by prefixing the namespace module and a double underscore, so we can tell by the existence of a double underscore that a module is intended to be hidden. The following predicate tests for that condition:

let is_hidden path = Astring.String.is_infix ~affix:"__" (Fpath.to_string path)

To build the documentation, we start with these files. With the following function, we'll call odoc compile-deps on the file to find all other compilation units upon which it depends:

type compile_deps = { digest : Digest.t; deps : (string * Digest.t) list }

let compile_deps f =
  let cmd = Cmd.(odoc % "compile-deps" % Fpath.to_string f) in
  OS.Cmd.(run_out cmd |> to_lines)
  >>|= List.filter_map (Astring.String.cut ~sep:" ")
  >>= fun l ->
  let basename = Fpath.(basename (f |> rem_ext)) |> String.capitalize_ascii in
  match List.partition (fun (n, _) -> basename = n) l with
  | [ (_, digest) ], deps -> Ok { digest; deps }
  | _ -> Error (`Msg "odd")

Let's now put together a list of all possible modules. We'll keep track of which library they're in, and whether that library is a part of odoc or a dependency library.

let odoc_all_unit_paths = find_units ".." |> get_ok

let odoc_units =
    (fun lib ->
        (fun p acc ->
          if Astring.String.is_infix ~affix:lib (Fpath.to_string p) then
            ("odoc", lib, p) :: acc
          else acc)
        odoc_all_unit_paths [])
let lib_units =
    (fun (lib, p) ->
        (fun p acc -> ("deps", lib, p) :: acc)
        (find_units p |> get_ok)

let all_units = odoc_units @ lib_units |> List.flatten

Now we'll compile all of the parent .mld files. To ensure that the parents are compiled before the children, we start with odoc.mld, then deps.mld, and so on. The result of this file is a list of the resulting odoc files.

let compile_mlds () =
  let mkpage x = "page-\"" ^ x ^ "\"" in
  let mkmod x = "module-" ^ String.capitalize_ascii x in
  let mkmld x = Fpath.(add_ext "mld" (v x)) in
    (compile (mkmld "odoc")
       ("page-deps" :: mkpage (odoc_libraries @ extra_docs)));
  ignore (compile (mkmld "deps") ~parent:"odoc" ( mkpage dep_libraries));
  let extra_odocs =
      (fun p ->
        ignore (compile (mkmld p) ~parent:"odoc" []);
        "page-" ^ p ^ ".odoc")
  let odocs =
      (fun library ->
        let parent = List.assoc library parents in
        let children =
            (fun (parent, lib, child) ->
              if lib = library then Some (Fpath.basename child |> mkmod)
              else None)
        ignore (compile (mkmld library) ~parent children);
        "page-" ^ library ^ ".odoc")
    (fun f -> (Fpath.v f, false))
    ("page-odoc.odoc" :: "page-deps.odoc" :: odocs @ extra_odocs)

Now we get to the compilation phase. For each unit, we query its dependencies, then recursively call to compile these dependencies. Once this is done we compile the unit itself. If the unit has already been compiled we don't do anything. Note that we aren't checking the hashes of the dependencies which a build system should do to ensure that the module being compiled is the correct one. Again we benefit from the fact that we're creating the docs for one leaf package and that there must be no module name clashes in its dependencies. The result of this function is a list of the resulting odoc files.

let compile_all () =
  let mld_odocs = compile_mlds () in
  let rec rec_compile parent lib file =
    let output = Fpath.(base (set_ext "odoc" file)) in
    if OS.File.exists output |> get_ok then []
      let deps = compile_deps file |> get_ok in
      let files =
          (fun acc (dep_name, digest) ->
                (fun (_, _, f) ->
                  Fpath.basename f |> String.capitalize_ascii = dep_name)
            | None -> acc
            | Some (parent, lib, dep_path) ->
                let file = best_file dep_path in
                rec_compile parent lib file @ acc)
          [] deps.deps
      let ignore_output = parent = "deps" in
      ignore (compile file ~parent:lib ~ignore_output []);
      (output, ignore_output) :: files
    (fun acc (parent, lib, dep) -> acc @ rec_compile parent lib (best_file dep))
    [] all_units
  @ mld_odocs

Linking is now straightforward. We only need to link non-hidden odoc files, as any hidden are almost certainly aliased inside the non-hidden ones (a result of namespacing usually, and these aliases will be expanded).

let link_all odoc_files =
  let not_hidden (f, _) = not (is_hidden f) in
    (fun (odoc_file, ignore_output) ->
      ignore (link ~ignore_output odoc_file);
      Fpath.set_ext "odocl" odoc_file)
    (List.filter not_hidden odoc_files)

Now we simply run odoc html-generate over all of the resulting odocl files.

let generate_all odocl_files =
  List.iter (fun f -> ignore(html_generate f)) odocl_files;
  support_files ()

The following code actually executes all of the above, and we're done!

let compiled = compile_all () in
let linked = link_all compiled in
generate_all linked

Let's see if there was any output from the odoc invocations:

# !compile_output;;
- : string list = [""]
# !link_output;;
- : string list = [""]
# !generate_output;;
- : string list =
 "'../src/odoc/bin/main.exe' 'html-generate' 'odoc_xref_test.odocl' '-o' 'html' '--theme-uri' 'odoc' '--support-uri' 'odoc'";
 "odoc_xref_test.odocl: Warning, resolved hidden path: Odoc_model__Lang.Signature.t";
 "'../src/odoc/bin/main.exe' 'html-generate' 'odoc_examples.odocl' '-o' 'html' '--theme-uri' 'odoc' '--support-uri' 'odoc'";
 "odoc_examples.odocl: Warning, resolved hidden path: Odoc_examples__Unexposed.t"]