The documentation generator (ocamldoc)

# Chapter 15 The documentation generator (ocamldoc)

This chapter describes OCamldoc, a tool that generates documentation from special comments embedded in source files. The comments used by OCamldoc are of the form `(**`...`*)` and follow the format described in section 15.2.
OCamldoc can produce documentation in various formats: HTML, LATEX, TeXinfo, Unix man pages, and `dot` dependency graphs. Moreover, users can add their own custom generators, as explained in section 15.3.
In this chapter, we use the word element to refer to any of the following parts of an OCaml source file: a type declaration, a value, a module, an exception, a module type, a type constructor, a record field, a class, a class type, a class method, a class value or a class inheritance clause.

## 15.1 Usage

### 15.1.1 Invocation

OCamldoc is invoked via the command `ocamldoc`, as follows:

            ocamldoc options sourcefiles

#### Options for choosing the output format

The following options determine the format for the generated documentation.

`-html`
  Generate documentation in HTML default format. The generated HTML pages are stored in the current directory, or in the directory specified with the `-d` option. You can customize the style of the generated pages by editing the generated `style.css` file, or by providing your own style sheet using option `-css-style`. The file `style.css` is not generated if it already exists.
`-latex`
  Generate documentation in LATEX default format. The generated LATEX document is saved in file `ocamldoc.out`, or in the file specified with the `-o` option. The document uses the style file `ocamldoc.sty`. This file is generated when using the `-latex` option, if it does not already exist. You can change this file to customize the style of your LATEX documentation.
`-texi`
  Generate documentation in TeXinfo default format. The generated LATEX document is saved in file `ocamldoc.out`, or in the file specified with the `-o` option.
`-man`
  Generate documentation as a set of Unix `man` pages. The generated pages are stored in the current directory, or in the directory specified with the `-d` option.
`-dot`
  Generate a dependency graph for the toplevel modules, in a format suitable for displaying and processing by `dot`. The `dot` tool is available from `http://www.research.att.com/sw/tools/graphviz/`. The textual representation of the graph is written to the file `ocamldoc.out`, or to the file specified with the `-o` option. Use `dot ocamldoc.out` to display it.
`-g` file.cm[o,a]
  Dynamically load the given file, which defines a custom documentation generator. See section 15.4.1. This option is supported by the `ocamldoc` command, but not by its native-code version `ocamldoc.opt`. If the given file is a simple one and does not exist in the current directory, then ocamldoc looks for it in the custom generators default directory, and in the directories specified with optional `-i` options.
`-customdir`
  Display the custom generators default directory.
`-i` directory
  Add the given directory to the path where to look for custom generators.

#### General options

`-d` dir
  Generate files in directory dir, rather than in the current directory.
`-dump` file
  Dump collected information into file. This information can be read with the `-load` option in a subsequent invocation of `ocamldoc`.
`-hide` modules
  Hide the given complete module names in the generated documentation modules is a list of complete module names are separated by '`,`', without blanks. For instance: `Pervasives,M2.M3`.
`-inv-merge-ml-mli`
  Inverse implementations and interfaces when merging. All elements in implementation files are kept, and the `-m` option indicates which parts of the comments in interface files are merged with the comments in implementation files.
`-keep-code`
  Always keep the source code for values, methods and instance variables, when available. The source code is always kept when a `.ml` file is given, but is by default discarded when a `.mli` is given. This option allows to always keep the source code.
`-load` file
  Load information from file, which has been produced by `ocamldoc -dump`. Several `-load` options can be given.
`-m` flags
  Specify merge options between interfaces and implementations. (see section 15.1.2 for details). flags can be one or several of the following characters:
  `d`
    merge description
  `a`
    merge @author
  `v`
    merge @version
  `l`
    merge @see
  `s`
    merge @since
  `o`
    merge @deprecated
  `p`
    merge @param
  `e`
    merge @raise
  `r`
    merge @return
  `A`
    merge everything
`-no-custom-tags`
  Do not allow custom @-tags (see section 15.2.5).
`-no-stop`
  Keep elements placed after the `(**/**)` special comment (see section 15.2).
`-o` file
  Output the generated documentation to file instead of `ocamldoc.out`. This option is meaningful only in conjunction with the `-latex`, `-texi`, or `-dot` options.
`-pp` command
  Pipe sources through preprocessor command.
`-sort`
  Sort the list of top-level modules before generating the documentation.
`-stars`
  Remove blank characters until the first asterisk ('`*`') in each line of comments.
`-t` title
  Use title as the title for the generated documentation.
`-intro` file
  Use content of file as ocamldoc text to use as introduction (HTML, LATEXand TeXinfo only). For HTML, the file is used to create the whole `index.html` file.
`-v`
  Verbose mode. Display progress information.
`-warn-error`
  Treat warnings as errors.

#### Type-checking options

OCamldoc calls the Objective Caml type-checker to obtain type informations. The following options impact the type-checking phase. They have the same meaning as for the `ocamlc` and `ocamlopt` commands.

`-I` directory
  Add directory to the list of directories search for compiled interface files (`.cmi` files).
`-nolabels`
  Ignore non-optional labels in types.
`-rectypes`
  Allow arbitrary recursive types. (See the `-rectypes` option to `ocamlc`.)

#### Options for generating HTML pages

The following options apply in conjunction with the `-html` option:

`-all-params`
  Display the complete list of parameters for functions and methods.
`-css-style` filename
  Use filename as the Cascading Style Sheet file.
`-colorize-code`
  Colorize the OCaml code enclosed in `[ ]` and `\{[ ]\}`, using colors to emphasize keywords, etc. If the code fragments are not syntactically correct, no color is added.
`-index-only`
  Generate only index files.

#### Options for generating LATEX files

The following options apply in conjunction with the `-latex` option:

`-latex-value-prefix` prefix
  Give a prefix to use for the labels of the values in the generated LATEX document. The default prefix is the empty string. You can also use the options `-latex-type-prefix`, `-latex-exception-prefix`, `-latex-module-prefix`, `-latex-module-type-prefix`, `-latex-class-prefix`, `-latex-class-type-prefix`, `-latex-attribute-prefix` and `-latex-method-prefix`.
  These options are useful when you have, for example, a type and a value with the same name. If you do not specify prefixes, LATEX will complain about multiply defined labels.
`-latextitle` n,style
  Associate style number n to the given LATEX sectioning command style, e.g. `section` or `subsection`. (LATEX only.) This is useful when including the generated document in another LATEX document, at a given sectioning level. The default association is 1 for `section`, 2 for `subsection`, 3 for `subsubsection`, 4 for `paragraph` and 5 for `subparagraph`.
`-noheader`
  Suppress header in generated documentation.
`-notoc`
  Do not generate a table of contents.
`-notrailer`
  Suppress trailer in generated documentation.
`-sepfiles`
  Generate one `.tex` file per toplevel module, instead of the global `ocamldoc.out` file.

#### Options for generating TeXinfo files

The following options apply in conjunction with the `-texi` option:

`-esc8`
  Escape accented characters in Info files.
`-info-entry`
  Specify Info directory entry.
`-info-section`
  Specify section of Info directory.
`-noheader`
  Suppress header in generated documentation.
`-noindex`
  Do not build index for Info files.
`-notrailer`
  Suppress trailer in generated documentation.

#### Options for generating `dot` graphs

The following options apply in conjunction with the `-dot` option:

`-dot-colors` colors
  Specify the colors to use in the generated `dot` code. When generating module dependencies, `ocamldoc` uses different colors for modules, depending on the directories in which they reside. When generating types dependencies, `ocamldoc` uses different colors for types, depending on the modules in which they are defined. colors is a list of color names separated by '`,`', as in `Red,Blue,Green`. The available colors are the ones supported by the `dot` tool.
`-dot-include-all`
  Include all modules in the `dot` output, not only modules given on the command line or loaded with the `-load` option.
`-dot-reduce`
  Perform a transitive reduction of the dependency graph before outputting the `dot` code. This can be useful if there are a lot of transitive dependencies that clutter the graph.
`-dot-types`
  Output `dot` code describing the type dependency graph instead of the module dependency graph.

#### Options for generating man files

The following options apply in conjunction with the `-man` option:

`-man-mini`
  Generate man pages only for modules, module types, clases and class types, instead of pages for all elements.
`-man-suffix`
  Set the suffix used for generated man filenames. Default is '`o`', like in `List.o`.

### 15.1.2 Merging of module information

Information on a module can be extracted either from the `.mli` or `.ml` file, or both, depending on the files given on the command line. When both `.mli` and `.ml` files are given for the same module, information extracted from these files is merged according to the following rules:

- Only elements (values, types, classes, ...) declared in the `.mli` file are kept. In other terms, definitions from the `.ml` file that are not exported in the `.mli` file are not documented.
- Descriptions of elements and descriptions in @-tags are handled as follows. If a description for the same element or in the same @-tag of the same element is present in both files, then the description of the `.ml` file is concatenated to the one in the `.mli` file, if the corresponding `-m` flag is given on the command line. If a description is present in the `.ml` file and not in the `.mli` file, the `.ml` description is kept. In either case, all the information given in the `.mli` file is kept.

### 15.1.3 Coding rules

The following rules must be respected in order to avoid name clashes resulting in cross-reference errors:

- In a module, there must not be two modules, two module types or a module and a module type with the same name. In the default HTML generator, modules `ab` and `AB` will be printed to the same file on case insensitive file systems.
- In a module, there must not be two classes, two class types or a class and a class type with the same name.
- In a module, there must not be two values, two types, or two exceptions with the same name.
- Values defined in tuple, as in `let (x,y,z) = (1,2,3)` are not kept by OCamldoc.
- Avoid the following construction:

      open Foo (* which has a module Bar with a value x *)
      module Foo =
        struct
          module Bar =
            struct
              let x = 1
            end
        end
        let dummy = Bar.x

  In this case, OCamldoc will associate `Bar.x` to the `x` of module `Foo` defined just above, instead of to the `Bar.x` defined in the opened module `Foo`.

## 15.2 Syntax of documentation comments

Comments containing documentation material are called special comments and are written between `(**` and `*)`. Special comments must start exactly with `(**`. Comments beginning with `(` and more than two `*` are ignored.

### 15.2.1 Placement of documentation comments

OCamldoc can associate comments to some elements of the language encountered in the source files. The association is made according to the locations of comments with respect to the language elements. The locations of comments in `.mli` and `.ml` files are different.

#### Comments in `.mli` files

A special comment is associated to an element if it is placed before or after the element.
A special comment before an element is associated to this element if :

- There is no blank line or another special comment between the special comment and the element. However, a regular comment can occur between the special comment and the element.
- The special comment is not already associated to the previous element.
- The special comment is not the first one of a toplevel module.

A special comment after an element is associated to this element if there is no blank line or comment between the special comment and the element.
There are two exceptions: for type constructors and record fields in type definitions, the associated comment can only be placed after the constructor or field definition, without blank lines or other comments between them. The special comment for a type constructor with another type constructor following must be placed before the '|' character separating the two constructors.
The following sample interface file `foo.mli` illustrates the placement rules for comments in `.mli` files.

    (** The first special comment of the file is the comment associated
        with the whole module.*)

    (** Special comments can be placed between elements and are kept
        by the OCamldoc tool, but are not associated to any element.
        @-tags in these comments are ignored.*)

    (*******************************************************************)
    (** Comments like the one above, with more than two asterisks,
        are ignored. *)

    (** The comment for function f. *)
    val f : int -> int -> int
    (** The continuation of the comment for function f. *)

    (** Comment for exception My_exception, even with a simple comment
        between the special comment and the exception.*)
    (* Hello, I'm a simple comment :-) *)
    exception My_exception of (int -> int) * int

    (** Comment for type weather  *)
    type weather =
    | Rain of int (** The comment for construtor Rain *)
    | Sun (** The comment for constructor Sun *)

    (** Comment for type weather2  *)
    type weather2 =
    | Rain of int (** The comment for construtor Rain *)
    | Sun (** The comment for constructor Sun *)
    (** I can continue the comment for type weather2 here
      because there is already a comment associated to the last constructor.*)

    (** The comment for type my_record *)
    type my_record = {
        val foo : int ;    (** Comment for field foo *)
        val bar : string ; (** Comment for field bar *)
      }
      (** Continuation of comment for type my_record *)

    (** Comment for foo *)
    val foo : string
    (** This comment is associated to foo and not to bar. *)
    val bar : string
    (** This comment is assciated to bar. *)

    (** The comment for class my_class *)
    class my_class :
      object
        (** A comment to describe inheritance from cl *)
        inherit cl

        (** The comment for attribute tutu *)
        val mutable tutu : string

        (** The comment for attribute toto. *)
        val toto : int

        (** This comment is not attached to titi since
            there is a blank line before titi, but is kept
            as a comment in the class. *)

        val titi : string

        (** Comment for method toto *)
        method toto : string

        (** Comment for method m *)
        method m : float -> int
      end

    (** The comment for the class type my_class_type *)
    class type my_class_type =
      object
        (** The comment for variable x. *)
        val mutable x : int

        (** The commend for method m. *)
        method m : int -> int
    end

    (** The comment for module Foo *)
    module Foo =
      struct
        (** The comment for x *)
        val x : int

        (** A special comment that is kept but not associated to any element *)
      end

    (** The comment for module type my_module_type. *)
    module type my_module_type =
      sig
        (** The comment for value x. *)
        val x : int

        (** The comment for module M. *)
        module M =
          struct
            (** The comment for value y. *)
            val y : int

            (* ... *)
          end

      end

#### Comments in `.ml` files

A special comment is associated to an element if it is placed before the element and there is no blank line between the comment and the element. Meanwhile, there can be a simple comment between the special comment and the element. There are two exceptions, for type constructors and record fields in type definitions, whose associated comment must be placed after the constructor or field definition, without blank line between them. The special comment for a type constructor with another type constructor following must be placed before the '|' character separating the two constructors.
The following example of file `toto.ml` shows where to place comments in a `.ml` file.

    (** The first special comment of the file is the comment associated
        to the whole module.*)

    (** The comment for function f *)
    let f x y = x + y

    (** This comment is not attached to any element since there is another
        special comment just before the next element. *)

    (** Comment for exception My_exception, even with a simple comment
        between the special comment and the exception.*)
    (* A simple comment. *)
    exception My_exception of (int -> int) * int

    (** Comment for type weather  *)
    type weather =
    | Rain of int (** The comment for constructor Rain *)
    | Sun (** The comment for constructor Sun *)

    (** The comment for type my_record *)
    type my_record = {
        val foo : int ;    (** Comment for field foo *)
        val bar : string ; (** Comment for field bar *)
      }

    (** The comment for class my_class *)
    class my_class =
        object
          (** A comment to describe inheritance from cl *)
          inherit cl

          (** The comment for the instance variable tutu *)
          val mutable tutu = "tutu"
          (** The comment for toto *)
          val toto = 1
          val titi = "titi"
          (** Comment for method toto *)
          method toto = tutu ^ "!"
          (** Comment for method m *)
          method m (f : float) = 1
        end

    (** The comment for class type my_class_type *)
    class type my_class_type =
      object
        (** The comment for the instance variable x. *)
        val mutable x : int
        (** The commend for method m. *)
        method m : int -> int
      end

    (** The comment for module Foo *)
    module Foo =
      struct
        (** The comment for x *)
        val x : int
        (** A special comment in the class, but not associated to any element. *)
      end

    (** The comment for module type my_module_type. *)
    module type my_module_type =
      sig
        (* Comment for value x. *)
        val x : int
        (* ... *)
      end

### 15.2.2 The Stop special comment

The special comment `(**/**)` tells OCamldoc to discard elements placed after this comment, up to the end of the current class, class type, module or module type. For instance:

    class type foo =
      object
        (** comment for method m *)
        method m : string

        (**/**)

        (** This method won't appear in the documentation *)
        method bar : int
      end

    (** This value appears in the documentation, since the Stop special comment
        in the class does not affect the parent module of the class.*)
    val foo : string

    (**/**)
    (** The value bar does not appear in the documentation.*)
    val bar : string

    (** The type t does not appear either. *)
    type t = string

The `-no-stop` option to `ocamldoc` causes the Stop special comments to be ignored.

### 15.2.3 Syntax of documentation comments

The inside of documentation comments `(**`...`*)` consists of free-form text with optional formatting annotations, followed by optional tags giving more specific information about parameters, version, authors, ... The tags are distinguished by a leading @ character. Thus, a documentation comment has the following shape:

    (** The comment begins with a description, which is text formatted
       according to the rules described in the next section.
       The description continues until the first non-escaped '@' character.
       @author Mr Smith
       @param x description for parameter x
    *)

Some elements support only a subset of all @-tags. Tags that are not relevant to the documented element are simply ignored. For instance, all tags are ignored when documenting type constructors, record fields, and class inheritance clauses. Similarly, a `@param` tag on a class instance variable is ignored.
At last, `(**)` is the empty documentation comment.

### 15.2.4 Text formatting

Here is the BNF grammar for the simple markup language used to format text descriptions.
text ::= (text_element)`+`
text_element ::=

`| {[0-9]+ text}` | format text as a section header; the integer following `{` indicates the sectioning level.
`| {[0-9]+:label text}` | same, but also associate the name label to the current point. This point can be referenced by its fully-qualified label in a `{!` command, just like any other element.
`| {b text}` | set text in bold.
`| {i text}` | set text in italic.
`| {e text}` | emphasize text.
`| {C text}` | center text.
`| {L text}` | left align text.
`| {R text}` | right align text.
`| {ul list}` | build a list.
`| {ol list}` | build an enumerated list.
`| {{:string}text}` | put a link to the given address (given as a string) on the given text.
`| [string]` | set the given string in source code style.
`| {[string]}` | set the given string in preformatted source code style.
`| {v string v}` | set the given string in verbatim style.
`| {% string %}` | take the given string as raw LATEX code.
`| {!string}` | insert a reference to the element named string. string must be a fully qualified element name, for example `Foo.Bar.t`. The kind of the referenced element can be forced (useful when various elements have the same qualified name) with the following syntax: `{!kind: Foo.Bar.t}` where `kind` can be `module`, `modtype`, `class`, `classtype`, `val`, `type`, `exception`, `attribute`, `method` or `section`.
`| {!modules: string string ...}` | insert an index table for the given module names. Used in HTML only.
`| {!indexlist}` | insert a table of links to the various indexes (types, values, modules, ...). Used in HTML only.
`| {^ text}` | set text in superscript.
`| {_ text}` | set text in subscript.
`| escaped_string` | typeset the given string as is; special characters ('`{`', '`}`', '`[`', '`]`' and '`@`') must be escaped by a '`\`'
`| blank_line` | force a new line.

list ::=
`| ({- text})+`
`| ({li text})+`
A shortcut syntax exists for lists and enumerated lists:

    (** Here is a {b list}
    - item 1
    - item 2
    - item 3

    The list is ended by the blank line.*)

is equivalent to:

    (** Here is a {b list}
    {ul {- item 1}
    {- item 2}
    {- item 3}}
    The list is ended by the blank line.*)

The same shortcut is available for enumerated lists, using '`+`' instead of '`-`'. Note that only one list can be defined by this shortcut in nested lists.
In the description of a value, type, exception, module, module type, class or class type, the first sentence is sometimes used in indexes, or when just a part of the description is needed. The first sentence is composed of the first characters of the description, until

- the first dot followed by a blank, or
- the first blank line

outside of the following text formatting : `{ul list}`, `{ol list}`, `[string]`, `{[string]}`, `{v string v}`, `{% string%}`, `{!string}`, `{^ text}`, `{_ text}`.

### 15.2.5 Documentation tags (@-tags)

#### Predefined tags

The folowing table gives the list of predefined @-tags, with their syntax and meaning.

`@author` string | The author of the element. One author by `@author` tag. There may be several `@author` tags for the same element.
`@deprecated` text | The text should describe when the element was deprecated, what to use as a replacement, and possibly the reason for deprecation.
`@param` id text | Associate the given description (text) to the given parameter name id. This tag is used for functions, methods, classes and functors.
`@raise Exc` text | Explain that the element may raise the exception `Exc`.
`@return` text | Describe the return value and its possible values. This tag is used for functions and methods.
`@see <url>` text | Add a reference to the URL between '`<`' and '`>`' with the given text as comment.
`@see 'filename'` text | Add a reference to the given file name (written between single quotes), with the given text as comment.
`@see "document name"` text | Add a reference to the given document name (written between double quotes), with the given text as comment.
`@since` string | Indicates when the element was introduced.
`@version` string | The version number for the element.

#### Custom tags

You can use custom tags in the documentation comments, but they will have no effect if the generator used does not handle them. To use a custom tag, for example `foo`, just put `@foo` with some text in your comment, as in:

    (** My comment to show you a custom tag.
    @foo this is the text argument to the [foo] custom tag.
    *)

To handle custom tags, you need to define a custom generator, as explained in section 15.3.2.

## 15.3 Custom generators

OCamldoc operates in two steps:

1. analysis of the source files;
2. generation of documentation, through a documentation generator, which is an object of class `Odoc_args.class_generator`.

Users can provide their own documentation generator to be used during step 2 instead of the default generators. All the information retrieved during the analysis step is available through the `Odoc_info` module, which gives access to all the types and functions representing the elements found in the given modules, with their associated description.
The files you can used to define custom generators are installed in the `ocamldoc` sub-directory of the OCaml standard library.

### 15.3.1 The generator class

A generator class is a class of type `Odoc_args.doc_generator`. It has only one method
`generator : Odoc_info.Module.t_module list -> unit`
This method will be called with the list of analysed and possibly merged `Odoc_info.t_module` structures. Of course the class can have other methods, but the object of this class must be coerced to `Odoc_args.doc_generator` before being passed to the function
`Odoc_args.set_doc_generator : Odoc_args.doc_generator -> unit`
which installs the new documentation generator.
The following example shows how to define and install a new documentation generator. See the `odoc_fhtml` generator (in the Ocamldoc Hump) for a complete example.

    class my_doc_gen =
      object
        (* ... *)

        method generate module_list =
          (* ... *)
          ()

        (* ... *)
      end

    let my_generator = new my_doc_gen
    let _ = Odoc_args.set_doc_generator (my_generator :> Odoc_args.doc_generator)

Note: The new class can inherit from `Odoc_html.html`, `Odoc_latex.latex`, `Odoc_man.man`, `Odoc_texi.texi` or `Odoc_dot.dot`, and redefine only some methods to benefit from the existing methods.

### 15.3.2 Handling custom tags

Making a custom generator handle custom tags (see 15.2.5) is very simple.

#### For HTML

Here is how to develop a HTML generator handling your custom tags.
The class `Odoc_html.html` inherits from the class `Odoc_html.info`, containing a field `tag_functions` which is a list pairs composed of a custom tag (e.g. 'foo') and a function taking a `text` and returning HTML code (of type `string`). To handle a new tag `bar`, create a HTML generator class from the existing one and complete the `tag_functions` field:

    class my_gen =
      object(self)
        inherit Odoc_html.html

        (** Return HTML code for the given text of a bar tag. *)
        method html_of_bar t = (* your code here *)

        initializer
          tag_functions <- ("bar", self#html_of_bar) :: tag_functions
      end

Another method of the class `Odoc_html.info` will look for the function associated to a custom tag and apply it to the text given to the tag. If no function is associated to a custom tag, then the method prints a warning message on `stderr`.

#### For other generators

As for the HTML custom generator, you can define a new LATEX(resp. `man`) generator by inheriting from the class `Odoc_latex.latex` (resp. `Odoc_man.man`) and adding your own tag handler to the field `tag_functions`.

## 15.4 Adding command line options

The command line analysis is performed after loading the module containing the documentation generator, thus allowing command line options to be added to the list of existing ones. Adding an option can be done with the function
`Odoc_args.add_option : string * Arg.spec * string -> unit`
Note: Existing command line options can be redefined using this function.

### 15.4.1 Compilation and usage

#### Defining a custom generator class in one file

Let `custom.ml` be the file defining a new generator class. Compilation of `custom.ml` can be performed by the following command :
`ocamlc -I +ocamldoc -c custom.ml`
The file `custom.cmo` is created and can be used this way :
`ocamldoc -g custom.cmo other-options source-files`
It is important not to give the `-html` or any other option selecting a built in generator to `ocamldoc`, which would result in using this generator instead of the one you just loaded.

#### Defining a custom generator class in several files

It is possible to define a generator class in several modules, which are defined in several files `file1.ml[i]`, `file2.ml[i]`, ..., `fileN.ml[i]`. A `.cma` library file must be created, including all these files.
The following commands create the `custom.cma` file from files `file1.ml[i]`, ..., `fileN.ml[i]` :
`ocamlc -I +ocamldoc -c file1.ml[i]
ocamlc -I +ocamldoc -c file2.ml[i]
...
ocamlc -I +ocamldoc -c fileN.ml[i]
ocamlc -o custom.cma -a file1.cmo file2.cmo ... fileN.cmo`
Then, the following command uses `custom.cma` as custom generator:
`ocamldoc -g custom.cma other-options source-files`
Again, it is important not to give the `-html` or any other option selecting a built in generator to `ocamldoc`, which would result in using this generator instead of the one you just loaded.