Erlang -- EEP-48: Documentation storage and format

# 5 EEP-48: Documentation storage and format

This User's Guide describes the documentation storage format initially described in EEP-48. By standardizing how API documentation is stored, it will be possible to write tools that work across languages.

To fetch the EEP-48 documentation for a module you can use code:get_doc/1.

To render the EEP-48 documentation for an Erlang module you can use shell_docs:render/2.

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5.1 the "Docs" storage

To look for documentation for a module name example, a tool should:

Look for example.beam in the code path, parse the BEAM file and retrieve the Docs chunk. If the chunk is not available, it should look for "example.beam" in the code path and find the doc/chunks/example.chunk file in the application that defines the example module. If a .chunk file is not available, then documentation is not available.

The choice of using a chunk or the filesystem is completely up to the language or library. In both cases, the documentation can be added or removed at any moment by stripping the Docs chunk or by removing the doc/chunks directory.

For example, languages like Elixir and LFE attach the Docs chunk at compilation time, which can be controlled via a compiler flag. On the other hand, projects like OTP itself will likely generate the doc/chunks entries on a separate command, completely unrelated from code compilation.

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5.2 the "Docs" format

In both storages, the documentation is written in the exactly same format: an Erlang term serialized to binary via term_to_binary/1. The term may be optionally compressed when serialized. It must follow the type specification below:

    {docs_v1,
     Anno :: erl_anno:anno(),
     BeamLanguage :: atom(),
     Format :: binary(),
     ModuleDoc :: #{DocLanguage := DocValue} | none | hidden,
     Metadata :: map(),
     Docs ::
       [{{Kind, Name, Arity},
         Anno :: erl_anno:anno(),
         Signature :: [binary()],
         Doc :: #{DocLanguage := DocValue} | none | hidden,
         Metadata :: map()
        }]} when DocLanguage :: binary(),
                 DocValue :: binary() | term()

where in the root tuple we have:

Anno
  annotation (line, column, file) of the definition itself (see erl_anno(3))
BeamLanguage
  an atom representing the language, for example: erlang, elixir, lfe, alpaca, etc
Format
  the mime type of the documentation, such as <<"text/markdown">> or <<"application/erlang+html">>. For details of the format used by Erlang see the EEP-48 Chapter in Erl_Docgen's User's Guide.
ModuleDoc
  a map with the documentation language as key, such as <<"en">> or <<"pt_BR">>, and the documentation as a binary value. It may be the atom none in case there is no documentation or the atom hidden if documentation has been explicitly disabled for this entry.
Metadata
  a map of atom keys with any term as value. This can be used to add annotations like the authors of a module, deprecated, or anything else a language or documentation tool may find relevant.
Docs
  a list of documentation for other entities (such as functions and types) in the module.

For each entry in Docs, we have:

{Kind, Name, Arity}
  the kind, name and arity identifying the function, callback, type, etc. The official entities are: function, type and callback. Other languages will add their own. For instance, Elixir and LFE may add macro.
Anno
  annotation (line, column, file) of the module documentation or of the definition itself (see erl_anno(3)).
Signature
  the signature of the entity. It is is a list of binaries. Each entry represents a binary in the signature that can be joined with a whitespace or a newline. For example, [<<"binary_to_atom(Binary, Encoding)">>, <<"when is_binary(Binary)">>] may be rendered as a single line or two lines. It exists exclusively for exhibition purposes.
Doc
  a map with the documentation language as key, such as <<"en">> or <<"pt_BR">>, and the documentation as a value. The documentation may either be a binary or any Erlang term, both described by Format. If it is an Erlang term, then the Format must be <<"application/erlang+SUFFIX",>> such as <<"application/erlang+html">> when the documentation is an Erlang representation of an HTML document. The Doc may also be atom none in case there is no documentation or the atom hidden if documentation has been explicitly disabled for this entry.
Metadata
  a map of atom keys with any term as value.

This shared format is the heart of the EEP as it is what effectively allows cross-language collaboration.

The Metadata field exists to allow languages, tools and libraries to add custom information to each entry. This EEP documents the following metadata keys:

authors := [binary()]
  a list of authors as binaries.
cross_references := [module() | {module(), {Kind, Name, Arity}}]
  a list of modules or module entries that can be used as cross references when generating documentation.
deprecated := binary()
  when present, it means the current entry is deprecated with a binary that represents the reason for deprecation and a recommendation to replace the deprecated code.
since := binary()
  a binary representing the version such entry was added, such as <<"1.3.0">> or <<"20.0">>.
edit_url := binary()
  a binary representing a URL to change the documentation itself.

Any key may be added to Metadata at any time. Keys that are frequently used by the community can be standardized in future versions.

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5.3 See Also

erl_anno(3), shell_docs(3), EEP-48 Chapter in Erl_Docgen's User's Guide, code:get_doc/1