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# EEP-48: Documentation storage and format

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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, use `code:get_doc/1`.

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

## The "Docs" storage

To look for documentation for a module named `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 no `.chunk` file exists, 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 (using `beam_lib`) or by removing the `doc/chunks` directory.

For example, languages such as Elixir and LFE attach the `Docs` chunk at compilation time, which can be controlled via a compiler flag, while other languages might want to generate the documentation separate from the compilation of the source code.

## 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 can 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`)

- `BeamLanguage` - an atom representing the language, for example: `erlang`, `elixir`, `lfe`, `alpaca`, and so on

- `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 EDoc'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 can be atom `none` if no documentation exists 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 finds 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, and so on. The official entities are: `function`, `type`, and `callback`. Other languages will add their own. For instance, Elixir and LFE might add `macro`.

- `Anno` - annotation (line, column, file) of the module documentation or of the definition itself (see `erl_anno`).

- `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 whitespace or newline. For example, `[<<"binary_to_atom(Binary, Encoding)">>, <<"when is_binary(Binary)">>]` can 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 can either be a binary or any Erlang term, both described by `Format`. If it is an Erlang term, then `Format` must be `<<"application/erlang+SUFFIX">>`, such as `<<"application/erlang+html">>` when the documentation is an Erlang representation of an HTML document. `Doc` can also be atom `none` if no documentation exists 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.

## See Also

`erl_anno`, `shell_docs`, `EEP-48 Chapter in EDoc's User's Guide`, `code:get_doc/1`

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