EEP-48: Documentation storage and format — kernel v10.6

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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 (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.

- `behaviours := [module()]` - a list of the behaviours implemented by this module.

- `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.

- `group := binary()` - when present, it allows tooling, such as shell autocompletion and documentation generators, to list all entries within the same group together, often using the group name as an indicator. It currently only applies to doc entries, not for module metadata.

- `since := binary()` - a binary representing the version such entry was added, such as `<<"1.3.0">>` or `<<"20.0">>`.

- `source_annos := [erl_anno:anno()]` - a list of source code locations. You may either store one for each clause or only for the first clause.

- `source_path := file:filename()` - the absolute location of the source file for this module. Applies only to the module metadata.

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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