			-*- html -*-

	EDoc overview page


@author Richard Carlsson <richardc@it.uu.se>
@copyright 2003-2006 Richard Carlsson
@version {@version}
@title Welcome to EDoc

@doc EDoc is the Erlang program documentation generator. Inspired by the
Javadoc<sup><font size="-3">TM</font></sup> tool for the Java<sup><font
size="-3">TM</font></sup> programming language, EDoc is adapted to the
conventions of the Erlang world, and has several features not found in
Javadoc.

== Contents ==

<ol>
  <li>{@section Introduction}</li>
  <li>{@section Running EDoc}</li>
  <li>{@section The overview page}</li>
  <li>{@section Generic tags}</li>
  <li>{@section Overview tags}</li>
  <li>{@section Module tags}</li>
  <li>{@section Function tags}</li>
  <li>{@section References}</li>
  <li>{@section Notes on XHTML}</li>
  <li>{@section Wiki notation}</li>
  <li>{@section Macro expansion}</li>
  <li>{@section Type specifications}</li>
  <li>{@section Acknowledgements}</li>
</ol>

== Introduction ==

EDoc lets you write the documentation of an Erlang program as
comments in the source code itself, using <em>tags</em> on the form
"`@Name ...'". A source file does not have to contain tags
for EDoc to generate its documentation, but without tags the result will
only contain the basic available information that can be extracted from
the module.

A tag must be the first thing on a comment line, except for leading
'`%'' characters and whitespace. The comment must be between
program declarations, and not on the same line as any program text. All
the following text - including consecutive comment lines - up until the
end of the comment or the next tagged line, is taken as the
<em>content</em> of the tag.

Tags are associated with the nearest following program construct "of
significance" (the module name declaration and function
definitions). Other constructs are ignored; e.g., in:
```
   %% @doc Prints the value X.

   -record(foo, {x, y, z}).

   print(X) -> ...
'''
the `@doc' tag is associated with the function `print/1'.

Note that in a comment such as:
```% % @doc ...'''
the tag is ignored, because only the first '`%'' character is
considered "leading". This allows tags to be "commented out".

Some tags, such as `@type', do not need to be associated
with any program construct. These may be placed at the end of the file,
in the "footer".


== Running EDoc ==

The following are the main functions for running EDoc:
    <ul>
      <li>{@link edoc:application/2}: Creates documentation for a
	  typical Erlang application.</li>
      <li>{@link edoc:packages/2}: Creates documentation for one or
	  more packages, automatically locating source files.</li>
      <li>{@link edoc:files/2}: Creates documentation for a
	  specified set of source files.</li>
      <li>{@link edoc:run/3}: General interface function; the common
          back-end for the above functions. Options are documented here.</li>
    </ul>

Note that the function {@link edoc:file/2} belongs to the old, deprecated
interface (from EDoc version 0.1), and should not be used.


== The overview page ==

When documentation is generated for an entire application, an overview
page, or "front page", is generated. (The page you are now reading is an
overview page.) This should contain the high-level description or user
manual for the application, leaving the finer details to the
documentation for individual modules. By default, the overview page is
generated from the file `overview.edoc' in the target directory
(typically, this is the `doc' subdirectory of the application
directory); see {@link edoc_doclet} for details.

The format of the overview file is the same as for EDoc documentation
comments (see {@section Introduction}), except that the lines do not
have leading '`%'' characters. Furthermore, all lines before the first
tag line are ignored, and can be used as a comment. All tags in the
overview file, such as `@@doc', `@@version', etc., refer to the
application as a whole; see {@section Overview tags} for details.

Here is an example of the contents of an overview file:
```** this is the overview.doc file for the application 'frob' **

   @@author R. J. Hacker <rjh@acme.com>
   @@copyright 2007 R. J. Hacker
   @@version 1.0.0
   @@title Welcome to the `frob' application!
   @@doc `frob' is a highly advanced frobnicator with low latency,
   ...
'''


== Generic tags ==

The following tags can be used anywhere within a module:
<dl>
  <dt><a name="gtag-clear">`@clear'</a></dt>

      <dd>This tag causes all tags above it (up to the previous program
	construct), to be discarded, including the `@clear'
	tag itself. The text following the tag
	is also ignored. <em>This is typically only useful in code
	containing conditional compilation, when preprocessing is turned
	on.</em> (Preprocessing is turned off by default.) E.g., in
```-ifdef(DEBUG).
   %% @doc ...
   foo(...) -> ...
   -endif.
   %% @clear

   %% @doc ...
   bar(...) -> ...'''
        the `@clear' tag makes sure that EDoc does not see
	two `@doc' tags before the function `bar',
	even if the code for function `foo' is removed by
	preprocessing. (There is no way for EDoc to see what the first
	`@doc' tag "really" belongs to, since preprocessing
	strips away all such information.)</dd>

  <dt><a name="gtag-docfile">`@docfile'</a></dt>
      <dd>Reads a plain documentation file (on the same format as an
      overview file - see {@section The overview page} for details), and
      uses the tags in that file as if they had been written in place of
      the `@docfile' tag. The content is the name of the file to be
      read; leading and trailing whitespace is ignored. See also <a
      href="#gtag-headerfile">`@headerfile'</a>.</dd>

  <dt><a name="gtag-end">`@end'</a></dt>
      <dd>The text following this tag is always ignored. Use this to
      mark the end of the previous tag, when necessary, as e.g. in:
```%% ----------------------------------
   %% ...
   %% @doc ...
   %% ...
   %% @end
   %% ----------------------------------'''
      to avoid including the last "ruler" line in the
      `@doc' tag.

      <em>Note: using some other "dummy" `@'-tag for
      the same purpose might work in a particular implementation of
      EDoc, but is not guaranteed to.  Always use `@end'
      to ensure future compatibility.</em></dd>

  <dt><a name="gtag-headerfile">`@headerfile'</a></dt>
      <dd>Similar to the <a href="#gtag-docfile">`@docfile' tag</a>, but
      reads a file containing Erlang source code - generally this should
      be a header file (with the extension `.hrl'). If the file turns
      out to contain one or more function definitions or a module
      declaration, all tags that occur above the last such definition or
      module declaration are ignored, and EDoc will print a
      warning. This tag allows you to write documentation in a header
      file and insert it at a specific place in the documentation, even
      if the header file is used (i.e., included) by several
      modules. The `includes' option can be used to specify a search
      path (see {@link edoc:read_source/2}).</dd>

  <dt><a name="gtag-todo">`@todo' (or `@TODO')</a></dt>
      <dd>Attaches a To-Do note to a function, module, package, or
      overview-page. The content can be any XHTML text describing
      the issue, e.g.:
```%% @TODO Finish writing the documentation.'''
      or
```%% @todo Implement <a href="http://www.ietf.org/rfc/rfc2549.txt">RFC 2549</a>.'''
      These tags can also be written as "`TODO:'", e.g.:
```%% TODO: call your mother'''
      see {@section Wiki notation} for more information. To-Do notes are
      normally not shown unless the `todo' option is turned on (see
      {@link edoc:get_doc/2}).</dd>

  <dt><a name="gtag-type">`@type'</a></dt>
      <dd>Documents an abstract data type or type alias. The content
      consists of a type declaration or definition, optionally
      followed by a period ('`.'') separator and XHTML
      text describing the type (i.e., its purpose, use, etc.). There
      must be at least one whitespace character between the '`.'' and
      the text. See {@section Type specifications} for syntax and
      examples.
      All data type descriptions are placed in a separate section of
      the documentation, regardless of where the tags occur.

      Instead of specifying the complete type alias in an EDoc
      documentation comment, type definitions from the actual
      Erlang code can be re-used for documentation.
      See {@section Type specifications} for examples.</dd>
</dl>


== Overview tags ==

The following tags can be used in an overview file.
<dl>
  <dt><a name="otag-author">`@author'</a></dt>
      <dd>See the <a href="#mtag-author">`@author' module tag</a> for
      details.</dd>

  <dt><a name="otag-copyright">`@copyright'</a></dt>
      <dd>See the <a href="#mtag-copyright">`@copyright' module tag</a>
      for details.</dd>

  <dt><a name="otag-doc">`@doc'</a></dt>
      <dd>See the <a href="#mtag-doc">`@doc' module tag</a> for
      details.</dd>

  <dt><a name="otag-reference">`@reference'</a></dt>
      <dd>See the <a href="#mtag-reference">`@reference' module tag</a>
      for details.</dd>

  <dt><a name="otag-see">`@see'</a></dt>
      <dd>See the <a href="#mtag-see">`@see' module tag</a> for
      details.</dd>

  <dt><a name="otag-since">`@since'</a></dt>
      <dd>See the <a href="#mtag-since">`@since' module tag</a> for
      details.</dd>

  <dt><a name="otag-title">`@title'</a></dt>
      <dd>Specifies a title for the overview page. This tag can
      <em>only</em> be used in an overview file. The content can be
      arbitrary text.</dd>

  <dt><a name="otag-version">`@version'</a></dt>
      <dd>See the <a href="#mtag-version">`@version' module
      tag</a> for details.</dd>

</dl>


== Module tags ==

The following tags can be used before a module declaration:
<dl>
  <dt><a name="mtag-author">`@author'</a></dt>
      <dd>Specifies the name of an author, along with contact
      information. An e-mail address can be given within `<...>'
      delimiters, and a URI within `[...]' delimiters. Both e-mail and
      URI are optional, and any surrounding whitespace is stripped from
      all strings.

      The name is the first nonempty string that is not within `<...>'
      or `[...]', and does not contain only whitespace. (In other words,
      the name can come before, between, or after the e-mail and URI,
      but cannot be split up; any sections after the first are ignored.)
      If an e-mail address is given, but no name, the e-mail string will
      be used also for the name. If no `<...>' section is present, but
      the name string contains an '`@'' character, it is assumed to be
      an e-mail address. Not both name and e-mail may be left out.

      Examples:
```%% @author Richard Carlsson'''

```%% @author Richard Carlsson <richardc@it.uu.se>
   %%   [http://user.it.uu.se/~richardc/]'''

```%% @author <richardc@it.uu.se>'''

```%% @author richardc@it.uu.se [http://user.it.uu.se/~richardc/]'''
      </dd>

<dt><a name="mtag-copyright">`@copyright'</a></dt>
      <dd>Specifies the module copyrights. The content can be
      arbitrary text; for example:
```
   %% @copyright 2001-2003 Richard Carlsson'''
      </dd>

  <dt><a name="mtag-deprecated">`@deprecated'</a></dt>
      <dd>Mark the module as deprecated, indicating that it should no
      longer be used. The content must be well-formed XHTML, and should
      preferably include a `@{@link}' reference to a
      replacement; as in:
```
   %% @deprecated Please use the module @{@link foo} instead.'''
      </dd>

  <dt><a name="mtag-doc">`@doc'</a></dt>
      <dd>Describes the module, using well-formed XHTML text. The
      first sentence is used as a summary (see the
      <a href="#ftag-doc">`@doc' function tag</a> for details). For
      example.:
```%% @doc This is a <em>very</em> useful module. It is ...'''</dd>

  <dt><a name="mtag-hidden">`@hidden'</a></dt>
      <dd>Marks the module so that it will not appear in the
      documentation (even if "private" documentation is generated).
      Useful for sample code, test modules, etc. The content can be
      used as a comment; it is ignored by EDoc.</dd>

  <dt><a name="mtag-private">`@private'</a></dt>
      <dd>Marks the module as private (i.e., not part of the public
      interface), so that it will not appear in the normal
      documentation. (If "private" documentation is generated, the
      module will be included.) The content can be used as a comment; it
      is ignored by EDoc.</dd>

  <dt><a name="mtag-reference">`@reference'</a></dt>
      <dd>Specifies a reference to some arbitrary external resource,
      such as an article, book, or web site. The content must be
      well-formed XHTML text. Examples:
```%% @reference Pratchett, T., <em>Interesting Times</em>,
   %% Victor Gollancz Ltd, 1994.'''

```%% @reference See <a href="www.google.com">Google</a> for
   %% more information.'''
      </dd>

  <dt><a name="mtag-see">`@see'</a></dt>
      <dd>See the <a href="#ftag-see">`@see' function tag</a>
      for details.</dd>

  <dt><a name="mtag-since">`@since'</a></dt>
      <dd>Specifies when the module was introduced, with respect to
      the application, package, release or distribution it is part
      of. The content can be arbitrary text.</dd>

  <dt><a name="mtag-version">`@version'</a></dt>
      <dd>Specifies the module version. The content can be arbitrary
      text.</dd>

</dl>



== Function tags ==

The following tags can be used before a function definition:
<dl>
  <dt><a name="ftag-deprecated">`@deprecated'</a></dt>
      <dd>See the <a href="#mtag-deprecated">`@deprecated'
      module tag</a> for details.</dd>

  <dt><a name="ftag-doc">`@doc'</a></dt>
      <dd>XHTML text describing the function. The first
      sentence of the text is used as a quick summary; this ends at
      the first period character ('`.'') or exclamation mark
      ('`!'') that is followed by a whitespace character, a
      line break, or the end of the tag text, and is not within XML
      markup. (As an exception, the first sentence may be within an
      initial paragraph element)</dd>

  <dt><a name="ftag-equiv">`@equiv'</a></dt>
      <dd>Specify equivalence to another function call/expression.
      The content must be a proper Erlang expression. If the
      expression is a function call, a cross-reference to the called
      function is created automatically. Typically, this tag is used
      instead of `@doc'. </dd>

  <dt><a name="ftag-hidden">`@hidden'</a></dt>
      <dd>Marks the function so that it will not appear in the
      documentation (even if "private" documentation is generated).
      Useful for debug/test functions, etc. The content can be
      used as a comment; it is ignored by EDoc.</dd>

  <dt><a name="ftag-private">`@private'</a></dt>
      <dd>Marks the function as private (i.e., not part of the public
      interface), so that it will not appear in the normal
      documentation. (If "private" documentation is generated, the
      function will be included.) Only useful for exported functions,
      e.g. entry points for `spawn'. (Non-exported functions are
      always "private".) The content can be used as a comment; it is
      ignored by EDoc.</dd>

  <dt><a name="ftag-see">`@see'</a></dt>
      <dd>Make a reference to a module, function, datatype, or
      application. (See {@section References}.)
      The content consists of a reference, optionally followed by a
      period ('`.''), one or more whitespace characters, and
      XHTML text to be used for the label; for example "`@see edoc'" or
      "`@see edoc. <b>EDoc</b>'". If no label text is specified, the
      reference itself is used as the label.</dd>

  <dt><a name="ftag-since">`@since'</a></dt>
      <dd>Specifies in what version of the module the function was
      introduced; cf. the
      <a href="#mtag-version">`@version'
      module tag</a>. The content can be arbitrary text.</dd>

  <dt><a name="ftag-spec">`@spec'</a></dt>
      <dd>Used to specify the function type; see {@section Type
      specifications} for syntax details. If the function name is
      included in the specification, it must match the name in the
      actual code. When parameter names are not given in the
      specification, suitable names will be taken from the source
      code if possible, and otherwise synthesized.

      Instead of specifying the complete function type in an EDoc
      documentation comment, specifications from the actual
      Erlang code can be re-used for documentation.
      See {@section Type specifications} for examples.</dd>

  <dt><a name="ftag-throws">`@throws'</a></dt>
      <dd>Specifies which types of terms may be thrown by the
      function, if its execution terminates abruptly due to a call to
      `erlang:throw(Term)'. The content is a type expression (see {@section
      Type specifications}), and can be a union type.

      Note that exceptions of type `exit' (as caused by calls to
      `erlang:exit(Term)') and `error' (run-time errors such as `badarg'
      or `badarith') are not viewed as part of the normal interface of
      the function, and cannot be documented with the `@throws' tag.</dd>

  <dt><a name="ftag-type">`@type'</a></dt>
      <dd>See the <a href="#gtag-type">`@type' generic tag</a>
      for details. Placing a `@type' tag by a function
      definition may be convenient, but does not affect where the
      description is placed in the generated documentation.</dd>
</dl>



== References ==

In several contexts (`@see' tags, `@link' macros, etc.), EDoc lets
you refer to the generated documentation for modules, functions,
datatypes, and applications, using a simple and compact syntax. The
possible formats for references are:
<table border="1" summary="reference syntax">
  <tr><th>Reference syntax</th><th>Example</th><th>Scope</th></tr>
  <tr><td>`Module'</td><td>{@link edoc_run}, `erl.lang.list'</td><td>Global</td></tr>
  <tr><td>`Package.*'</td><td>`erl.lang.*'</td><td>Global</td></tr>
  <tr><td>`Function/Arity'</td><td>`file/2'</td><td>Within module</td></tr>
  <tr><td>`Module:Function/Arity'</td><td>{@link edoc:application/2}</td><td>Global</td></tr>
  <tr><td>`Type()'</td><td>`filename()'</td><td>Within module</td></tr>
  <tr><td>`Module:Type()'</td><td>{@link edoc:edoc_module()}</td><td>Global</td></tr>
  <tr><td>`//Application'</td><td>{@link //edoc}</td><td>Global</td></tr>
  <tr><td>`//Application/Module'</td><td>{@link //edoc/edoc_doclet}</td><td>Global</td></tr>
  <tr><td>`//Application/Module:Function/Arity'</td><td>{@link //edoc/edoc_run:file/1}</td><td>Global</td></tr>
  <tr><td>`//Application/Module:Type()'</td><td>{@link //edoc/edoc:edoc_module()}</td><td>Global</td></tr>
</table>


EDoc will resolve references using the information it finds in
`edoc-info'-files at the locations specified with the `doc_path'
option. EDoc will automatically (and somewhat intelligently) try to find
any local `edoc-info'-files using the current code path, and add them to
the end of the `doc_path' list. The target doc-directory is also
searched for an existing info file; this allows documentation to be
built incrementally. (Use the `new' option to ignore any old info
file.)

Note that if the name of a module, function or datatype is explicitly
qualified with an application (as in "`//edoc/edoc_run'"), this
overrides any other information about that name, and the reference will
be made relative to the location of the application (if it can be
found). This makes it possible to refer to e.g. a module "`fred'" as
"`//foo/fred'" without accidentally getting a reference to
"`//bar/fred'". You should not use this form of explicit references for
names that are local to the application you are currently creating -
they will always be resolved correctly.

Note that module-local references such as `file/2' only work properly
within a module. In an overview-page like this (i.e., the one you are
currently reading), no module context is available.

== Notes on XHTML ==

In several places, XHTML markup can be used in the documentation
text, in particular in `@doc' tags. The main differences from
HTML are the following:
<ul>
  <li>All elements must have explicit start and end tags, and be
      correctly nested. This means that you cannot e.g. write a
      `<li>' tag without also writing a corresponding `</li>'
      tag in the right place. This could be an annoyance
      at times, but has the great advantage that EDoc can report all
      malformed XHTML in your source code, rather than propagate the
      errors to the generated documentation.</li>
  <li>XHTML tag and attribute names should always be lower-case.</li>
  <li>Attributes must be quoted, as in e.g. `<a
      name="top">'.</li>
</ul>
To write an element like the HTML `<br>', which has no actual content,
you can write either the full `<br></br>', or better, use the XHTML
abbreviated form `<br/>'.

Since the purpose of EDoc is to document programs, there is also a
limited form of "wiki"-syntax available for making program code easier
to write inline (and to make the doc-comments easier to read).
See {@section Wiki notation} for details.

The HTML heading tags `h1' and `h2' are reserved for use by EDoc.
Headings in documentation source code should start at `h3'. There is
however a special syntax for writing headings which avoids using
specific level numbers altogether; see {@section Headings} for details.

EDoc uses {@link //xmerl. XMerL} to parse and export XML markup.


== Wiki notation ==

When EDoc parses XHTML, it does additional pre- and post-processing of
the text in order to expand certain notation specific to EDoc into
proper XHTML markup. This "wiki" ([http://en.wikipedia.org/wiki/Wiki])
notation is intended to make it easier to write source code
documentation.

 === Empty lines separate paragraphs ===

Leaving an empty line in XHTML text (i.e., a line which except for
any leading start-of-comment '<tt>%</tt>' characters contains only
whitespace), will make EDoc split the text before and
after the empty line into separate paragraphs. For example:
```%% @doc This will all be part of the first paragraph.
   %% It can stretch over several lines and contain <em>any
   %% XHTML markup</em>.
   %% 
   %% This is the second paragraph. The above line is
   %% regarded as "empty" by EDoc, even though it ends with
   %% a space.'''
will generate the following text:
<blockquote><p>This will all be part of the first paragraph. It can
stretch over several lines and contain <em>any XHTML markup</em>.</p>
This is the second paragraph. The above line is regarded as "empty" by
EDoc, even though it ends with a space.</blockquote>

Paragraph splitting takes place after the actual XHTML parsing. It only
affects block-level text, and not e.g., text within `<pre>' markup, or
text that is already within `<p>' markup.

 === Headings ===

Section headings, sub-headings, and sub-sub-headings, can be written
using the following notation:
```== Heading ==
   === Sub-heading ===
   ==== Sub-sub-heading ===='''
Such a heading must be alone on a line, except for whitespace, and
cannot be split over several lines. A link target is automatically
created for the heading, by replacing any whitespace within the text by
a single underscore character. E.g.,
```== Concerning Hobbits =='''
is equivalent to
```<h3><a name="Concerning_Hobbits">Concerning Hobbits</a></h3>'''
Thus, headings using this notation should not contain characters that
may not be part of URL labels, except for whitespace. If you need to
create such headings, you have to use the explicit XHTML markup.

A hypertext link to a heading written this way can be created using the
`@section' macro, which transforms the argument text into a label as
described above. E.g.,
```@{@section Concerning Hobbits}'''
is equivalent to writing
```<a href="#Concerning_Hobbits">Concerning Hobbits</a>'''

The above expansions take place before XML parsing.

 === External links ===

Writing a URL within brackets, as in "`[http://www.w3c.org/]'", will
generate a hyperlink such as [http://www.w3c.org/], using the URL both
for the destination and the label of the reference, equivalent to writing
"`<a href="http://www.w3c.org/"><tt>http://www.w3c.org/</tt></a>'". This
short-hand keeps external URL references short and readable. The
recognized protocols are `http', `ftp', and `file'. This expansion takes
place before XML parsing.

 === TODO-notes ===

Lines that begin with the text "`TODO:'" (the colon is required) are
recognized as tags, as if they had been written as "`@todo ...'" (see <a
href="#gtag-todo">@todo tags</a> for further details).

 === Verbatim quoting ===

In XHTML text, the '<code>&#x60;</code>' character (Unicode `000060',
known as "grave accent" or "back-quote") can be used for verbatim
quoting. This expansion takes place before XML parsing.

<ul>
  <li>A character sequence "<code>&#x60;...'</code>" or
      "<code>&#x60;&#x60;...''</code>" will be expanded to
      "`<code>...</code>'", where all occurrences of the special XML
      characters '`<'' and '`&'' (and for completeness, also '`>'') in
      the quoted text have been escaped to "`&lt;'", "`&amp;'", and
      "`&gt;'", respectively.
      All whitespace is stripped from the beginning and end of the
      quoted text.

      Double back-quotes "<code>&#x60;&#x60;...''</code>" can be used
      to quote text containing single '`` ' ''' characters. The automatic
      stripping of any surrounding whitespace makes it possible to write
      things like "<code>&#x60;&#x60; 'foo@bar' ''</code>".

      To quote text containing "<code>''</code>" verbatim,
      explicit `<code>' markup or similar must be used.</li>

  <li>A character sequence "<code>&#x60;&#x60;&#x60;...'''</code>"
      will be expanded to "`<pre><![CDATA[...]]></pre>'", which disables
      all XML markup within the quoted text, and displays the result in
      fixed-font with preserved indentation. Whitespace is stripped from
      the end of the quoted text, but not from the beginning, except for
      whole leading lines of whitespace. This is
      useful for multi-line code examples, or displayed
      one-liners.</li>

  <li>To produce a single '<code>&#x60;</code>'-character in XML
      without beginning a new quote, you can write "<code>&#x60;'</code>"
      (no space between the '<code>&#x60;</code>' and the '<code>'</code>').
      You can of course also use the XML character entity
      "`&#x60;'".</li>
</ul>

Examples:
  ```%% @doc ...where the variable `Foo' refers to... '''

  ```%% @doc ...returns the atom `` 'foo@erlang.org' ''... '''

  <pre>
     %% @doc ...use the command &#x60;&#x60;&#x60;erl -name foo''' to...</pre>

  <pre>
     %% @doc ...as in the following code:
     %% &#x60;&#x60;&#x60;f(X) ->
     %%       case X of
     %%          ...
     %%       end'''</pre>

  <pre>
     %% @doc ...or in the following:
     %% &#x60;&#x60;&#x60;
     %%     g(X) ->
     %%       fun () -> ... end
     %% '''</pre>


== Macro expansion ==

Before the content of a tag is parsed, the text undergoes <em>macro
expansion</em>. The syntax for macro calls is:
<pre>
    @{@<em>name</em>}</pre>
or
<pre>
    @{@<em>name</em> <em>argument</em>}</pre>
where <em>name</em> and <em>argument</em> are separated by one or more
whitespace characters. The argument can be any text, which may contain
other macro calls. The number of non-escaped "<code>@{@</code>" and
"`}'" delimiters must be balanced.

 The argument text is first expanded in the current environment, and
the result is bound to the <em>macro parameter</em>, written
<code>@{@?}</code>. (If no argument is given, <code>@{@?}</code> is
bound to the empty string.)  The macro definition is then substituted
for the call, and expansion continues over the resulting text. Recursive
macro expansions are not allowed.

 === User-defined macros ===

Users can define their own macros by using the `def' EDoc
option; see {@link edoc:file/2} and {@link edoc:get_doc/2} for more
information. User-defined macros override predefined macros.

 === Predefined macros ===

<dl>
  <dt><a name="predefmacro-date"><code>@{@date}</code></a></dt>
      <dd>Expands to the current date, as "<tt>Month Day Year</tt>",
      e.g. "{@date}".</dd>

  <dt><a name="predefmacro-docRoot"><code>@{@docRoot}</code></a></dt>
      <dd>Expands to the relative URL path (such as
      `"../../.."') from the current page to the root
      directory of the generated documentation. This can be used to
      create XHTML references such as `<img
      src="@{@docRoot}/images/logo.jpeg">' that are independent of how
      deep down in a package structure they occur. If packages are not
      used (i.e., if all modules are in the "empty" package),
      <code>@{@docRoot}</code> will always resolve to the empty
      string.</dd>

  <dt><a name="predefmacro-link"><code>@{@link <em>reference</em>.
		    <em>description</em>}</code></a></dt>
      <dd>This creates a hypertext link; cf. the
      <a href="#ftag-see">`@see' function tag</a> above for
      details. The description text (including the period separator)
      is optional; if no text is given, the reference itself is
      used. For example, <code>@{@link edoc:file/2}</code> creates the
      link {@link edoc:file/2}, and `@{@link edoc:file/2. <em>this link</em>}'
      creates {@link edoc:file/2. <em>this link</em>}.</dd>

  <dt><a name="predefmacro-module"><code>@{@module}</code></a></dt>
      <dd>Expands to the name of the current module. Only defined when a
      module is being processed.</dd>

  <dt><a name="predefmacro-package"><code>@{@package}</code></a></dt>
      <dd>Expands to the name of the current package.</dd>

  <dt><a name="predefmacro-section"><code>@{@section
        <em>heading</em>}</code></a></dt>
      <dd>Expands to a hypertext link to the specified section heading;
      see {@section Headings} for more information.</dd>

  <dt><a name="predefmacro-time"><code>@{@time}</code></a></dt>
      <dd>Expands to the current time, as "<tt>Hr:Min:Sec</tt>",
      e.g. "{@time}".</dd>

  <dt><a name="predefmacro-type"><code>@{@type
      <em>type-expression</em>}</code></a></dt>
      <dd>Formats a type expression within `<code>...</code>'
      markup and with hypertext links for data types. For example,
      <code>@{@type {options, List::edoc:option_list()@@}}</code>
      generates "{@type {options, List::edoc:option_list()@}}". (Cf.
      {@section Escape sequences}.)</dd>

  <dt><a name="predefmacro-version"><code>@{@version}</code></a></dt>
      <dd>Intended for use in <a href="#mtag-version">`@version'
      tags</a>. Defaults to a timestamp using `@{@date}' and `@{@time}'.
      Typically, this macro is redefined by the user when an official
      release of the application is generated.</dd>
</dl>

 === Escape sequences ===

To prevent certain characters from being interpreted as delimiters,
for example to produce the text "<code>@{@</code>" in the output, or use a
'`}'' character in the argument text of a macro call, the
following escape sequences may be used: <dl>
  <dt><code>@@{</code></dt>
      <dd>Expands to "`{'". Example:
```
   %% @doc A macro call starts with the sequence "@@@{@".'''
      </dd>
  <dt><code>@@}</code></dt>
      <dd>Expands to "`}'". Example:
```
   %% @doc ...@{@foo ...{Key, Value@@}...}...'''
      </dd>
  <dt><code>@@@@</code></dt>
      <dd>Expands to "`@'". Example:
```
   %% @doc Contact us at support@@@@@{@hostname}'''
      Will generate the text "Contact us at support@vaporware.acme.com"
      if the macro `hostname' is bound to
      "`vaporware.acme.com'". Also:
```
   %% @doc You might want to write something like
   %% @@@@foo that will expand to @@foo and does not start
   %% a new tag even if it appears first in a line.'''
      </dd>
</dl>


== Type specifications ==

 === Function specifications ===

<note>Although the syntax described in the following can still be used
for specifying functions we recommend that Erlang specifications as
described in <seealso marker="doc/reference_manual:typespec"> Types
and Function Specification</seealso> should be added to the source
code instead. This way the analyses of <seealso
marker="dialyzer:dialyzer">Dialyzer</seealso>'s can be utilized in the
process of keeping the documentation consistent and up-to-date.
Erlang specifications will be used unless there is also a function
specification (a `@spec' tag followed by a type) with the same name.
</note>

The following grammar describes the form of the specifications following
a `@spec' tag. A '`?'' suffix implies that the element is optional.
Function types have higher precedence than union types; e.g., "`(atom())
-> atom() | integer()'" is parsed as `((atom()) -> atom()) | integer()',
not as `(atom()) -> (atom() | integer())'.

<table summary="specification syntax grammar">
<tbody valign="baseline">
  <tr>
    <td><code>Spec</code></td>
    <td>::=</td>
    <td><code>FunType "where"? DefList?
	  <br/>| FunctionName FunType "where"? DefList?</code></td>
  </tr>
  <tr>
    <td><code>FunctionName</code></td>
    <td>::=</td>
    <td><code>Atom</code></td>
  </tr>
  <tr>
    <td><code>FunType</code></td>
    <td>::=</td>
    <td><code>"(" UnionTypes? ")" "->" UnionType</code></td>
  </tr>
  <tr>
    <td><code>UnionTypes</code></td>
    <td>::=</td>
    <td><code>UnionType
	  <br/>| UnionType "," UnionTypes</code></td>
  </tr>
  <tr>
    <td><code>UnionType</code></td>
    <td>::=</td>
    <td><code>UnionList
	  <br/>| Name "::" UnionList</code></td>
  </tr>
  <tr>
    <td><code>Name</code></td>
    <td>::=</td>
    <td><code>Variable</code></td>
  </tr>
  <tr>
    <td><code>UnionList</code></td>
    <td>::=</td>
    <td><code>Type
	  <br/>| Type "+" UnionList
	  <br/>| Type "|" UnionList</code></td>
  </tr>
  <tr>
    <td><code>Type</code></td>
    <td>::=</td>
    <td><code>TypeVariable
	  <br/>| Atom
	  <br/>| Integer
	  <br/>| Float
          <br/>| Integer ".." Integer
	  <br/>| FunType
          <br/>| "fun(" FunType ")"
          <br/>| "fun(...)"
	  <br/>| "{" UnionTypes? "}"
          <br/>| "#" Atom "{" Fields? "}"
	  <br/>| "[" "]"
	  <br/>| "[" UnionType "]"
          <br/>| "[" UnionType "," "..." "]"
	  <br/>| "(" UnionType ")"
          <br/>| BinType
	  <br/>| TypeName "(" UnionTypes? ")"
	  <br/>| ModuleName ":" TypeName "(" UnionTypes? ")"
	  <br/>| "//" AppName "/" ModuleName ":" TypeName "(" UnionTypes? ")"</code></td>
  </tr>
  <tr>
    <td><code>Fields</code></td>
    <td>::=</td>
    <td><code>Field
          <br/>| Fields "," Fields</code></td>
  </tr>
  <tr>
    <td><code>Field</code></td>
    <td>::=</td>
    <td><code>Atom "=" UnionList</code></td>
  </tr>
  <tr>
    <td><code>BinType</code></td>
    <td>::=</td>
    <td><code>"&lt;&lt;&gt;&gt;"
          <br/>| "&lt;&lt;" BaseType "&gt;&gt;"
          <br/>| "&lt;&lt;" UnitType "&gt;&gt;"
          <br/>| "&lt;&lt;" BaseType "," UnitType "&gt;&gt;"</code></td>
  </tr>
  <tr>
    <td><code>BaseType</code></td>
    <td>::=</td>
    <td><code>"_" ":" Integer</code></td>
  </tr>
  <tr>
    <td><code>UnitType</code></td>
    <td>::=</td>
    <td><code>"_" ":" "_" "*" Integer</code></td>
  </tr>
  <tr>
    <td><code>TypeVariable</code></td>
    <td>::=</td>
    <td><code>Variable</code></td>
  </tr>
  <tr>
    <td><code>TypeName</code></td>
    <td>::=</td>
    <td><code>Atom</code></td>
  </tr>
  <tr>
    <td><code>ModuleName</code></td>
    <td>::=</td>
    <td><code>Atom
          <br/>| ModuleName "." Atom</code></td>
  </tr>
  <tr>
    <td><code>AppName</code></td>
    <td>::=</td>
    <td><code>Atom</code></td>
  </tr>
  <tr>
    <td><code>DefList</code></td>
    <td>::=</td>
    <td><code>Def
	  <br/>| DefList Def
	  <br/>| DefList "," Def</code></td>
  </tr>
  <tr>
    <td><code>Def</code></td>
    <td>::=</td>
    <td><code>TypeVariable "=" UnionList
	  <br/>| TypeName "(" TypeVariables? ")" "=" UnionType</code></td>
  </tr>
  <tr>
    <td><code>TypeVariables</code></td>
    <td>::=</td>
    <td><code>TypeVariable
	  <br/>| TypeVariable "," TypeVariables</code></td>
  </tr>
</tbody>
</table>


Examples:
```
    -spec my_function(X :: integer()) -> integer().
    %% @doc Creates ...'''
```
    %% @spec my_function(X::integer()) -> integer()'''
```
    %% @spec (X::integer()) -> integer()'''
```
    %% @spec sqrt(float()) -> float()'''
```
    %% @spec pair(S, T) -> {S, T}'''
```
    %% @spec append(List, List) -> List
    %%       List = [term()]'''
```
    %% @spec append(A::List, B::List) -> List
    %%       List = [Item]
    %%       Item = term()'''
```
    %% @spec open(File::filename()) -> FileDescriptor
    %% where
    %%       filename() = string() + atom(),
    %%       FileDescriptor = term()'''
```
    %% @spec close(graphics:window()) -> ok'''

The first example shows the recommended way of specifying functions.

In the above examples, `X', `A', `B',
and `File' are parameter names, used for referring to the
parameters from the documentation text. The <em>type variables</em>
`S', `T' and `List' are used to
simplify the type specifications, and may be supplied with
definitions. It is also possible to give definitions for named types,
which means that the name is simply an alias. (Use the
`@type' tag to document abstract data types.) If a named type
is defined in another module, it can be referred to as
`Module:TypeName(...)'. Note that the keyword '`where'' is optional
before a list of definitions, and that the definitions in the list may
optionally be separated by '`,''.

Both the '`|'' and the '`+'' character may be
used to separate alternatives in union types; there is no semantic
difference. Note that the notation `[Type]' means "proper
(nil-terminated) list whose elements all belong to `Type'";
For example, `[atom()|integer()]' means the same thing as
`[atom()+integer()]', i.e., a proper list of atoms and/or
integers.

If only a type variable is given for a parameter, as in
"`pair(S, T) -> ...'", the same variable name may implicitly
be used as the parameter name; there is no need to write
"`pair(S::S, T::T) -> ...'".

EDoc automatically extracts possible parameter names from the source
code, to be used if no parameter name is given in the specification (or
if the specification is missing altogether). If this fails, EDoc will
generate a dummy parameter name, such as `X1'. This way, EDoc
can often produce helpful documentation even for code that does not
contain any annotations at all.

 === Type definitions ===

<note>Although the syntax described in the following can still be used
for specifying types we recommend that Erlang types as described in
<seealso marker="doc/reference_manual:typespec"> Types and Function
Specification</seealso> should be added to the source code instead.
Erlang types will be used unless there is a type alias with the same
name.</note>

The following grammar (see above for auxiliary definitions) describes
the form of the definitions that may follow a `@type' tag:

<table summary="type definition grammar">
<tbody valign="baseline">
  <tr>
    <td><code>Typedef</code></td>
    <td>::=</td>
    <td><code>TypeName "(" TypeVariables? ")" DefList?
	  <br/>| TypeName "(" TypeVariables? ")" "=" UnionList DefList?</code></td>
  </tr>
</tbody>
</table>

(For a truly abstract data type, no equivalence is specified.) The main
definition may be followed by additional local definitions. Examples:
```
    -type my_list(X) :: [X]. %% A special kind of lists ...'''
```
    -opaque another_list(X) :: [X].
    %% another_list() is a kind of list...'''
```
    %% @type myList(X). A special kind of lists ...'''
```
    %% @type filename() = string(). Atoms not allowed!'''
```
    %% @type thing(A) = {thong, A}
    %%           A = term().
    %%   A kind of wrapper type thingy.'''

The first two examples show the recommended way of specifying types.

 === Pre-defined data types ===

The following data types are predefined by EDoc, and may not be
redefined:
```
    any()
    arity()
    atom()
    binary()
    bitstring()
    bool()        (allowed, but use boolean() instead)
    boolean()
    byte()
    char()
    cons()
    deep_string()
    float()
    function()
    integer()
    iodata()
    iolist()
    list()
    maybe_improper_list()
    mfa()
    module()
    nil()
    neg_integer()
    node()
    non_neg_integer()
    nonempty_improper_list()
    nonempty_list()
    nonempty_maybe_improper_list()
    nonempty_string()
    none()
    number()
    pid()
    port()
    pos_integer()
    reference()
    string()
    term()
    timeout()
    tuple()
'''
Details:
<ul>
  <li>`any()' means "any Erlang data type".
      `term()' is simply an alias for `any()'.</li>
  <li>`atom()', `binary()',
      `float()', `function()',
      `integer()', `pid()', `port()'
      and `reference()' are primitive data types of
      the Erlang programming language.</li>
  <li>`boolean()' is the subset of `atom()' consisting
      of the atoms `true' and `false'.</li>
  <li>`char()' is a subset of
      `integer()' representing character codes.</li>
  <li>`tuple()' is the set of all tuples `{...}'.</li>
  <li>`list(T)' is just an alias for `[T]'.</li>
  <li>`nil()' is an alias for the empty list `[]'.</li>
  <li>`cons(H,T)' is the list constructor. This is usually not
      used directly. It is possible to recursively define `list(T)
      := nil()+cons(T,list(T))'.</li>
  <li>`string()' is an alias for `[char()]'.</li>
  <li>`deep_string()' is recursively defined as
      `[char()+deep_string()]'.</li>
  <li>`none()' means "no data type". E.g., a function
      that never returns has type `(...) -> none()'</li>
</ul>


== Acknowledgements ==

Since the first version of EDoc, several people have come up with
suggestions (Luke Gorrie, Joe Armstrong, Erik Stenman, Sean Hinde, Ulf
Wiger, ...), and some have even submitted code to demonstrate their
ideas (Vlad Dumitrescu, Johan Blom, Vijay Hirani, ...). None of that
code was actually included in the Great Rewriting that followed the
initial public release (EDoc version 0.1), but most of the central
points were addressed in the new system, such as better modularization
and possibility to plug in different layout engines, and making EDoc
understand the application directory layout.

It is now getting too hard to keep track of all the people who have made
further suggestions or submitted bug reports, but your input is always
appreciated. Thank you.
