LDoc2tl/docs/doc.md

# LDoc, a Lua Documentation Tool

## Introduction

LDoc is a second-generation documentation tool that can be used as a replacement for
[LuaDoc](http://keplerproject.github.com/luadoc/). It arose out of my need to document my
own projects and only depends on the [Penlight](https://github.com/stevedonovan/Penlight)
libraries.

It is mostly compatible with LuaDoc, except that certain workarounds are no longer needed.
For instance, it is not so married to the idea that Lua modules should be defined using the
`module` function; this is not only a matter of taste since this has been deprecated in Lua
5.2.

Otherwise, the output is very similar, which is no accident since the HTML templates are
based directly on LuaDoc. You can ship your own customized templates and style sheets with
your [own project](http://nilnor.github.com/textui/docs/), however. You have an option to
use Markdown to process the documentation, which means no ugly HTML is needed in doc
comments.  C/C++ extension modules may be documented in a similar way, although function
names cannot be inferred from the code itself.

LDoc can provide integrated documentation, with traditional function comments, any documents
in Markdown format, and specified source examples. Lua source in examples and the documents
will be prettified.

Although there are a fair number of command-line options, the preferred route is to write a
`config.ld` configuration file in Lua format. By convention, if LDoc is simply invoked as
`ldoc .` it will read this file first. In this way, the aim is to make it very easy for
end-users to build your documentation using this simple command.

## Commenting Conventions

LDoc follows the conventions established by Javadoc and later by LuaDoc.

Only 'doc comments' are parsed; these can be started with at least 3 hyphens, or by a empty
comment line with at least 3 hypens:

    --- summary.
    -- Description; this can extend over
    -- several lines

    -----------------
    -- This will also do.

You can also use Lua block comments:

    --[[--
     Summary. A description
     ...;
    ]]

Any module or script must start with a doc comment; any other files are ignored and a
warning issued. The only exception is if the module starts with an explicit `module`
statement.

All doc comments start with a summary sentence, that ends with a period or a question mark.
An optional description may follow. Normally the summary sentence will appear in the module
contents.

After this descriptive text, there will typically be _tags_. These follow the convention
established by Javadoc and widely used in tools for other languages.

    --- foo explodes text.
    -- It is a specialized splitting operation on a string.
    -- @param text the string
    -- @return a table of substrings
    function foo (text)
    ....
    end

There are also 'tparam' and 'treturn' which let you [specify a type](#Tag_Modifiers):

    -- @tparam string text the string
    -- @treturn {string,...} a table of substrings

There may be multiple 'param' tags, which should document each formal parameter of the
function. For Lua, there can also be multiple 'return' tags

    --- solvers for common equations.
    module("solvers", package.seeall)

    --- solve a quadratic equation.
    -- @param a first coeff
    -- @param b second coeff
    -- @param c third coeff
    -- @return first root, or nil
    -- @return second root, or imaginary root error
    function solve (a,b,c)
        local disc = b^2 - 4*a*c
        if disc < 0 then
            return nil,"imaginary roots"
        else
           disc = math.sqrt(disc)
           return (-b + disc)/2*a,
                  (-b - disc)/2*a
        end
    end

    ...

This is the common module style used in Lua 5.1, but it's increasingly common to see less
'magic' ways of creating modules in Lua. Since `module` is deprecated in Lua 5.2, any
future-proof documentation tool needs to handle these styles gracefully:

    --- a test module
    -- @module test

    local test = {}

    --- first test.
    function test.one()
    ...
    end

    ...

    return test

Here the name of the module is explicitly given using the 'module' tag. If you leave this
out, then LDoc will infer the name of the module from the name of the file and its relative
location in the filesystem; this logic is also used for the `module(...)` idiom. (How this
works and when you need to provide extra information is discussed later.)

It is common to use a local name for a module when declaring its contents. In this case the
'alias' tag can tell LDoc that these functions do belong to the module:

    --- another test.
    -- @module test2
    -- @alias M

    local M = {}

    -- first test.
    function M.one()
    ..
    end

    return M

`M` and `_M` are used commonly enough that LDoc will recognize them as aliases
automatically, but 'alias' allows you to use any identifier.

LDoc tries to deduce the function name and the formal parameter names from examining the
code after the doc comment. It also recognizes the 'unsugared' way of defining functions as
explicit assignment to a variable:

    --- second test.
    M.two = function(...) ... end

Apart from exported functions, a module usually contains local functions. By default, LDoc
does not include these in the documentation, but they can be enabled using the `--all` flag.
They can be documented just like 'public' functions:

    --- it's clear that boo is local from context.
    local function boo(...) .. end

    local foo

    --- we need to give a hint here for foo
    -- @local here
    function foo(...) .. end

Modules can of course export tables and other values. The classic way to document a table
looks like this:

    --- a useful table of constants
    -- @field alpha first correction
    -- @field beta second correction
    -- @field gamma fudge factor
    -- @table constants

Here the kind of item is made explicit by the 'table' tag; tables have 'fields' in the same
way as functions have parameters.

This can get tedious, so LDoc will attempt to extract table documentation from code:

    --- a useful table of constants
    M.constants = {
        alpha = 0.23, -- first correction
        beta = 0.443, -- second correction
        gamma = 0.01  -- fudge factor
    }

The rule followed here is `NAME = <table-constructor>`. If LDoc can't work out the name and
type from the following code, then a warning will be issued, pointing to the file and
location.

Another kind of module-level type is 'field', such as follows:

    --- module version.
    M._VERSION = '0.5'

That is, a module may contain exported functions, local functions, tables and fields.

When the code analysis would lead to the wrong type, you can always be explicit.

    --- module contents.
    -- @field _CONTENTS
    M._CONTENTS = {constants=true,one=true,...}

The order of tags is not important, but as always, consistency is useful.  Tags like 'param'
and 'return' can be specified multiple times, whereas a type tag like 'function' can only
occur once in a comment. The basic rule is that a single doc comment can only document one
entity.

By default, LDoc will process any file ending in '.lua' or '.luadoc' in a specified
directory; you may point it to a single file as well. A 'project' usually consists of many
modules in one or more _packages_. The generated `index.html` will point to the generated
documentation for each of these modules.

If only one module or script is documented for a project, then the `index.html` generated
contains the documentation for that module, since an index pointing to one module would be
redundant.

(If you want to document a script, there is a project-level type 'script' for that.)

## See References

The tag 'see' is used to reference other parts of the documentation, and 'usage' can provide
examples of use:

    ---------
    -- split a string in two.
    -- @param s the string
    -- @param delim the delimiter (default space)
    -- @return first part
    -- @return second part
    -- @usage local hello,world = split2("hello world")
    -- @see split
    funtion split2(s,delim) .. end

Here it's assumed that 'split' is a function defined in the same module. If you wish to link
to a function in another module, then the reference has to be qualified.

References to methods use a colon: `myclass:method`; this is for instance how you would
refer to members of a `@type` section.

The example at `tests/complex` shows how @see references are interpreted:

    complex.util.parse
    complex.convert.basic
    complex.util
    complex.display
    complex

You may of course use the full name of a module or function, but can omit the top-level
namespace - e.g. can refer to the module `util` and the function `display.display_that`
directly. Within a module, you can directly use a function name, e.g. in `display` you can
say `display_this`.

What applies to functions also applies to any module-level item like tables. New
module-level items can be defined and they will work according to these rules.

If a reference is not found within the project, LDoc checks to see if it is a reference to a
Lua standard function or table, and links to the online Lua manual. So references like
'table.concat' are handled sensibly.

References may be made inline using the @\{ref} syntax. This may appear anywhere in the
text, and is more flexible than @see. In particular, it provides one way to document the
type of a parameter or return value when that type has a particular structure:

    ------
    -- extract standard variables.
    -- @param s the string
    -- @return @\{stdvars}
    function extract_std(s) ... end

    ------
    -- standard variables.
    -- Use @\{extract_std} to parse a string containing variables,
    -- and @\{pack_std} to make such a string.
    -- @field length
    -- @field duration
    -- @field viscosity
    -- @table stdvars

@\{ref} is very useful for referencing your API from code samples and readme text. (I've had
to throw in a spurious backspace to stop expansion in this example.)

The link text can be changed from the default by the extended syntax @\{ref|text}.

You can also put references in backticks, like `\`stdvars\``. This is commonly used in
Markdown to indicate code, so it comes naturally when writing documents. It is controlled by
the configuration variable `backtick_references`; the default is `true` if you use Markdown
in your project, but can be specified explicitly in your `config.ld`.

### Custom @see References

It's useful to define how to handle references external to a project. For instance, in the
[luaposix](https://github.com/luaposix/luaposix) project we wanted to have `man` references
to the corresponding C function:

    ------------
    -- raise a signal on this process.
    -- @see raise(3)
    -- @int nsig
    -- @return integer error cod
    function raise (nsig)
    end

These see references always have this particular form, and the task is to turn them into
online references to the Linux manpages. So in `config.ld` we have:

    local upat = "http://www.kernel.org/doc/man-pages/online/pages/man%s/%s.%s.html"

    custom_see_handler('^(%a+)%((%d)%)$',function(name,section)
        local url = upat:format(section,name,section)
        local name = name .. '(' ..section..')'
        return name, url
    end)

'^(%a+)%((%d)%)$' both matches the pattern and extracts the name and its section. THen it's
a simple matter of building up the appropriate URL.  The function is expected to
return _link text_ and _link source_ and the patterns are checked before LDoc tries to resolve
project references. So it is best to make them match as exactly as possible.

## Sections

LDoc supports _explicit_ sections. By default, the sections correspond to the pre-existing
types in a module: 'Functions', 'Tables' and 'Fields' (There is another default section
'Local Functions' which only appears if LDoc is invoked with the `--all` flag.) But new
sections can be added; the first mechanism is when you define a new type (say 'macro') a new
section ('Macros') is created to contain these types. There is also a way to declare ad-hoc
sections using the `@section` tag.

The need occurs when a module has a lot of functions that need to be put into logical
sections.

    --- File functions.
    -- Useful utilities for opening foobar format files.
    -- @section file

    --- open a file
    ...

    --- read a file
    ...

    --- Encoding operations.
    -- Encoding foobar output in different ways.
    -- @section encoding

    ...

A section doc-comment has the same structure as a normal doc-comment; the summary is used as
the new section title, and the description will be output at the start of the function
details for that section.

In any case, sections appear under 'Contents' on the left-hand side. See the
[winapi](http://stevedonovan.github.com/winapi/api.html) documentation for an example of how
this looks.

Arguably a module writer should not write such very long modules, but it is not the job of
the documentation tool to limit the programmer!

A specialized kind of section is `type`: it is used for documenting classes. The functions
(or fields) within a type section are considered to be the methods of that class.

    --- A File class.
    -- @type File

    ....
    --- get the modification time.
    -- @return standard time since epoch
    function File:mtime()
    ...
    end

(In an ideal world, we would use the word 'class' instead of 'type', but this would conflict
with the LuaDoc usage.)

A section continues until the next section is found, `@section end`, or end of file.

You can put items into an implicit section using the @within tag. This allows you to put
adjacent functions in different sections, so that you are not forced to order your code
in a particular way.

Sometimes a module may logically span several files. There will be a master module with name
'foo' and other files which when required add functions to that module. If these files have
a @submodule tag, their contents will be placed in the master module documentation. However,
a current limitation is that the master module must be processed before the submodules.

See the `tests/submodule` example for how this works in practice.

## Differences from LuaDoc

LDoc only does 'module' documentation, so the idea of 'files' is redundant.

One added convenience is that it is easier to name entities:

    ------------
    -- a simple module.
    -- (LuaDoc)
    -- @class module
    -- @name simple

    ------------
    -- a simple module.
    -- (LDoc)
    -- @module simple

This is because type names (like 'function', 'module', 'table', etc) can function as tags.
LDoc also provides a means to add new types (e.g. 'macro') using a configuration file which
can be shipped with the source. If you become bored with typing 'param' repeatedly then you
can define an alias for it, such as 'p'. This can also be specified in the configuration file.

LDoc will also work with C/C++ files, since extension writers clearly have the same
documentation needs as Lua module writers.

LDoc gives the documenter the option to use Markdown to parse the contents of comments.

## Adding new Tags

LDoc tries to be faithful to LuaDoc, but provides some extensions. Aliases for tags can be
defined, and new types declared.

    --- zero function. Two new ldoc features here; item types
    -- can be used directly as tags, and aliases for tags
    -- can be defined in config.ld.
    -- @function zero_fun
    -- @p k1 first
    -- @p k2 second

Here an alias for 'param' has been defined. If a file `config.ld` is found in the source,
then it will be loaded as Lua data. For example, the configuration for the above module
provides a title and defines an alias for 'param':

    title = "testmod docs"
    project = "testmod"
    alias("p","param")

Extra tag _types_ can be defined:

    new_type("macro","Macros")

And then used as any other type:

    -----
    -- A useful macro. This is an example of a custom type.
    -- @macro first_macro
    -- @see second_function

This will also create a new module section called 'Macros'.

## Inferring more from Code

The qualified name of a function will be inferred from any `function` keyword following the
doc comment. LDoc goes further with this kind of code analysis, however.

Instead of:

    --- first table.
    -- @table one
    -- @field A alpha
    -- @field B beta
    M.one = {
        A = 1,
        B = 2;
    }

you can write:

    --- first table
    -- @table one
    M.one = {
        A = 1, -- alpha
        B = 2; -- beta
    }

Simularly, function parameter comments can be directly used:

    ------------
    -- third function. Can also provide parameter comments inline,
    -- provided they follow this pattern.
    function mod1.third_function(
        alpha, -- correction A
        beta, -- correction B
        gamma -- factor C
        )
        ...
    end

As always, explicit tags can override this behaviour if it is inappropriate.

## Extension modules written in C

LDoc can process C/C++ files:

    @plain
    /***
    Create a table with given array and hash slots.
    @function createtable
    @param narr initial array slots, default 0
    @param nrec initial hash slots, default 0
    @return the new table
    */
    static int l_createtable (lua_State *L) {
    ....

Both `/**` and `///` are recognized as starting a comment block. Otherwise, the tags are
processed in exactly the same way. It is necessary to specify that this is a function with a
given name, since this cannot be reliably be inferred from code. Such a file will need a
module comment, which is treated exactly as in Lua.

An unknown extension can be associated with a language using a call like
`add_language_extension('lc','c')` in `config.ld`. (Currently the language can only be 'c'
or 'lua'.)

See 'tests/examples/mylib.c' for the full example.

## Basic Usage

For example, to process all files in the 'lua' directory:

    $ ldoc lua
    output written to docs/

Thereafter the `docs` directory will contain `index.html` which points to individual modules
in the `modules` subdirectory.  The `--dir` flag can specify where the output is generated,
and will ensure that the directory exists. The output structure is like LuaDoc: there is an
`index.html` and the individual modules are in the `modules` subdirectory. This applies to
all project-level types, so that you can also get `scripts`, `examples` and `topics`
directories.

If your modules use `module(...)` then the module name has to be deduced. If `ldoc` is run
from the root of the package, then this deduction does not need any help - e.g. if your
package was `foo` then `ldoc foo` will work as expected. If we were actually in the `foo`
directory then `ldoc -b .. .` will correctly deduce the module names. An example would be
generating documentation for LuaDoc itself:

    $ ldoc -b .. /path/to/luadoc

Without the `-b` setting the base of the package to  the _parent_ of the directory, implicit
modules like `luadoc.config` will be incorrectly placed in the global namespace.

For new-style modules, that don't use `module()`, it is recommended that the module comment
has an explicit `@module PACKAGE.NAME`. If it does not, then `ldoc` will still attempt to
deduce the module name, but may need help with `--package/-b` as above.

`format = 'markdown'` can be used in your `config.ld` and will be used to process summaries
and descriptions. This requires [markdown.lua](http://www.frykholm.se/files/markdown.lua) by
Niklas Frykholm to be installed (this can be most easily done with `luarocks install
markdown`.)  A much faster alternative is
[lua-discount](http://asbradbury.org/projects/lua-discount/) which you can use by setting
`format` to 'discount' after installing using `luarocks install lua-discount`)  The
[discount](http://www.pell.portland.or.us/~orc/Code/discount/) Markdown processor
additionally has more features than the pure Lua version, such as PHP-Extra style tables.
As a special case, LDoc will fall back to using `markdown.lua` if it cannot find `discount`.

`format = 'markdown'` can be used in your `config.ld` and will be used to process summaries
and descriptions. This requires a markdown processor.
LDoc knows how to use:

   - [markdown.lua](http://www.frykholm.se/files/markdown.lua) a pure Lua processor by
Niklas Frykholm (this can be installed easily with `luarocks install markdown`.)
  - [lua-discount](http://asbradbury.org/projects/lua-discount/), a faster alternative
(installed with `luarocks install lua-discount`).  lua-discount uses the C
[discount](http://www.pell.portland.or.us/~orc/Code/discount/) Markdown processor which has
more features than the pure Lua version, such as PHP-Extra style tables.
  - [lunamark](http://jgm.github.com/lunamark/), another pure Lua processor,  faster than
markdown, and with extra features (`luarocks install lunamark`).

You can request the processor you like with `format = 'markdown|discount|lunamark'`, and
LDoc will attempt to use it.  If it can't find it, it will look for one of the other
markdown processors.  If it can't find any markdown processer, it will fall back to text
processing.


A special case is if you simply say 'ldoc .'. Then there _must_ be a `config.ld` file
available in the directory, and it can specify the file:

    file = "mymod.lua"
    title = "mymod documentation"
    description = "mymod does some simple but useful things"

`file` can of course point to a directory, just as with the `--file` option. This mode makes
it particularly easy for the user to build the documentation, by allowing you to specify
everything explicitly in the configuration.

In `config.ld`, `file` may be a Lua table, containing file names or directories; if it has
an `exclude` field then that will be used to exclude files from the list, for example
`{'examples', exclude = {'examples/slow.lua'}}`.


## Processing Single Modules

`--output` can be used to give the output file a different name. This is useful for the
special case when a single module file is specified. Here an index would be redundant, so
the single HTML file generated contains the module documentation.

    $ ldoc mylib.lua --> results in docs/index.html
    $ ldoc --output mylib mylib.lua --> results in docs/mylib.html
    $ ldoc --output mylib --dir html mylib.lua --> results in html/mylib.html

The default sections used by LDoc are 'Functions', 'Tables' and 'Fields', corresponding to
the built-in types 'function', 'table' and 'field'. If `config.ld` contains something like
`new_type("macro","Macros")` then this adds a new section 'Macros' which contains items of
'macro' type - 'macro' is registered as a new valid tag name.  The default template then
presents items under their corresponding section titles, in order of definition.

## Getting Help about a Module

There is an option to simply dump the results of parsing modules. Consider the C example
`tests/example/mylib.c':

    @plain
    $ ldoc --dump mylib.c
    ----
    module: mylib   A sample C extension.
    Demonstrates using ldoc's C/C++ support. Can either use /// or /*** */ etc.

    function        createtable(narr, nrec)
    Create a table with given array and hash slots.
    narr     initial array slots, default 0
    nrec     initial hash slots, default 0

    function        solve(a, b, c)
    Solve a quadratic equation.
    a        coefficient of x^2
    b        coefficient of x
    c        constant
    return  {"first root","second root"}

This is useful to quickly check for problems; here we see that `createable` did not have a
return tag.

LDoc takes this idea of data dumping one step further. If used with the `-m` flag it will
look up an installed Lua module and parse it. If it has been marked up in LuaDoc-style then
you will get a handy summary of the contents:

    @plain
    $ ldoc -m pl.pretty
    ----
    module: pl.pretty       Pretty-printing Lua tables.
    * read(s) - read a string representation of a Lua table.
    * write(tbl, space, not_clever) - Create a string representation of a Lua table.

    * dump(t, ...) - Dump a Lua table out to a file or stdout.

You can specify a fully qualified function to get more information:

    @plain
    $ ldoc -m pl.pretty.write

    function        write(tbl, space, not_clever)
    create a string representation of a Lua table.
    tbl      {table} Table to serialize to a string.
    space    {string} (optional) The indent to use.
                   Defaults to two spaces.
    not_clever       {bool} (optional) Use for plain output, e.g {['key']=1}.
                   Defaults to false.

LDoc knows about the basic Lua libraries, so that it can be used as a handy console reference:

    @plain
    $> ldoc -m assert

    function        assert(v, message)
    Issues an error when the value of its argument `v` is false (i.e.,
     nil or false); otherwise, returns all its arguments.
    `message` is an error
     message; when absent, it defaults to "assertion failed!"
    v
    message

Thanks to mitchell's [TextAdept](http://code.google.com/p/textadept/) project, LDoc has a
set of `.luadoc` files for all the standard tables, plus
[LuaFileSystem](http://keplerproject.github.com/luafilesystem/) and
[LPeg](http://www.inf.puc-rio.br/~roberto/lpeg/lpeg.html).

    @plain
    $> ldoc -m lfs.lock

    function        lock(filehandle, mode, start, length)
    Locks a file or a part of it.
    This function works on open files; the file
     handle should be specified as the first argument. The string mode could be
     either r (for a read/shared lock) or w (for a write/exclusive lock). The
     optional arguments start and length can be used to specify a starting point
     and its length; both should be numbers.
     Returns true if the operation was successful; in case of error, it returns
     nil plus an error string.
    filehandle
    mode
    start
    length

## Anatomy of a LDoc-generated Page

[winapi](http://stevedonovan.github.com/winapi/api.html) can be used as a good example of a
module that uses extended LDoc features.

The _navigation section_ down the left has several parts:

  - The project name ('project' in the config)
  - A project description ('description')
  - ''Contents'' of the current page
  - ''Modules'' listing all the modules in this project

Note that `description` will be passed through Markdown, if it has been specified for the
project. This gives you an opportunity to make lists of links, etc; any '##' headers will be
formatted like the other top-level items on the navigation bar.

'Contents' is automatically generated. It will contain any explicit sections, if they have
been used. Otherwise you will get the usual categories: 'Functions', 'Tables' and 'Fields'.

'Modules' will appear for any project providing Lua libraries; there may also be a 'Scripts'
section if the project contains Lua scripts. For example,
[LuaMacro](http://stevedonovan.github.com/LuaMacro/docs/api.html) has a driver script `luam`
in this section. The
[builtin](http://stevedonovan.github.com/LuaMacro/docs/modules/macro.builtin.html) module
only defines macros, which are defined as a _custom tag type_.

The _content section_ on the right shows:

  - The module summary and description
  - The contents summary, per section as above
  - The detailed documentation for each item

As before, the description can use Markdown. The summary contains the contents of each
section as a table, with links to the details. This is where the difference between an
item's summary and an item's description is important; the first will appear in the contents
summary. The item details show the item name and its summary again, followed by the
description. There are then sections for the following tags: 'param', 'usage', 'return' and
'see' in that order. (For tables, 'Fields' is used instead of 'Parameters' but internally
fields of a table are stored as the 'param' tag.)

You can of course customize the default template, but there are some parameters that can
control what the template will generate. Setting `one` to `true` in your configuration file
will give a _one-column_ layout, which can be easier to use as a programming reference.  You
can suppress the contents summary with `no_summary`.

## Customizing the Page

Setting `no_return_or_parms` to `true` will suppress the display of 'param' and 'return'
tags. This may appeal to programmers who dislike the traditional @tag soup xDoc style and
prefer to comment functions just with a description. This is particularly useful when using
Markdown in a stylized way to specify arguments:

    ---------
    -- This extracts the shortest common substring from the strings _s1_ and _s2_
    function M.common_substring(s1,s2)

Here I've chosen to italicise parameter names; the main thing is to be consistent.

This style is close to the Python [documentation
standard](http://docs.python.org/library/array.html#module-array), especially when used with
`no_summary`.

It is also very much how the Lua documentation is ordered. For instance, this configuration
file formats the built-in documentation for the Lua global functions in a way which is close
to the original:

    project = 'Lua'
    description = 'Lua Standard Libraries'
    file = {'ldoc/builtin',exclude = {'ldoc/builtin/globals.lua'}}
    no_summary = true
    no_return_or_parms = true
    format = 'discount'


Generally, using Markdown gives you the opportunity to structure your documentation in any
way you want; particularly if using lua-discount and its [table
syntax](http://michelf.com/projects/php-markdown/extra/#table); the desired result can often
be achieved then by using a custom style sheet.

## Examples

It has been long known that documentation generated just from the source is not really
adequate to explain _how_ to use a library.  People like reading narrative documentation,
and they like looking at examples.  Previously I found myself dealing with source-generated
and writer-generated documentation using different tools, and having to match these up.

LDoc allows for source examples to be included in the documentation. For example, see the
online documentation for [winapi](http://stevedonovan.github.com/winapi/api.html). The
function `utf8_expand` has a `@see` reference to 'testu.lua' and following that link gives
you a pretty-printed version of the code.

The line in the `config.ld` that enables this is:

    examples = {'examples', exclude = {'examples/slow.lua'}}

That is, all files in the `examples` folder are to be pretty-printed, except for `slow.lua`
which is meant to be called from one of the examples. The see-reference to `testu.lua`
resolves to 'examples/testu.lua.html'.

Examples may link back to the API documentation, for instance the example `input.lua` has a
@\{spawn_process} inline reference.

##  Readme files

Like all good Github projects, Winapi has a `readme.md`:

    readme = "readme.md"

This goes under the 'Topics' global section; the 'Contents' of this document is generated
from the second-level (##) headings of the readme.

Readme files are always processed with Markdown, but may also contain @\{} references back
to the documentation and to example files. Any symbols within backticks will be expanded as
references, if possible. As with doc comments, a link to a standard Lua function like
@\{os.execute} will work as well. Any code sections will be pretty-printed as Lua, unless
the first indented line is '@plain'. (See the source for this readme to see how it's used.)

Another name for `readme` is `topics`, which is more descriptive. From LDoc 1.2,
`readme/topics` can be a list of documents. These act as a top-level table-of-contents for
your documentation. Currently, if you want them in a particular order, then use names like
`01-introduction.md` etc which sort appropriately.

The first line of a document may be a Markdown `#` title. If so, then LDoc will regard the
next level as the subheadings, normally second-level `##`. But if the title is already
second-level, then third-level headings will be used `###`, and so forth. The implication is
that the first heading must be top-level relative to the headings that follow, and must
start at the first line.

A reference like @\{string.upper} is unambiguous, and will refer to the online Lua manual.
In a project like Penlight, it can get tedious to have to write out fully qualified names
like @\{pl.utils.printf}.  The first simplification is to use the `package` field to resolve
unknown references, which in this case is 'pl'. (Previously we discussed how `package` is
used to tell LDoc where the base package is in cases where the module author wishes to
remain vague, but it does double-duty here.)  A further level of simplification comes from
the @lookup directive in documents, which must start at the first column on its own line.
For instance, if I am talking about `pl.utils`, then I can say "@lookup utils" and
thereafter references like @\{printf} will resolve correctly.

Remember that the default is for references in backticks to be resolved; unlike @
references, it is not an error if the reference cannot be found.

The _sections_ of a document (the second-level headings) are also references. This
particular section can be refered to as @\{doc.md.Resolving_References_in_Documents} - the
rule is that any non-alphabetic character is replaced by an underscore.


## Tag Modifiers

Ay tag may have _tag modifiers_. For instance, you may say
@\param[type=number] and this associates the modifier `type` with value `number` with this
particular param tag. A shorthand can be introduced for this common case, which is "@tparam
<type> <parmname> <comment>"; in the same way @\treturn is defined.

This is useful for larger projects where you want to provide the argument and return value
types for your API, in a structured way that can be easily extracted later. There is a
useful function for creating new tags that can be used in `config.ld`:

    tparam_alias('string','string')

That is, "@string" will now have the same meaning as "@tparam string".

From 1.3, the following standard type aliases are predefined:

  * `string`
  * `number`
  * `int`
  * `bool` Lua 'boolean' type
  * `func` 'function' (using 'function' would conflict with the type)
  *  `tab` 'table'
  * `thread`

The exact form of `<type>` is not defined, but here is a suggested scheme:

    number  -- a plain type
    Bonzo   -- a known type; a reference link will be generated
    {string,number}  -- a 'list' tuple, built from type expressions
    {A=string,N=number} -- a 'struct' tuple, ditto
    {Bonzo,...}   -- an array of Bonzo objects
    {[string]=Bonzo,...} -- a map of Bonzo objects with string keys
    Array(Bonzo) -- (assuming that Array is a container)

Currently the `type` modifier is the only one known and used by LDoc when generating HTML
output. However, any other modifiers are allowed and are available for use with your own
templates or for extraction by your own tools.

The `alias` function within configuration files has been extended so that alias tags can be
defined as a tag plus a set of modifiers.  So `tparam` is defined as:

    alias('tparam',{'param',modifiers={type="$1"}})

As an extension, you're allowed to use '@param' tags in table definitions. This makes it
possible to use type alias like '@string' to describe fields, since they will expand to
'param'.

## Fields allowed in `config.ld`

These mostly have the same meaning as the corresponding parameters:

  - `file`  a file or directory containing sources. In `config.ld` this can also be a table
of files and directories.
  - `project` name of project, used as title in top left
  - `title` page title, default 'Reference'
  - `package ` explicit base package name; also used for resolving references in documents
  - `all` show local functions, etc as well in the docs
  - `format` markup processor, can be 'plain' (default), 'markdown' or 'discount'
  - `output` output name (default 'index')
  - `dir` directory for output files (default 'docs')
  - `ext` extension for output (default 'html')
  - `one` use a one-column layout
  - `style`, `template`: together these specify the directories for the style and and the
template. In `config.ld` they may also be `true`, meaning use the same directory as the
configuration file.

These only appear in `config.ld`:

  - `description` a project description used under the project title
  - `examples` a directory or file: can be a table
  - `readme` name of readme file (to be processed with Markdown)
  - `no_return_or_parms` don't show parameters or return values in output
  - `backtick_references` whether references in backticks will be resolved
  -  `manual_url` point to an alternative or local location for the Lua manual, e.g.
'file:///D:/dev/lua/projects/lua-5.1.4/doc/manual.html'


Available functions are:

  - `alias(a,tag)` provide an alias `a` for the tag `tag`, for instance `p` as short for
`param`
  - `add_language_extension(ext,lang)` here `lang` may be either 'c' or 'lua', and `ext` is
an extension to be recognized as this language
  - `add_section`
  - `new_type(tag,header,project_level)` used to add new tags, which are put in their own
section `header`. They may be 'project level'.
  - `tparam_alias(name,type)` for instance, you may wish that `string` means `@\tparam
string`.

## Annotations and Searching for Tags

Annotations are special tags that can be used to keep track of internal development status.
The known annotations are 'todo', 'fixme' and 'warning'. They may occur in regular
function/table doc comments, or on their own anywhere in the code.

    --- Testing annotations
    -- @module annot1
    ...
    --- first function.
    -- @todo check if this works!
    function annot1.first ()
        if boo then

        end
        --- @fixme what about else?
    end

Although not currently rendered by the template as HTML, they can be extracted by the
`--tags` command, which is given a comma-separated list of tags to list.

    @plain
    D:\dev\lua\LDoc\tests> ldoc --tags todo,fixme annot1.lua
    d:\dev\lua\ldoc\tests\annot1.lua:14: first: todo check if this works!
    d:\dev\lua\ldoc\tests\annot1.lua:19: first-fixme1: fixme what about else?


## Generating HTML

LDoc, like LuaDoc, generates output HTML using a template, in this case `ldoc_ltp.lua`. This
is expanded by the powerful but simple preprocessor devised originally by [Rici
Lake](http://lua-users.org/wiki/SlightlyLessSimpleLuaPreprocessor) which is now part of
Penlight. There are two rules - any line starting with '#' is Lua code, which can also be
embedded with '$(...)'.

    <h2>Contents</h2>
    <ul>
    # for kind,items in module.kinds() do
    <li><a href="#$(no_spaces(kind))">$(kind)</a></li>
    # end
    </ul>

This is then styled with `ldoc.css`. Currently the template and stylesheet is very much
based on LuaDoc, so the results are mostly equivalent; the main change that the template has
been more generalized. The default location (indicated by '!') is the directory of `ldoc.lua`.

You may customize how you generate your documentation by specifying an alternative style
sheet and/or template, which can be deployed with your project. The parameters are `--style`
and `--template`, which give the directories where `ldoc.css` and `ldoc.ltp` are to be
found. If `config.ld` contains these variables, they are interpreted slightly differently;
if they are true, then it means 'use the same directory as config.ld'; otherwise they must
be a valid directory relative to the ldoc invocation.

An example of fully customized documentation is `tests/example/style`: this is what you
could call 'minimal Markdown style' where there is no attempt to tag things (except
emphasizing parameter names). The narrative alone _can_ to be sufficient, if it is written
appropriately.

Of course, there's no reason why LDoc must always generate HTML. `--ext` defines what output
extension to use; this can also be set in the configuration file. So it's possible to write
a template that converts LDoc output to LaTex, for instance. The separation of processing
and presentation makes this kind of new application possible with LDoc.

## Internal Data Representation

The `--dump` flag gives a rough text output on the console. But there is a more
customizeable way to process the output data generated by LDoc, using the `--filter`
parameter. This is understood to be a fully qualified function (module + name). For example,
try

    $ ldoc --filter pl.pretty.dump mylib.c

to see a raw dump of the data. (Simply using `dump` as the value here would be a shorthand
for `pl.pretty.dump`.)  This is potentially very powerful, since you may write arbitrary Lua
code to extract the information you need from your project.

For instance, a file `custom.lua` like this:

    return {
       filter = function (t)
          for _, mod in ipairs(t) do
             print(mod.type,mod.name,mod.summary)
          end
       end
    }

Can be used like so:

    ~/LDoc/tests/example$ ldoc --filter custom.filter mylib.c
    module  mylib   A sample C extension.

The basic data structure is straightforward: it is an array of 'modules' (project-level
entities, including scripts) which each contain an `item` array (functions, tables and so
forth).

For instance, to find all functions which don't have a @return tag:

    return {
       filter = function (t)
          for _, mod in ipairs(t) do
             for _, item in ipairs(mod.items) do
                if item.type == 'function' and not item.ret then
                   print(mod.name,item.name,mod.file,item.lineno)
                end
             end
          end
       end
    }

The internal naming is not always so consistent; `ret` corresponds to @return, and `params`
corresponds to @param.  `item.params` is an array of the function parameters, in order; it
is also a map from these names to the individual descriptions of the parameters.

`item.modifiers` is a table where the keys are the tags and the values are arrays of
modifier tables.  The standard tag aliases `tparam` and `treturn` attach a `type` modifier
to their tags.