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SexpCode

Towards a Better BBCode

SexpCode is a specialised mark-up language designed to replace
BBCode and, to a lesser extent, user-generated HTML in general.
It is not as yet intended as a general-purpose mark-up
language, or to replace HTML for general web design.
SexpCode does away with the verbosity that has plagued HTML and
XML since SGML, without sacrificing power and ease of use, and
without succumbing to the various problems that plague other
solutions, like Markdown.

This document provides an informal description of its syntax,
pending the outcome of the ISO standardisation process.

Table of Contents

 • 1. Simple Functions
 • 2. Higher-Arity Functions
 • 3. Verbatim
 • 4. Function Definition
 • Appendix 1: Spoiler
 • Appendix 2: Images
 • Appendix 3: Variants

1. Simple functions

Most SexpCode functions are quite simple, and are applied like
this:

   {b This is bold text.}

   This is bold text.

The first word in the S-expression is the function, everything
else is the text it applies to. The S-expression is delimited
by curly braces rather than the traditional parentheses,
because they do not occur in natural language.

If you do need a literal curly brace in your text, you can
escape it with a backslash.

   {b These \}\} curly braces will not generate a syntax
   error, because they are ignored.}

   These }} curly braces will not generate a syntax error,
   because they are ignored.

If you need a literal backslash, you can escape it with a
backslash as well (though it will generally be ignored unless
it precedes a curly brace).

S-expressions can be nested, of course:

   {b This text is bold. {i This text is italic and bold.}}

   This text is bold. This text is italic and bold.

Some functions can be iterated. sup is such a function:

   {sup*2 This text is superscripted two levels.}

   ^^This text is superscripted two levels.

You may also use an accent circonflexe instead, in keeping with
traditional mathematical notation:

   {sub^2 This text is subscripted two levels.}

   [[This text is subscripted two levels.]]

And finally, functions can be composed with a dot:

   {b.i This text is both bold and italic.}

   This text is both bold and italic.

Your expressions can get arbitrarily complicated:

   {b.sup*2.i Way to be a dick, {sub*2.u.o dick}.}

   ^^Way to be a dick, [[dick]].

It should be noted the iteration operator has precedence over
function composition.

Table 1 details the simple functions a SexpCode implementation
can be expected to support.

    Table 1: Simple functions
Function   Description    Example
b        Bold text       Text.
i        Italic text     Text.
u        Underlined text Text.
o        Overlined text  Text.
s        Strike-through  Text.
m        Monospaced text Text.
tt       Teletype        Text.
spoiler  Obfuscated text Text.†
sup      Superscript     ^Text.‡
sub      Subscript       [Text.]‡
quote    Blockquote          Text.‡

† See appendix 1.
‡ May be iterated.

2. Higher-Arity Functions

Some functions can take arguments. url is an example of a
function of arity 1:

   {url http://example.com/ Click here!}

   Click here!

This syntax is equivalent to the following:

   {{url http://example.com/} Click here!}

   Click here!

This second syntax can be thought of as a partial application
of the function, returning another function of lower arity.
This is particularly useful if you want to compose a
higher-arity function int a 0-arity function expression:

   {{url http://example.com/}.b.i Bold italic link!}

   Bold italic link!

If two higher-arity functions are composed, the arguments apply
in the order of composition:

   {url.code http://www.example.org/ Python Contrived
   example.}

   Contrived example.

If you need to pass an argument that has whitespace in it, use
the following syntax:

   {code '{Algorithmic Language Scheme} (fibs 10)}

   (fibs 10)

Alternatively, you could just enclose it in a regular function
application, as functions will strip functions from their
arguments when it makes sense to do so. If you cannot be
certain functions will be stripped, you could perhaps verbatim,
as that does not modify its contents. This is considered poor
form, however, and the above syntax is preferred.

The following is a list of higher-arity functions SexpCode can
be expected to support:

    Table 2: Higher-arity functions
Function Arguments      Description
url      target    Hyperlinks
code     lang      Code
img      source    Images; see appendix 2

3. Verbatim

Sometimes you want to turn off parsing of SexpCode for some
text, perhaps because you want to post some C code. You could
manually escape every curly brace as mentioned, or you could
use the verbatim function:

   {verbatim This {b text} will {{not}} be parsed.}

   This {b text} will {{not}} be parsed.

The downside of this is that it requires that the curly braces
inside the text be balanced. If you cannot guarantee this,
there is an alternative syntax:

   {- This }} text can {{{ have as many unbalanced braces as
   it likes. -}

   This }} text can {{{ have as many unbalanced braces as it
   likes.

Here, the dash may be any expression (of arbitrary length) that
starts with a non-alphanumeric character. The closing tag must
be preceded by a space followed by the same expression.

4. Function Definition

It is possible to define your own functions:

   {define em i}{em This text is highly semantic.}

   This text is highly semantic.

Any function may be freely redefined, including built-in ones.
Functions defined like this only have meaning to text after
their initial definition.

   {b This text is bold.} {define b i}{b This text is not.}

   This text is bold. This text is not.

It goes without saying that the second argument of your
function composition can be an arbitrarily complex function
expression:

   {define expert b.i.o.u.{code SexpCode}}{expert Like a boss}

   Like a boss

Arity is passed transparently:

   {define link url.i}{link http://www.example.net/ Link!}

   Link!

The only restriction is that defined functions cannot be
iterated.

   {define super sup*3}{super*3 Only three sups, or maybe an
   error.}

   ^^^Only three sups, or maybe an error

Function definition may be particularly useful to call a
higher-arity function with the same set of arguments every
time, or to ensure you don't accidentally forget the code you
are posting can't be parsed as SexpCode:

   {define code verbatim.{code C}}
   {code int buffa[] = {};}
   Normal text.
   {code more({code});}

   int buffa[] = {};
   Normal text.
   more({code});

To undo function definitions, undefine works as you would
expect.

   {define a b}{undefine a}

Note that it is impossible to undefine built-in functions, and
undefining a defined function with the name of a built-in
function will revert that function to its original behavior:

   {define b i}{b This text is in italics.} {undefine b}{b Now
   it's bold again.}

   This text is in italics. Now it's bold again.

Appendix 1: Spoiler

The spoiler function is one of the few that doesn't have a
direct HTML equivalent. It is unsurprising that many BBCode
dialects don't bother with it.
The usual implementation is as black text on a black
background. On mouse-over, the text turns white.

   This is a typical spoiler.

An alternative could be the following, with or without
automatic decoding:

   Guvf vf n fcbvyre uvqqra ol EBG13.

These are merely suggestions, however, and the SexpCode
standard has no specific requirements about how exactly the
spoiler function is implemented. It only requires that it is
implemented.

Appendix 2: Images

The img function is perhaps an odd one, in that you might not
expect it to apply to text. However, not applying it to any
text (outside of use in a composition of functions) is a syntax
error.

When SexpCode is translated to HTML, the text the function is
applied to is expected to be used for the alt attribute. XHTML
has made this attribute compulsory, and even for older
standards it is generally considered good form to use it.
Additionally, the same text may also be used for the title
attribute, and perhaps to provide a caption.

   {img http://cairnarvon.rotahall.org/pics/wug.png This is a
   wug.}

   This is a wug.

When SexpCode is translated to something other than HTML, the
text may be used for a caption, for description of the image in
some sort of image index, or it may be discarded altogether.
This is up to the implementation.

When captions are not used, it may not make sense to allow the
img function to be composable. Implementations may disallow
composition in this case, or may opt to ignore the functions
img is composed with.

Some webmasters may wish to use SexpCode for user comments, and
may consider it inappropriate to allow users to post images. If
they wish to disable img support entirely, this is allowed by
the standard.

Appendix 3: Variants

We appreciate that SexpCode may be too powerful a language for
a lot of applications, and that implementing it is more
difficult than implementing BBCode. For this reason, the
standard allows for reduced syntax implementations,
collectively known as SexpCode^−.

There is no strict definition of SexpCode^−; it is a catch-all
term referring to any SexpCode variant which does not implement
all features detailed in this spec. To still be recognised as
SexpCode, however, implementations must support at least the
following items:

1. Simple functions (all of them)
2. Functions of arity 1, with the first argument syntax (url
   and code)
3. Backslash escaping

Everything else (function composition, iteration, definition, &
c.) is optional. Implementations that do implement all of these
things may refer to themselves as SexpCode^+, to make the
distinction explicit.
The plain term SexpCode may refer to either SexpCode^− or
SexpCode^+. In this document, as in all documents of the
SexpCode Working Group, it refers exclusively to SexpCode^+. In
case of ambiguity, we recommend being explicit.

This document was created by the SexpCode Working Group, and is
made available under the GNU Free Documentation License.
Last modified on June 20, 2010.
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