– eXtended S-eXpressions

Repository moved to codeberg.org

import "git.fractalqb.de/fractalqb/xsx"

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Package XSX provides tools for parsing something I call eXtended S-eXpressions. Extended means the following things compared to SEXP S-expressions:

1. Nested structures are delimited by balanced braces '()', '[]' or '{}’ – not only by '()'.

2. XSX provides a notation for "Meta Values", i.e. XSXs that provide some sort of meta information that is not part of the "normal" data.

On the other hand some properties from SEXP were dropped, e.g. typing of the so called "octet strings". Things like that are completely left to the application.

Somewhat more formal description

Frist of all, XSX is not about datatypes, in this it is comparable to e.g. XML (No! don't leave… its much simpler). Instead its building block is the atom, i.e. nothing else than a sequence of characters, aka a 'string'. Atoms come as quoted atoms and as unquoted atoms. One needs to quote an atom when the atom's string contains characters that have a special meaning in XSX: ()[]{}\ and white-space.

Regexp style definition of Atom

atom     := nq-atom | q-atom
nq-atom  := ([^()[]{}]|\s)+
q-atom   := "([^"\]|(\")(\\))+"
XSX      := atom

I.e. x is an atom and foo, bar and baz are atoms. An atom that contains a '"' or '' would be "quote: \" and backslash: \\ in a quoted atom". Also "(" is an atom but ( is not an atom. We need '(' for other things!

Sequences now BNF Style

Each atom is an XSX and from XSX'es one can build sequences:

XSX  ::= atom | seq1 | seq2 | seq3
seq1 ::= '(' ws* ')' | '(' ws* xsxs ws* ')'
seq2 ::= '[' ws* ']' | '[' ws* xsxs ws* ']'
seq3 ::= '{' ws* '}' | '{' ws* xsxs ws* '}'
xsxs ::= XSX | XSX ws* xsxs
ws   ::= “Unicode's White Space”

Out-Of-Band Information with Meta XSXs

You can prefix each XSX with a backslash to make that expression a meta-expression. A meta-expression is not considered to be a XSX, i.e. you cannot create meta-meta-expressions or meta-meta-meta-expressions… hmm… and not event meta-meta-meta-meta-expressions! I think it became clear?

E.g. \4711 is a meta-atom and \{foo 1 bar false baz 3.1415} is a meta-sequence. What meta means is completely up to the application. Imagine e.g. (div hiho) and (div \{class green} hiho) to be a translation from <div>hiho</div> and <div class="green">hiho</div>.

Rationale

None! … despite the fact that I found it to be fun – and useful in some situations.

Because XSX syntax so simple it is easy to use the PullParser as a tokenizer to build customized parsers for proprietary data files. E.g. see the table sub-package. On the other hand the low level parser and scanner API is inspired by the expat streaming parser that allows one to push some data into the paring machinery and it will fire appropriate callbacks when tokes are detected.

So, if you are looking for something that's even simpler than JSON or YAML you might give it a try… Happy coding!