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(I'm new enough to Lisp not to know how to do this, but familiar enough to know there just must be a simple way.)

I was intrigued by an article that I read recently which advocated storing log files as Lisp-style S-expressions, so that the log files could be turned into a DSL easily. This got me thinking about similar grammars for other DSLs, but I've run into a snag.

Parsing s-expressions where the delimiters are parentheses is easy 

"(my-function foo bar)"

can be read from a file/stream and evaluated trivially.

And if XML is really just an S-expression, shouldn't there be an easy way to transform something as simple as

<function>foo bar etc-1 etc-2</function>

into

(function foo bar etc-1 etc-2)

I can fake this behavior with string manipulation techniques (heck, a regex could take care of this, really). But that seems like Lisp blasphemy - these are just S-expressions! The same applies for the Lisp-based versions of lex/yacc - I can see the need for more complex syntax, but really, this is just Lisp syntax disguised in a less efficient manner.

This should, in theory, extend beyond XML, to any context-free grammar that is a homomorphism of the Lisp grammar. (I know that Lisp is supposedly not perfectly defined by a context-free grammar as simple as the S-expression, but obviously the latter is a strict subset of the former, so my statement still stands).

So, in short: Is there a simple way to define the syntax for a context-free grammar homomorphic to S-expressions (like XML, for example) and parse that grammar within Lisp (or a Lisp-based DSL)?

chimeracoder
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  • I thought it was implied, but maybe it wasn't clear since I didn't phrase it as a question. Edited to clarify it (at the end). – chimeracoder Mar 01 '12 at 05:24

2 Answers2

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Yes you can do that by implementing "reader macros" in lisp that will take your XML and convert them to S-expr. Once in S-expr everything is simple in LISP world.

Ex: http://www.agentsheets.com/lisp/XMLisp/

Ankur
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It might be possible to parse XML with reader macros but I'm not aware of anyone having done so.

S-XML is a simple XML parser and builder, but if I remember it doesn't use reader macros.

Part of the reason reader macros haven't been used to solve this problem conclusively is that XML is in fact, despite initial appearances, really complicated.

Samuel Edwin Ward
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