[Freeciv-Dev] Re: XML (OT)

[Andrew Sutton <ansutton@xxxxxxx>]

On Friday 07 December 2001 11:00 am, Petr Baudis wrote:
> They say something about impossibility of parsing C or perl syntax by yacc.
> Well, dunno about yacc but it's then really funny that both perl and gcc
> use bison, isn't it? And, honestly, even when writing IMHO relatively
> complex grammar using bison, I never got into those problems.

having worked extensively with compilers in the last month or two, i can say 
*without* doubt that there are just some grammars that yacc/bison are 
incapable of parsing. it has to do with the algorithm used to for descent. 
yacc/bison generates bottum-up LALR(1) parser - it's just insufficient for 
sufficiently complex grammars. don't forget that you can manipulate a grammar 
specification to help resolve shift/reduce, reduce/reduce and other various 
ambiguities or infite recursion issues. you can also approximate some aspects 
of the grammar thru code. so they're probably right. it may be impossible to 
parse the C and perl syntaxes as specified, but for lack of a better tool, 
they make up for the shortcomings with more complex grammar implementations 
(like a fixed token lookahead).

> > actually, on a side note flex isn't even GNU or being actively
> > maintained. i don't know about bison.
>
> That matters?

that's why it was a side note.

> > both cases have their weak points. xml isn't very expressive and it's not
> > really mean to be. writing your own parser will provide a more
> > specialized language but is going to be a pain in the ass - regardless of
> > how easy it is or how much experience anybody has.
>
> I don't think so :).

alot of it is going to have to deal with specification of the grammar itself. 
it's not easy to write the perfect grammar first time out, because you're 
going to miss something. then, going back and adding might prove to be 
somewhat difficult and its possible that you could break all the existing 
scripts/files by changing a rule to accomodate your addition.

> I would maybe use XML, depending on the style of the language. If it's only
> config file, I wouldn't use it, as it's pretty useless then, IMHO.

i'd disagree here... if its just data, XML would probably be a better idea 
for the simple fact that it's meant for that and its widely supported. 
however, as soon as you want to introduce something a little more complicated 
like functional specifications, XML kind of sucks. the only exception would 
be the embedding of a scripting language in the XML specification - which 
could be pretty useful.

anyway, this is getting pretty far off topic now :)

andy