Re: [Caml-list] Context Free Grammars?

[skaller <skaller@users.sourceforge.net>]

On Fri, 2004-08-13 at 05:25, Paul Snively wrote:

> 1) Only one token at a time is taken into account in reductions.
> 2) Right-recursion will screw you over.

Actually Ocamlyacc handles right recursion just fine.
I have no idea why .. but it does, in fact its the
prefered form in many cases: here's an example
of BOTH left and right recursion being used:
 
tlelement:
  | lelement COLON factor { $1,Some (typecode_of_expr $3) } 
  | lelement { $1,None }

lexprs:
  | tlelement COMMA lexprs { $1 :: $3 }
  | tlelement { [$1] }


This works -- its part of the Felix grammar: tlelement
is left recursive, lexprs is right recursive,
and the reason is clear -- its the most efficient
way to build a list. 

Perhaps someone can explain why this actually works ..??

-- 
John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
snail: PO BOX 401 Glebe NSW 2037 Australia
Checkout the Felix programming language http://felix.sf.net



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