From: Gregory Seidman <gsslist+ruby@...>
Date: 2006-01-08T23:24:49+09:00
Subject: Re: [QUIZ] Dice Roller (#61)

On Sun, Jan 08, 2006 at 10:48:01PM +0900, Pierre Barbier de Reuille wrote:
} Matthew Moss a ?crit :
} > Wow...  You guys are just having too much fun with
} > "(5d5-4)d(16/d4)+3", I think.  Heck, if y'all know Ruby so well (more
} > than me, cause I'm still such a n00b), you'd be able to swap in loaded
} > dice for random dice and get your min's and max's.  =)
} > 
} > Anyway, I just wanted to add a couple of notes.  It was made aware to
} > me that the simplified BNF (in the original post) is slightly in
} > error, in that it allows expressions like this:
} > 
} >    3dddd6
} > 
} > which is invalid.
} > 
} > A couple possible fixes:
} > 
} > 1. Use the expanded BNF I posted, which doesn't have this fault.
} > 2. Implement your dice parser using right-associativity for 'd'
} > operators (but maintain left-assoc for the other binary operators).
} > 
} > Feel free to do what you like.
} > 
} > 
} 
} Well, why do you say it's invalid ? Given the simplified BNF it must be
} read as :
} 
} 3d(d(d(d6)))
} 
} and it is perfectly valid as there is no other way to understand that ...

That is only true if the d operator is right-associative. According to the
original spec, it is left-associative and, therefore, 3dddd6 is a syntax
error. Mind you, the second option Matthew gave is to make it
right-associative, which is what you have done. I chose to treat it as a
syntax error.

} Pierre
--Greg