[ragel-users] Problem generating code.
husam at senussi.com
Mon May 10 17:23:49 UTC 2010
Thanks for the pointer Brian for the pointer looking again at the RFC2822 grammar looks like there is ambiguity,
I will need to see if I can fix it and keeping the parser complaint with the spec
On 9 May 2010, at 22:13, Brian Pane wrote:
> On Sun, May 9, 2010 at 1:43 PM, Husam Senussi <husam at senussi.com> wrote:
>> I'm trying to create RFC2822 parer but I'm having problem generating the code for
>> some reason ragel keep running until I press CTRL C "was running for 20 minutes",
>> below the grammar I'm trying to use.
> Whenever I've encountered very long Ragel processing times with my own
> grammars, the reason usually has been nondeterminism. For example, with
> a grammar like this:
> word = space* alpha+ space*;
> number = space* digit+ space*;
> main = ( space+ | word | number )*;
> there is an ambiguity: if the first input character is a space, it might
> be the start of the "space*" option in main, but it might also be the
> start of the "word" option in main or the start of the "number" option
> in main, since those can start with a space also.
> Internally, Ragel has to build a state graph that models those
> nondeterministic states. The more ambiguity there is in the
> grammar, the bigger this graph becomes, and the longer it
> takes for Ragel to run. With my own grammars, I've found
> that the run time of Ragel and the subsequent C compilation
> is a good estimator of how nondeterministic my grammar is.
> I've found that it helps, when using the "|" operator, to make
> the different options start with distinct prefix strings. In
> the case of my example grammar above, a less ambiguous
> alternative would be:
> word: alpha+;
> number: digit+;
> main := ( space+ | word | number )+;
> For more complicated languages, another good pattern I've
> learned from other people's grammars is to put optional space
> at the end of each rule and never at the start.
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