[ragel-users] How do I act on eof in state charts

[Antony Blakey]

On 22/10/2009, at 12:10 AM, Antony Blakey wrote:

> string_literal_body =
>  start: (
>    "'" -> seen_quote |
>    [^'] -> start
>  ),
>  seen_quote: (
>    "'" -> start |
>    [^'] @{ fhold; } -> final
>  );
> string_literal = "'" string_literal_body %{ puts "string_literal" } ;
>
> The problem occurs when a string literal ends at eof. How do I  
> specify the eof 'match' in the seen_quote state such that all the  
> leaving-transition actions that are in place above the  
> string_literal are executed, such as the 'puts' on the  
> 'string_literal' machine. I don't want to manually duplicate the  
> parent code because multiple machines reference string_literal, with  
> different leaving-transition actions.
>
> I couldn't get it to work using priorities - the terminator needs  
> lookahead to disambiguate; the following doesn't work:
>
> string_literal = "'" [^']* ( "''" [^']* )* '"'

I ended up doing this:

   string_literal_unqoted = "'" [^']* "'" ;
   string_literal = string_literal_unqoted+ $(longest, 1) %(longest,  
0) % { puts "string_literal" };

which works. I would have thought that this:

   string_literal = string_literal_unqoted string_literal_unqoted** %  
{ puts "string_literal" };

would work, but '**' doesn't work for me - if I use a main like this:

   main := space* ( string_literal space* )* ;

then I get two string_literals from 'a''b' rather than one, while the  
explicit priorities do give me one string_literal.

Of course this works as well:

   main := space* ( string_literal <: space* )* ;

but given the pervasiveness of whitespace handling in my grammar (a  
full Smalltalk parser) it's a real PITA because everything you want  
greedy consumption for has to be thus annotated wherever it is used.

Still, I'm interested in how you specify EOF transitions in an  
explicit state machine. I also had success appending a 0 onto the  
input for use as an EOF marker that can be matched in the state  
machine, but I'm not sure yet if/how that will interfere with true EOF  
functioning.

Antony Blakey
-------------
CTO, Linkuistics Pty Ltd
Ph: 0438 840 787

In anything at all, perfection is finally attained not when there is  
no longer anything to add, but when there is no longer anything to  
take away.
   -- Antoine de Saint-Exupery