[ragel-users] Parsing a template language

[Laslavic, Alex]

Keep in mind you can always muck with the location of 'p' in your input string.  There are ragel psudo functions ( fhold, fexec ) that do this for you, or you can always just do:

  p = p - 2

in your action code.  The above is in C, actually, i'm not sure how to do that with ruby.  Decrementing 'p' in ruby probably won't back it up along the string, but maybe 'fhold' or 'fexec' can help you.

I have used the ( any* -- token ) trick to advance to the token, and then modify 'p' to get it back to the beginning of the token.

Perhaps other in this list know a cleaner way to do this, but its what I've found that works.

-----Original Message-----
From: ragel-users-bounces at complang.org on behalf of Tobias Lütke
Sent: Thu 7/29/2010 12:40 AM
To: ragel-users at complang.org
Subject: Re: [ragel-users] Parsing a template language
 
Thanks Alex,

I modified the code to your clever example. This almost works, however
after running the any* scanner p will be advanced all the way to the
end of {{, so the other rule will not match the tag correctly. Here is
my current machine:


      machine parser;

      action start      { tokstart = p; }
      action on_tag     { results << [:tag,    data[tokstart..p]] }
      action on_static  { results << [:static, data[tokstart..p]] }

      tag  = '{{' lower+ '}}' >start @on_tag;
      html = (any* -- '{{')   >start @on_static;
      EOF = 0;

      main := |*
        tag;
        html;
        EOF;
      *|;

Regards
-- tobi



On Tue, Jul 27, 2010 at 9:52 PM, Laslavic, Alex
<Alex.Laslavic at turner.com> wrote:
> I'm actually working on a similar sounding task.
>
> Try the strong subtraction operator
> Untested:
>
> main := |*
>   '[[' lower+ ']]' => action
>   ( any* -- '[[' ) => action
> *|;
>
>
> ( any* -- '[[' ) will match the longest possible string that doesn't have
> '[[' as a substring.
>
> -----Original Message-----
> From: ragel-users-bounces at complang.org on behalf of Tobias Lütke
> Sent: Tue 7/27/2010 6:54 PM
> To: ragel-users at complang.org
> Subject: Re: [ragel-users] Parsing a template language
>
> Depends on the answers in this thread I suppose :-)
>
>
>
> On Tue, Jul 27, 2010 at 3:42 AM, Magnus Holm <judofyr at gmail.com> wrote:
>> (A little off-topic, but whatever:
>>
>> So Liquid will finally get a proper parser? :-))
>>
>> // Magnus Holm
>>
>>
>>
>> On Tue, Jul 27, 2010 at 03:15, Tobias Lütke <tobi at leetsoft.com> wrote:
>>> I've been working on a parser for simple template language. I'm using
>>> Ragel.
>>>
>>> The requirements are modest. I'm trying to find [[tags]] that can be
>>> embedded anywhere in the input string.
>>>
>>> I'm trying to parse a simple template language, something that can
>>> have tags such as {{foo}} embedded within HTML. I tried several
>>> approaches to parse this but had to resort to using a Ragel scanner
>>> and use the inefficient approach of only matching a single character
>>> as a "catch all". I feel this is the wrong way to go about this. I'm
>>> essentially abusing the longest-match bias of the scanner to implement
>>> my default rule ( it can only be 1 char long, so it should always be
>>> the last resort ).
>>>
>>> %%{
>>>
>>>  machine parser;
>>>
>>>  action start      { tokstart = p; }
>>>  action on_tag      { results << [:tag, data[tokstart..p]] }
>>>  action on_static  { results << [:static, data[p..p]] }
>>>
>>>  tag  = ('[[' lower+ ']]') >start @on_tag;
>>>
>>>  main := |*
>>>    tag;
>>>    any      => on_static;
>>>  *|;
>>>
>>> }%%
>>>
>>> ( actions written in ruby, but should be easy to understand ).
>>>
>>> How would you go about writing a parser for such a simple language? Is
>>> Ragel maybe not the right tool? It seems you have to fight Ragel tooth
>>> and nails if the syntax is unpredictable such as this.
>>>
>>>
>>> Regards
>>> -- tobi
>>>
>>> _______________________________________________
>>> ragel-users mailing list
>>> ragel-users at complang.org
>>> http://www.complang.org/mailman/listinfo/ragel-users
>>>
>>
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