[ragel-users] Parsing a template language

[Tobias Lütke]

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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