PetitParser: refer to previous values

>> Is there a way to do something like:
>> #digit asParser, 'same digit that I just parsed' asParser ?
>>
> I'm not sure if there is a possibility in declarative way. But you can work on the products of the individual parsers. You can do it like this:
>
> twoEqualDigits
>        ^ #digit asParser, #digit asParser ==> [:nodes|
>                (nodes first = nodes second)
>                        ifTrue: [ nodes ]
>                        ifFalse: [ PPFailure message: 'not the same value' at: 0]]

>>> Is there a way to do something like:
>>> #digit asParser, 'same digit that I just parsed' asParser ?
>>>
>> I'm not sure if there is a possibility in declarative way. But you can work on the products of the individual parsers. You can do it like this:
>>
>> twoEqualDigits
>>        ^ #digit asParser, #digit asParser ==> [:nodes|
>>                (nodes first = nodes second)
>>                        ifTrue: [ nodes ]
>>                        ifFalse: [ PPFailure message: 'not the same value' at: 0]]
>
> Yes, this is essentially the trick.
>
> Have a look at some of the other references to PPFailure, for example
>
> - in PPXmlParser>>element we ensure that the open and close tag are the same,
> - in PPSmalltalkGrammar>>number an external parser is used to validate
> and extract the number, and
> - in PPObjectTest>>testFibonacci the input must be a fibonacci
> sequence (quite complicated code).
>
> Now this seems to be quite a common pattern, so it might be worth to
> add a helper method to PetitParser. I imagine something along the
> lines of
>
>    (#digit asParser , #digit asParser)
>         condition: [ :nodes | nodes first = nodes second ]
>         message: 'not the same value'
>
>>> I snooped around and saw PPMemoizedParser, but didn't exactly understand the
>>> test for it.
>>
>> I would like to know exactly, too :)
>
> Did you see the class comment and the method comment in the factory
> method of PPMemoizedParser?
>
> PPParser>>memoized
> "Answer a new memoized parser, for refraining redundant computations.
> This ensures polynomial time O(n^4) for left-recursive grammars and
> O(n^3) for non left-recursive grammars in the worst case. Not
> necessary for most grammars that are carefully written and in O(n)
> anyway."
>
> ^ PPMemoizedParser on: self
>
> So, this is to cache repeated parses of the same rule, at the same
> position, and in the same input stream. In almost all cases it is not
> useful and actually slows things down. In rare cases it is necessary,
> but then you should probably rather fix your grammar.
>
> The basic idea behind memoization is explained in "Packrat parsing:
> simple, powerful, lazy, linear time" by Bryan Ford
> (http://arxiv.org/pdf/cs/0603077). The implementation of PetitParser
> is actually based on ideas presented in "Packrat Parsers Can Support
> Left Recursion" by Alessandro Warth
> (http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.99.7466&rep=rep1&type=pdf).
> Note that the PetitParser implementation of PPMemoizedParser has the
> same problem as the one of Warth leading to invalid parses on some
> PEGs. Details can be found in "Direct Left-Recursive Parsing
> Expressing Grammars" by Laurence Tratt
> (http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.156.7945&rep=rep1&type=pdf).