[Moose-dev] Re: [Pharo-project] omit support in PetitParser

Hi Camillo, I don't really see use of #omit. The element is still in a collection and you need to extract it from there. What is the benefit other than it is now at index 1 instead of 2? Also the implementation has some problems: 1. #omit makes it really hard to compose grammars, because parsers suddenly get very asymmetric and start to affect each others behavior depending on how you compose them. 2. For the same reason grammar transformation becomes impossible, one of the design goals of PetitParser. 3. #omit makes a typical grammar (the smalltalk one) roughly 15% slower without even using the new feature. 4. There are various methods (#-, #match..., #copy..., ...) that need to be fixed when you add state to a parser. 5. There is no need for eager (#initialize) as well as lazy initialization (kills the possibility for a quick invocation). 6. Please do not remove PPParser class>>new and do not change how initialization works, otherwise you break GemStone, VisualWorks, and GNU Smalltalk. The Seaside wiki has some nice documentation of how to correctly initialize objects (basically following the pattern of Objective-C). I think, problems 1, 3 (!), 4, 5 and to some extent also 2 could be solved if the implementation was a bit more object-oriented. That is, if you added a PPOmitParser that would dispatch from #parseOn: to some other parsing method #parseOmittedOn: you wouldn't need all those #ifTrue:ifFalse:. Did you see #map: and #permutation:? Both allow you do the same in a less invasive way: parser := ($" asParser , $" asParser negate star flatten, $" asParser). parser map: [ :begin :inner :end | inner ] or parser := ($" asParser , $" asParser negate star flatten, $" asParser). parser permutation: #(2) Additionally, did you see the experimental binding/capturing functionality in PetitBeta? This kind of provides something much more general and (I believe) much more powerful without the above drawbacks. Bind-Capture allows you to bind certain parse results anywhere down the tree to names and directly access them later on in a capture block. It also deals with unpacking and reshuffling collections automatically, check the tests for that. Your example would look like:   parser := ($" asParser , ($" asParser negate star flatten bind: #inner) , $" asParser). parser capture: [ :inner | inner ] Cheers, Lukas -- Lukas Renggli www.lukas-renggli.ch