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' 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=re…)ype=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=r…)ype=pdf).