Perhaps this will help? http://www.dalnefre.com/wp/2011/05/parsing-expression-grammars-part-4/#expression
A different perspective, in any case. On Mon, Sep 7, 2015 at 4:27 PM, Nikita Nemkin <nik...@nemkin.ru> wrote: > Hello! > > I wonder if someone could help me with a certain assertion made in this > paper: http://arxiv.org/abs/1207.0443v3 (Medeiros, Mascarenhas, > Ierusalimschy. "Left Recursion in Parsing Expression Grammars" v3). > > The last paragraph on p.14: > > > We could impose left-associativity as the default in case of a grammar > that > > mixes left and right recursion just by changing k ≥ k' in rule lvar.4 to > k > k' > > and by changing k < k' in rule lvar.5 to k ≤ k'. The disadvantage is > that this > > makes specifying precedence levels with right-associative operators a > little > > harder > > I can't see how it's supposed to work. Consider this grammar: > > E = E[1] "+" E[1] / "n" > > where [1] is precedence and the initial precedence is also 1. > > Default precedence check is "fail if current rule precedence <= memoized > precedence," leading to right associative parse tree, as expected. > > Modified precedence check is "fail if current rule precedence < memoized > precedence" and it does NOT lead to left associativity. Instead, recursive > E[1] invocation always fails due to precedence check, reducing the grammar > to E = "n" "+" "n" / "n". > > General algorithm producing left association by default would be much > preferred from a usability perspective, if only it was possible. > > -Nikita > > _______________________________________________ > PEG mailing list > PEG@lists.csail.mit.edu > https://lists.csail.mit.edu/mailman/listinfo/peg >
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