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How Janet's PEG module works

81 points by behnamoh

by mplanchard

2 subcomments

I did Advent of Code in Janet the year before last I think, and really loved the PEG support. Essentially every day started out with making a quick grammar to parse the problem into whatever data structures I was using. It's intuitive, pretty easy to pick up and adjust, and powerful.

by norir

2 subcomments

I am not a fan of PEG. It is straightforward to write a fast parser generator for languages that require just one character of lookahead to disambiguate any alternation in the grammar. This gets you most of the expressivity of PEG and nearly optimal performance (since you only need to look at one character to disambiguate and there is no backtracking). Just as importantly, it avoids the implicit ambiguities that PEG's resolution algorithm can hide from the grammar author that lead to unexpected parse results that can be quite difficult to debug and/or fix in the grammar.
It does require a bit more thought to design an unambiguous language but I think it's worth it. While there is a learning curve for designing such languages, it becomes natural with practice and it becomes hard to go back to ambiguous languages.