Porting Smarty to Twig with PEGs

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I'm not an expert on parsing, but I used PEG.js recently and found it a lot of fun to work with - a good way for a parsing noob to learn, or if you just need something with a little more rigor than regular expressions.

In the future I'd like to use a PEG with an additional tokenization step, as the parser generated from a pure top-down approach will give fairly useless error messages when you have a typo deep in a nested expression, and I think that could be mitigated if I could point those errors out during tokenization.

> as the parser generated from a pure top-down approach will give fairly useless error messages when you have a typo deep in a nested expression,

Errors in general are a hard part of PEG to solve. What you really want to do is save the "most correct" failed descent as you iterate across a rule. Usually they define it as the "deepest" failed subexpression. If your implementation doesn't implement error preservation like this, it's time to add it. :)

I don't think tokenization would really help here, and the preference by most PEGs to not have a separate class of token expressions is often cited as one of the major strengths of PEGs.

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