Last weekend, my brain was unexpectedly hijacked—very thoroughly hijacked—by my discovery of a Rust library called pest. Pest’s basic concept is relatively simple: it’s a general-purpose parser which, instead of parsing Insert Specific Language Here (in the manner of e.g. the various Serde formats, quick-xml, et cetera), lets you define your own grammar for the parser to use and parse any input stream captured by that grammar.

The grammar-definition part of pest took me a decent number of hours to learn—learning that was the really seriously brain-hijacking bit—but it was conveniently facilitated by the highly-convenient editor located at the bottom of the pest website (linked above). I only stumbled my way into a left-recursive loop once, during that time. And now I’ve got a grammar defined which ought to dramatically simplify the back-end text-parsing section of my old dice-roller project (which I ultimately chose to use as the test case for teaching myself pest, since it’s neither excessively simple nor excessively complicated and since I’d been meaning to do a full backend rewrite of it anyway for unrelated reasons).

The output trees pest produces once the input has been slurped up and parsed, on the other hand, leave something to be desired. They work—they successfully represent the parsed information in a useful way—but they’re relatively code-intensive to work through and drill down into, without much in the way of easy simplifying methods built into them. (At least unless you bring in an external library like pest_consume, but when I tried that one out, I found it to be pretty inconvenient in its own right, and ultimately abandoned it in favor of default pest.)

I haven’t yet finished rewriting my dice roller’s back end. Maybe it’ll turn out there’s some horrible impediment, or just a pile of lots and lots of small inconveniences, such that pest is less of a convenience-boost than I currently think it is. Or maybe the dice roller will go fine, but I’ll discover major limitations of some sort when I try to throw pest at a more ambitious project. But, tentatively, for the moment, I’m inclined to say: I like pest. Its grammar side is beautiful and powerful and something I’m likely to continue admiring for A While. Its tree outputs are… substantially less beautiful… but they still, a far as I can tell, basically work. And its overall functionality is very broadly applicable, something I anticipate having a use for in lots of different programs I write in the future. So, overall, at least for the moment, I am A Fan.

Tags: Rust