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Comment by Mathiciann | Hacker News Reader

Thanks for sharing! Especially day 13 is really compact and elegant. Do I understand correctly that you define parser combinators to parse the packages? (I'm not really familiar with Haskell nor parser combinators)

For me the parsing was the hardest part when trying without mutable state so this helps a lot.

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Yup! Parser combinators are incredibly useful once you learn the basics well. They compose predictably and you can test the individual parts pretty succinctly (I usually use an inline eval'd comment in vscode to smoke test, as you can see) and allow you to build an arbitrarily complicated data structure at parse time.

I'll admit first few times were a bit of a headache learning the library (https://hackage.haskell.org/package/attoparsec-0.14.4/docs/D...) and idiomatic patterns, plus how to test/troubleshoot. But now Haskell is my go-to language for parsing and I can parse pretty much any AoC input into a suitable representation, with the forethought taking no longer than reading the brief, by composing parsers for the different parts of the input string into a larger parser.

Also, disclaimer, since I'm the only one reading my code I often make oneliner "pointfree" parsers using dense syntax/tricks (as a little extra mental puzzle) instead of vastly more readable "do notation", so don't let the special character soup put you off.

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