For the last few days I've been rewritting the parsing in Hoogle to use parsec. As an end result, the parser is a lot more powerful, and more maintainable, and extendable - on the downside its longer and more complex.
The thing that most impressed me about parsec was its compositionality. In Hoogle there are type signatures, which are both on database lines (map :: (a -> b) -> [a] -> [b]), and there are user queries which might have a type signature in them, amongst other junk. Thanks to parsec I can use the same type signature parser in both of them, with extensions for the relevant bits. I couldn't really do this with a traditional yacc/bison/happy parser generator. Its also great for the fuzzy nature of user searches - you don't want to parse error if there is any sensible user interpretation of what they wrote.
Thanks to this rewrite, I now get a few query goodies that I always wanted but could never properly parse. Included in this are multiple words "concat map", names and type signatures "map :: [a] -> [a]" and the search parser now checks for command line options, which stops the bugs with things like "->" being misinterpretted.