I am writing a parser for a big, ugly, standard language and I need to decide between using either Happy or Parsec. I currently have a priliminary LALR(1) grammar, so a port to Happy would be relatively easy. But, I'm wondering if life would be easier if I chose Parsec's combinator parsing instead. It's error reporting seems to be top notch and it's "optional", "many", and "sepBy1" combinators are very elegant. However, I have a few concerns with Parsec. First is performance; what factor of slow-down should I expect? Second is bug prevention. I don't have much experience writing LL(n) grammars, so how easy is it to introduce bugs in a Parsec grammar? Even though I hate debugging LALR(1) parsing ambiguities, it prevents problems. Also, it appears the GHC developers chose Happy for Haskell. What was their rational? Thanks for your recommendations, one way or another. -Tom