Hi Fernando Which version of Parsec are you using, the one that ships with GHC or Parsec 3? I would imagine whichever you one you are using you have the imports wrong for these modules: import Text.Parsec.Pos import Text.Parsec.Prim should be ... import Text.ParserCombinators.Parsec.Pos import Text.ParserCombinators.Parsec.Prim Also it seems like an applicative instance for Parsec's GenParser monad is missing. This isn't defined for parsec-2.1.0.1, people seem to define it themselves: -- The Applicative instance for every Monad looks like this. instance Applicative (GenParser s a) where pure = return (<*>) = ap ... you will need `ap` from Control.Monad in scope. Also there is a pdf document for Parsec (should be available from Daan Leijen's home page) that covers separate scanners, it has quite a lot more documentation than Parsec's Haddock docs. For semantic analysis I'd highly recommend UUAG, it is well documented and used for a large compiler (EHC). There is also a version of Andrew Appel's Tiger language written with it that is much smaller and more comprehensible, the version on Hackage doesn't seem to contain the attribute grammar source but it is available here: http://www.cs.uu.nl/wiki/bin/view/HUT/TigerCompiler http://www.cs.uu.nl/wiki/bin/view/HUT/AttributeGrammarSystem Best wishes Stephen