Parsec is designed from scratch as an industrial-strength parser
library.  It is simple, safe, well documented (on the package
homepage), has extensive libraries and good error messages, and is
also fast.  It is defined as a monad transformer that can be stacked
on arbitrary monads, and it is also parametric in the input stream
type.

WWW: http://legacy.cs.uu.nl/daan/parsec.html