GHC error messages and their meaning.
1 "foo' is not a (visible) method of class `Bar'"
This error message occurs when one tries to instantiate a class, but did not import the functions one tries to implement.
import Prelude hiding ((==)) data Foo = Foo instance Eq Foo where (==) a b = True
2 "Cannot match a monotype with `Foo'"
See this Haskell-Cafe thread.
3 "Parse error in pattern"