GHC/Error messages

From HaskellWiki

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.

Example:

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"

TODO

Example: TODO