GHC/Error messages

GHC error messages and their meaning.

"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

"Cannot match a monotype with `Foo'"

See this Haskell-Cafe thread.

"Parse error in pattern"

TODO

Example: TODO