Why doesn't this function signature typecheck?
data MyEither a b = MyLeft a | MyRight b
                    deriving (Read, Show)

extractEither :: MyEither a b -> c
extractEither (MyLeft p) = p

Couldn't match type `a' with `c'
  `a' is a rigid type variable bound by
      the type signature for extractEither :: MyEither a b -> c
      at /Users/tongmuchenxuan/playground/test.hs:5:1
  `c' is a rigid type variable bound by
      the type signature for extractEither :: MyEither a b -> c
      at /Users/tongmuchenxuan/playground/test.hs:5:1
In the expression: p
In an equation for `extractEither': extractEither (MyLeft p) = p

extractEither :: MyEither a a -> a
extractEither (MyLeft p) = p
extractEither (MyRight p) = p

extractMyLeft :: MyEither a b -> Maybe a
extractMyRight :: MyEither a b -> Maybe b

extractEither :: (a -> c) -> (b -> c) -> MyEither a b -> c
extractEither f g (MyLeft x)  = f x
extractEither f g (MyRight y) = g y

extractEither :: MyEither a b -> c
extractEither (MyLeft p) = p

public Object blah(Foo whatever);