Here's a test case for the problem I'm having; I'm using runhaskell from ghc v6.6.

Problem #1) Without -fallow-undecidable-instances, I get the following error:

    Constraint is no smaller than the instance head
      in the constraint: ConvertToInt a
    (Use -fallow-undecidable-instances to permit this)
    In the instance declaration for `ConvertToIntList a'

Problem #2) With -fallow-undecidable-instances, I get this error instead:

    Overlapping instances for ConvertToIntList ()
      arising from use of `convl' at testcase.hs:28:6-15
    Matching instances:
      instance (ConvertToInt a) => ConvertToIntList a
-- Defined at testcase.hs:15:0
      instance ConvertToIntList () -- Defined at testcase.hs:18:0
    In the expression: convl [()]
    In the definition of `xl2': xl2 = convl [()]

I don't understand why there is an overlapping instances error; () is not an instance of ConvertToInt so how could that instance ever apply?

Is there something basic about type-classes that I'm not understanding here? My actual problem is more complicated than this, but this test-case covers the basic issue; something being an instance of class A means that I can derive an instance of class B for it, but I want to implement other instances of class B as well.

Code below:

{-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}

module TestCase

where

class ConvertToInt a where
conv :: a -> Int

class ConvertToIntList a where
convl :: [a] -> [Int]

instance ConvertToInt Int where
conv = id

instance ConvertToInt a => ConvertToIntList a where
convl = map conv

instance ConvertToIntList () where
convl x = []

x :: Int
x = 5

xl :: [Int]
xl = convl [x]

xl2 :: [Int]
xl2 = convl [()]