Following is a simplification of some code which I have written. I have an overlapping Show instance for A, which is more specific than the general instance for arrays, so I would expect it to be acceptable as an overlapping instance. Nevertheless, I get the following compiler error: Overlapping instances for Show A arising from a use of `show' at 13:17-22 Matching instances: instance (Ix a, Show a, Show b) => Show (Array a b) -- Defined in GHC.Arr instance [overlap ok] Show A -- Defined at 9:9-14 In the expression: show a In the definition of `show': show (B a) = show a In the instance declaration for `Show B' Compilation failed. I've tried UndecidableInstances and IncoherentInstances, but they don't seem to help. What am I doing wrong? Many thanks in advance for any assistance. -John {-# LANGUAGE TypeSynonymInstances, OverlappingInstances #-} import Data.Array type A = Array Int Bool data B = B A instance Show A where show a = "foo" instance Show B where show (B a) = show a