Hi John,
from what I gather this is because Show instance for "Array a b",
which you are overlapping, is defined in a module without the
OverlappingInstances declaration. Check the last few paragraphs of
this from the GHC's user's guide:
http://www.haskell.org/ghc/docs/6.12.2/html/users_guide/type-class-extension...
Markus Läll
On Sun, Oct 24, 2010 at 4:54 PM, John Smith
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
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