[Haskell-beginners] Overlapping Instances

[Markus Läll]

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-extensions.html#instance-overlap


Markus Läll

On Sun, Oct 24, 2010 at 4:54 PM, John Smith <voldermort at hotmail.com> wrote:
> 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
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>