[Haskell-beginners] Re: Overlapping Instances

[Markus Läll]

I think the IncoherentInstances doesn't solve the problem. It does
also not seem to be the problem (just) of Show B -- the overlap-error
arises whenever you actually try to use show on "Array Int Bool".
Like, when you specifiy the type of a as:
> a = show $ listArray (1,3 :: Int) $ repeat True
even when the Show B instance is commented out, you get the same error...

I can't exactly tell, because I'm quite new for Haskell myself, but it
might be, that TypeSynonymInstances works only for those cases, where:
- no Show instance for that type exists, or
- a Show instance for that type exists, but the module where it was
defined also uses the OverlappingInstances flag (try it with a custom
type!)

The end of the GHC docs section also says, that this choice of how
things happen is open for dispute, but it seems it has not came up
recently enouugh to change it.

With the two ways to implement it, right now we have the promise, that
when you define an instance for a type you created, it cannot be
overlapped by others (unless wrapped in a newtype). But when the
system is changed, the user user of some module gets to create more
specific instances for types in that module.


--
Markus Läll

On Mon, Oct 25, 2010 at 11:00 PM, John Smith <voldermort at hotmail.com> wrote:
> Thank you. Why does this code succeed for a, but stills fails on instance
> Show B? Do they not both invoke the same Show A?
>
> {-# LANGUAGE TypeSynonymInstances, OverlappingInstances, IncoherentInstances
> #-}
>
> 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
>
> a = show $ listArray (1,3) $ repeat True
>
> On 24/10/2010 16:37, Markus Läll wrote:
>>
>> 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
>
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