[Haskell-cafe] duplicate instance declarations. Why?
Ryan Ingram
ryani.spam at gmail.com
Fri Oct 24 05:40:22 EDT 2008
Instance instantiation is *not* search, and *not* similar to
subclassing in OO languages.
Both your instance declarations simply add constraints to functions that use it.
Here's a more concrete example:
class A a where
doA :: a -> String
class R a where
doR :: a -> String
instance R Int where doR = show
instance R a => A a where
doA = doR
question x = doA x
The question is, what is the type of "question"? Here are two valid
choices with this program:
question :: (A a) => a -> String
question :: (R a) => a -> String
The instance R a => A a says that every type "a" is an instance of
"A"; if an instance for A is needed, the compiler says "OK, I know how
to make one of those. But I now add a new constraint, R a."
Adding another instance S a => A a makes the choice of what constraint
to add ambiguous. In particular the following code does *not* work:
class S a where
doS :: a -> String
instance S String where
doS = id
instance S a => A a where
doA = doS
question2 = question (2::Int)
question3 = question "3"
In my experience, if you are turning undecidable instances on and you
don't know exactly why it's safe to do so, there is probably a mistake
in your design.
-- ryan
2008/10/24 Alberto G. Corona <agocorona at gmail.com>:
> with:
>
>>{-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}
>
>>class A a
>>class R a
>
>>class S a
>
>>instance R a => A a
>
>>instance S a => A a
> ----------
>
> GHC gives
>
> Duplicate instance declarations
> instance R a => A a
> instance S a => A a
>
> Why?
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