[Haskell-cafe] datatype contexts

[Gregory Crosswhite]

Oh, now I see!  I knew about (and have used) existential contexts, but
somehow I hadn't made the connection that in a sense they are already
equivalent to our intuition for Haskell 98 contexts "done right".  :-) 
Thanks!

Any chance of seeing them in Haskell'11?

Cheers,
Greg

On 07/26/10 10:44, Ryan Ingram wrote:
> There are two types of datatype contexts; haskell'98 contexts (which I
> think are terrible), and GHC existential contexts (which I like):
>
> class C a where runC :: a -> Int
> data C a => T1 a = D1 a
>
> All this does is add a context to the D1 *constructor*; that is:
> -- D1 :: C a => a -> T1 a
>
> But extracting a value of this type does nothing:
>
> foo :: T1 a -> Int
> foo (D1 a) = runC a -- compile error
>
> However, putting the context on the RHS as you have done works in GHC
> and does "the right thing"; pattern matching on that constructor now
> brings the class into scope.  You can think of the datatype has having
> another field which is "proof that a is a member of C":
>
> {-# LANGUAGE ExistentialQuantification #-}
> data T2 a = C a => D2 a
> -- D2 :: C a => a -> T2 a  -- same as D1
>
> bar :: T2 a -> Int
> bar (D2 a) = runC a -- works
>
>   -- ryan
>
> On Mon, Jul 26, 2010 at 7:48 AM, Gregory Crosswhite
> <gcross at phys.washington.edu> wrote:
>   
>>  I agree with prior discussion on this list that adding contexts to datatype
>> declarations seems to be more trouble than its worth, since these contexts
>> just have to be added again to every function using the datatype.  However,
>> I have often wondered:  why do function *have* to have these contexts?  What
>> would it affect in the language if we made the contexts be implicit, so that
>> if we have
>>
>>    data Datatype a = Context a => Datatype a
>>
>> then for function declarations
>>
>>    f :: D a -> ...
>>
>> the context "Context a" is automatically asserted by the compiler?
>>
>> Cheers,
>> Greg
>>
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