[Haskell-cafe] Class constraints on associated types ...

[aditya siram]

Hi,
Unfortunately while this compiles it does not work correctly. For example
the code:
data D1
data D2
class Dispatch a b where
   type Impl_ a b :: *
   runF :: a -> b -> Impl_ a b
instance (Show a, Impl_ D1 D2 ~ (a -> IO ())) => Dispatch D1 D2 where
   runF _ _ = print

compiles with the warning:
    No explicit associated type or default declaration for ‘Impl_’
    In the instance declaration for ‘Dispatch D1 D2’

But however when I run it I see that the type function does not resolve:
*Main> runF (undefined :: D1) (undefined :: D2)

<interactive>:8:1:
    Couldn't match expected type ‘a0 -> IO ()’
                with actual type ‘Impl_ D1 D2’
    The type variable ‘a0’ is ambiguous
    In the first argument of ‘print’, namely ‘it’
    In a stmt of an interactive GHCi command: print it

Thanks!
-deech



On Sat, Jan 3, 2015 at 8:59 AM, aditya siram <aditya.siram at gmail.com> wrote:

> Hi,
> That seemed to compile! I had no idea this kind of construction was even
> possible!
>
> However it did spew out a bunch of warnings like:
> "No explicit associated type or default declaration for ‘Impl’ in instance
> ..."
>
> Thanks!
> -deech
>
> On Fri, Jan 2, 2015 at 1:38 PM, Erik Hesselink <hesselink at gmail.com>
> wrote:
>
>> Could you do something like this?
>>
>>     instance (Impl D1 D2 ~ a -> IO (), C a) => Dispatch D1 D2
>>
>> Erik
>>
>> On Fri, Jan 2, 2015 at 7:59 PM, aditya siram <aditya.siram at gmail.com>
>> wrote:
>> > Hi all,
>> > I'd like to be able to constrain an associated type.
>> >
>> > I used to have an instance that looked like:
>> > class Dispatch a b c | a b -> c where
>> >    runF :: a -> b -> c
>> > instance (C a) => Dispatch D1 D2 ( a -> IO ()) where
>> >    runF d1 d2 = (\_ -> return ())
>> >
>> > Since I upgraded to 7.8 from 7.5 that instance declaration is no longer
>> > accepted is no longer accepted since it violates FD's.
>> >
>> > I have been updating my code to use type families like so:
>> > class Dispatch a b where
>> >    type Impl a b :: *
>> >    runF :: a -> b -> Impl a b
>> > instance (C a) => Dispatch D1 D2 where
>> >    type Impl D1 D2 = a -> IO ()
>> >    runF d1 d2 = (\_ return ())
>> >
>> > Unfortunately the `type Impl ...` line in the instance is rejected
>> because
>> > it uses `a` on the RHS.
>> >
>> > In this one case I could just package it up into a newtype or something
>> but
>> > I would ideally like to be able to constrain any number of arguments
>> like:
>> > instance (C a, C b ... C z) => Dispatch D1 D2 where
>> >    type Impl D1 D2 = a -> b -> ... -> z -> IO ()
>> >    ...
>> >
>> > This was something I could do in 7.6 (although I realize this is way
>> safer).
>> > How do I go about getting that constraint back?
>> >
>> > Thanks!
>> > -deech
>> >
>> > _______________________________________________
>> > Haskell-Cafe mailing list
>> > Haskell-Cafe at haskell.org
>> > http://www.haskell.org/mailman/listinfo/haskell-cafe
>> >
>>
>
>
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