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

[Erik Hesselink]

Ah, you're right, I was confused. You can't give an associated type
like this (with the forall). The best I can think of is to use a GADT,
something like this:

{-# LANGUAGE TypeFamilies, MultiParamTypeClasses, GADTs #-}

data D1
data D2
data Showable where
  Showable :: Show a => a -> Showable
class Dispatch a b where
   type Impl_ a b :: *
   runF :: a -> b -> Impl_ a b
instance Dispatch D1 D2 where
   type Impl_ D1 D2 = Showable -> IO ()
   runF _ _ (Showable x) = print x

Now you can run it like this:

*Main> runF (undefined :: D1) (undefined :: D2) (Showable True)
True

Regards,

Erik

On Sun, Jan 4, 2015 at 3:33 PM, aditya siram <aditya.siram at gmail.com> wrote:
> 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
>>> >
>>
>>
>