[Haskell-cafe] Class constraints on associated types ...
aditya.siram at gmail.com
Sun Jan 4 14:33:42 UTC 2015
Unfortunately while this compiles it does not work correctly. For example
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)
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
On Sat, Jan 3, 2015 at 8:59 AM, aditya siram <aditya.siram at gmail.com> wrote:
> That seemed to compile! I had no idea this kind of construction was even
> However it did spew out a bunch of warnings like:
> "No explicit associated type or default declaration for ‘Impl’ in instance
> On Fri, Jan 2, 2015 at 1:38 PM, Erik Hesselink <hesselink at gmail.com>
>> Could you do something like this?
>> instance (Impl D1 D2 ~ a -> IO (), C a) => Dispatch D1 D2
>> On Fri, Jan 2, 2015 at 7:59 PM, aditya siram <aditya.siram at gmail.com>
>> > 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
>> > it uses `a` on the RHS.
>> > In this one case I could just package it up into a newtype or something
>> > I would ideally like to be able to constrain any number of arguments
>> > 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
>> > 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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