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

4 Jan 2015


      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  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 
wrote:
...
Could you do something like this?
instance (Impl D1 D2 ~ a -> IO (), C a) => Dispatch D1 D2
Erik
...
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
On Fri, Jan 2, 2015 at 7:59 PM, aditya siram 
wrote:
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
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