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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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 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 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

On Fri, Jan 2, 2015 at 7:59 PM, aditya siram 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

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