[Haskell-cafe] extensible effects and open unions

[adam vogt]

Hi Suhail,

On Fri, Nov 28, 2014 at 4:52 AM, Suhail Shergill
<suhailshergill at gmail.com> wrote:
> having recently taken over as maintainer for the extensible-effects library, i'm
> looking to address some of the current implementation concerns. specifically:
>
> 1] the use/need for Typeable in Data.OpenUnion
> how does the TList.hs implementation compare with, say, OpenUnion2.hs? neither
> require OverlappingInstances, and the TList implementation also does away with
> the Typeable constraint. are there reasons why it might not make sense to use
> TList.hs as the only/default implementation of Data.OpenUnion?

You need to write an instance of TCode for every different "effect"
included in the union for the lookup to work. Check out this example
usage of TList.hs:

mkV :: Int -> ([] :> Maybe :> Void) Int
mkV 1 = H [1,2,3]
mkV 2 = T (H (Just 5))
mkV 3 = T (T (undefined :: Void Int))

-- | >>> test1
-- [Just [1,2,3],Nothing,Nothing]
test1 :: [Maybe [Int]]
test1 = map (prj . mkV) [1 .. 3]

-- | >>> test2
-- [Nothing,Just (Just 5),Nothing]
test2 :: [Maybe (Maybe Int)]
test2 = map (prj . mkV) [1 .. 3]

type instance TCode [] = Z
type instance TCode Maybe = S Z


If you instead had

type instance TCode [] = Z
type instance TCode Maybe = Z

then test2 would not typecheck, and the type error doesn't suggest (to
me) that the TCode instances are wrong.


I think you're better off depending on a

type family Eq :: Bool where
 Eq x x = True
 Eq x y = False

Or the equivalent with overlapping instances if the code is supposed
to work with ghc-7.6.


Another objection about TList is that it is a linked list, so
operations with types at the "end" of the union are probably
relatively slow at runtime, since you end up pattern matching on "n" T
constructors in some cases. It might be faster to have more of that
traversal done at compile time as in:

 http://code.haskell.org/HList/Data/HList/Variant.hs

Or with unions that use Typeable.


I'm not sure about your other questions.

Regards,
Adam