[Haskell-cafe] open-union

[adam vogt]

Hi Ben,

http://hackage.haskell.org/package/HList-0.3.4.1/docs/Data-HList-TIC.html
is very similar, though it uses -XDataKinds for the type-level list
and has fewer operations. For the next release we're changing the
type-indexed collections (TIP, TIC) to share most of the
implementation with Record and Variant respectively.

I believe it's unnecessary to involve Typeable. Since there's a list
of possible types, you can store (Int, Any), where the Int is an index
into that list, instead of using TypeRep for that index.
http://code.haskell.org/~aavogt/HList/Data/HList/Variant.hs has that
implementation. Leaving out Typeable makes a difference as far as what
is allowed, because Records using promoted strings as the labels do
not have a Typeable instance in ghc-7.8.2.

Regards,
Adam

On Sat, Jun 7, 2014 at 3:47 PM, Ben Foppa <benjamin.foppa at gmail.com> wrote:
> Thanks, I wasn't aware of vinyl and compdata. I'll check them out.
> I'm not very familiar with HList, but I was under the impression that it
> provided intersection, not union, i.e. for every type in { x1 x2 ..}, an
> HList has an element vs for SOME type  in {x1 x2 ..} a Union has an
> element..
>
>
> On Sat, Jun 7, 2014 at 3:18 PM, Carter Schonwald
> <carter.schonwald at gmail.com> wrote:
>>
>> There's also compdata
>>
>>
>> On Saturday, June 7, 2014, Ben Foppa <benjamin.foppa at gmail.com> wrote:
>>>
>>> Hi cafe. On a use case whim, I made the open-union package
>>> (https://github.com/RobotGymnast/open-union), copying the basic idea from
>>> extensible-effects:Data.OpenUnion1. I haven't uploaded to hackage yet, on
>>> the chance that there's already something like this around. Here's the basic
>>> functionality:
>>>
>>> {-# LANGUAGE TypeOperators #-}
>>> {-# LANGUAGE ScopedTypeVariables #-}
>>> import Data.OpenUnion
>>>
>>> type MyUnion = Union (Char :| Int :| [()] :| Void)
>>>
>>> showMyUnion :: MyUnion -> String
>>> showMyUnion
>>>     =  (\(c :: Char) -> "char: " ++ show c)
>>>     @> (\(i :: Int) -> "int: " ++ show i)
>>>     @> (\(l :: [()]) -> "list length: " ++ show (length l))
>>>     @> typesExhausted
>>>
>>> main :: IO ()
>>> main = do
>>>     putStrLn $ showMyUnion $ liftUnion (4 :: Int)
>>>     putStrLn $ showMyUnion $ liftUnion 'a'
>>>     putStrLn $ showMyUnion $ liftUnion [(), ()]
>>>
>>>
>>> If any of the (@>) cases is omitted, a compile-time error occurs. If you
>>> try to lift a bad value to the union, a compile-time error occurs.
>>>
>>> Any thoughts? Is there already something like this around?
>
>
>
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