[Haskell-cafe] open-union

[Carter Schonwald]

Id be surprised if there's not a way to encode a union like structure using
hlist.

On Saturday, June 7, 2014, 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
> <javascript:_e(%7B%7D,'cvml','carter.schonwald at gmail.com');>> wrote:
>
>> There's also compdata
>>
>>
>> On Saturday, June 7, 2014, Ben Foppa <benjamin.foppa at gmail.com
>> <javascript:_e(%7B%7D,'cvml','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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