Re: [Haskell-cafe] open-union

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 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 wrote:

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There's also compdata

On Saturday, June 7, 2014, Ben Foppa 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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