* adam vogt [2014-02-13 19:51:34-0500]

On Thu, Feb 13, 2014 at 4:36 PM, Omari Norman wrote:

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If there is already something like this out there (or some elegant way to do this that doesn't involve hairy extensions), please let me know and I will be all too happy to deprecate this.

Omari

Hi Omari,

You could use:

type S3 a b c = (Maybe a, Maybe b, Maybe c)

And then prove to yourself that only one entry is Just. This lets you use of classes that work with tuples such as Field1 through Field9 from lens or things from http://hackage.haskell.org/package/tuple.

At least that's the strategy in < http://hackage.haskell.org/package/HList-0.3.2.0/docs/Data-HList-Variant.htm...

...

.

Yes, this solution is highly recommended... by @EvilHaskellTips :-) https://twitter.com/EvilHaskellTips/status/433403451054096384 Roman

On Fri, Feb 14, 2014 at 11:02 PM, Tikhon Jelvis wrote:

Yeah, S0 and S1 are probably worth including for the sake of completion. We don't have a 1-tuple, and I believe that's caused a bit of trouble for people writing generic libraries. Enough trouble so that somebody wrote a OneTuple package[1] anyhow.

S1 is easy enough; I added that in. http://hackage.haskell.org/package/anonymous-sums-0.2.2.0 S0 takes some more thought as there's more than one way to do that. I could use EmptyDataDecls or do it more like ekmett's Data.Void. Then what do you get for, e.g., partitionS0? partitionS0 :: [S0] -> () I suppose that makes sense if you look at partition as returning the same number of lists as there are type variables. No type variables, no lists. Or should it be partitionS0 :: [S0] -> Void which also makes me wonder if this is a special case better left to Data.Void. Thoughts?

On Sat, Feb 15, 2014 at 9:08 AM, Roman Cheplyaka wrote:

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partitionS0 :: [S0] -> ()

I suppose that makes sense if you look at partition as returning the same number of lists as there are type variables. No type variables, no lists. Or should it be

partitionS0 :: [S0] -> Void

which also makes me wonder if this is a special case better left to Data.Void.

The result should be a 0-tuple, i.e. (), of course.

Otherwise, what would paritionS0 [] return?

OK, so with EmptyDataDecls this yields: data S0 deriving (Generic, Typeable) instance Eq S0 where _ == _ = undefined instance Ord S0 where compare _ _ = undefined instance Read S0 where readsPrec _ = undefined instance Show S0 where show _ = undefined partitionS0 :: [S0] -> () partitionS0 _ = () caseS0 :: S0 -> z caseS0 = undefined mapS0 :: S0 -> S0 mapS0 = id mapS0f :: Functor ftr => S0 -> ftr S0 mapS0f = undefined