On Tue, Jun 21, 2011 at 3:36 PM, Matthew Steele <mdste...@alum.mit.edu> wrote: > On Jun 21, 2011, at 4:02 PM, Malcolm Wallace wrote: > >> On 21 Jun 2011, at 20:53, Elliot Stern wrote: >> >>> A tuple is basically an anonymous product type. It's convenient to not >>> have to spend the time making a named product type, because product types >>> are so obviously useful. >>> >>> Is there any reason why Haskell doesn't have anonymous sum types? If >>> there isn't some theoretical problem, is there any practical reason why they >>> haven't been implemented? >> >> The Either type is the nearest Haskell comes to having anonymous sum >> types. >> >> If you are bothered because Either has a name and constructors, it does >> not take long before you realise that (,) has a name and a constructor too. > > Yes, Either is to sum types what (,) is to product types. The difference is > that there is no "anonymous" sum type equivalent to (,,) and (,,,) and > (,,,,) and so on, which I think is what the original question is getting at. > Indeed, I sometimes wish I could write something like (straw-man syntax): > > foo :: (Int | Bool | String | Double) -> Int > foo x = > case x of > 1stOf4 i -> i + 7 > 2ndOf4 b -> if b then 1 else 0 > 3rdOf4 s -> length s > 4thOf4 d -> floor d > bar :: Int > bar = foo (2ndOf4 True) > > and have that work for any size of sum type. But I can't. >
Why not just do what we do for tuples? Define a bunch of generic types up front: data Choice2 a b = OneOf2 a | TwoOf2 b data Choice3 a b c = OneOf3 a | TwoOf3 b | ThreeOf3 c . . . Then you wouldn't need any special compiler support, and you can release it on its own library on Hackage. I don't think I would use it, be to each their own! > Cheers, > -Matt > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
