On Tue, Jun 21, 2011 at 1:36 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. > The syntax is truly awful, but this doesn't seem that far from foo :: Either Int (Either Bool (Either String Double)) -> Int foo (Left e) = e + 7 foo (Right (Left e )) = if e then 1 else 0 foo (Right (Right (Left e))) = length e foo (Right (Right (Right e))) = floor e foo . Right . Left $ True Mike
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