[Haskell-cafe] Why aren't there anonymous sum types in Haskell?
dmbarbour at gmail.com
Wed Jun 22 01:34:28 CEST 2011
On Tue, Jun 21, 2011 at 1:36 PM, Matthew Steele <mdsteele at alum.mit.edu>wrote:
> 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.
Haskell operators are pretty flexible. You can get something really close
without much effort. Consider:
type (:|:) a b = Either a b
(???) = either
foo :: (Int :|: Bool :|: String :|: Double) -> Int
\ i -> i + 7 ???
\ b -> if b then 1 else 0 ???
\ s -> length s ???
\ d -> floor d
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