[Haskell-cafe] Why aren't there anonymous sum types in Haskell?

[Antoine Latter]

On Tue, Jun 21, 2011 at 3:36 PM, Matthew Steele <mdsteele at 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
>
>
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