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

[Mike Erickson]

On Tue, Jun 21, 2011 at 1:36 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.
>

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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