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

Matthew Steele mdsteele at alum.mit.edu
Tue Jun 21 22:36:52 CEST 2011

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.


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