[Haskell-cafe] Why is the no tuple syntax for sum types

[Silvio Frischknecht]

> For sum types you need a way to construct them and pattern match them.
> Without a way to _name_ them, it would probably look like this:
> 
>     showError :: (Result | Error3 | Error8) -> String
>     showError (result ||)   = "no error"
>     showError (| error3 |)  = "error3: " ++ show error3
>     showError (|| error8 |) = "error8: " ++ show error8
> 
> It's not the most readable code, so I'd generally avoid using it for
> anything greater than 3 (same applies to tuples).

I doubt that functions usually cause more than one or two different
errors. Unless you count undefined, asyncronous ... But you don't want
those in your type anyway (too verbose).

> 
> It also suffers the same flaw as tuples: no way to extend them easily.
> Imagine adding a new error: you have to edit _all_ of the patterns:
> 
>     showError :: (Result | Error3 | Error8 | Error11 ) -> String
>     showError (result |||)   = "no error"
>     showError (| error3 ||)  = "error3: "  ++ show error3
>     showError (|| error8 |)  = "error8: "  ++ show error8
>     showError (||| error11 ) = "error11: " ++ show error11
> 
> Extensible records and variants might be able to solve these problems,
> but that's a rabbit-hole of its own though.
> 

Yes of course, but wouldn't you still agree that tuples are useful. So
would sum tuples be.

Additionally, tuple like structures can easily be simulated using type
operators

data a :' b = a :' b

However, the pattern matching for even a 3-way sum type using Either
will be a nightmare. And I see no way of simulating this without using
Template Haskell.

type ... a = a `Either` Int `Either` Char

Left (Right int) ???

or

Right (Left int) ???

Silvio