I guess the point is that you can't put type names into the declaration of some other type.

data A = ...

data B = ...

-- No good
data AorB = A | B

f :: Int -> AorB
f x

  | even x = Aconstructor x
  | otherwise = Bconstructor x

-- OK

data AorB = AType A | BType B
f :: Int -> AorB
f x
  | even x = AType $ Aconstructor x
  | otherwise = BType $ Bconstructor x

What's confusing is that

data AorB = A | B

compiles with error.

That raises the question of what it really means!

-- Russ

On Tue, Dec 14, 2010 at 12:35 PM, Tobias Brandt <tob.brandt@googlemail.com> wrote:

On 14 December 2010 21:26, Russ Abbott <russ.abbott@gmail.com> wrote:
> Isn't "Either" the same thing as AorB in
>
> data AorB = Aconstructor Int | Bconstructor Int
>
> I want two separate types A and B along with a third type which is their
> Union. Is that not possible?

That's exactly what either is:

data A = ...
data B = ...

f :: Int -> Either A B

> In my actual case, I have more than two types. So I would like a way to
> take the union of an arbitrarily number of types.
>
> data Union = A1 | A2 | ...
>
> where each of A1, A2, ... has its own data declaration.

You can create a new data type:

data MyUnion = First A1 | Second A2 | Third A3

and use it like this:

f :: Int -> MyUnion