Re: [Haskell-beginners] Type unions

Tobias you replied at the same time I was answering, you did explained what is happening, however, you said something which isn't right. As I mentioned before, "Type Constructors" and "Value Constructors" are in different scopes, and so, their names can be used again... if you don't believe me, give it a try with this example. I didn't even try it, but I asure you it will compile without any trouble: data A = C Int data B = D Int data AorB = A A | B B Greeting, Héctor Guilarte On Tue, Dec 14, 2010 at 4:22 PM, Tobias Brandt wrote:

On 14 December 2010 21:44, Russ Abbott wrote:

...

What's confusing is that

data AorB = A | B

compiles with error. That raises the question of what it really means!

You have to distinguish between type and value constructors. On the left hand side of a data declaration you have a type constructor (AorB) and possibly some type variables. On the right hand side you have value constructors followed by their arguments (types or type variables). E.g.:

data TypeConstr a b c = ValueConstr1 a b | ValueConstr2 c | ValueConstr3 Int

But in your example A and B were already declared as type constructors, so they can't be used again as value constructors. That's why you get an error. If you remove

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

then

data AorB = A | B

compiles.

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