[Haskell-beginners] Type unions

[Russ Abbott]

Thanks. I had assumed that in

 data A = ...
data B = ...
data AorB = A | B


the A | B in the definition of AorB referred to the types A and B. But now I
gather that they are completely unrelated -- except for the fact that they
are spelled the same.

Thanks.
  *
-- Russ *
*
*


On Tue, Dec 14, 2010 at 1:29 PM, Hector Guilarte <hectorg87 at gmail.com>wrote:

> But did you already understand what's happening?
>
> > data AorB = A | B
>
> is pretty much like an enumeration the same as
>
> > data DayOfTheWeek = Monday | Tuesday | Wednesday | ... | Sunday
>
> it stores no values of any type, you could even have DayOfTheWeek as
> a constructor in the same declaration of the data DayOfTheWeek, because
> they are
> in different scopes:
>
> > data DayOfTheWeek = Monday | Tuesday | Wednesday | ... | Sunday
> | DayOfTheWeek
>
> and by no means, that A or B on the right hand side of your data AorB are
> referencing the
> data A nor the data B...
>
> In the end, you could have:
>
> data A = Aconstructor Int
> data B = Bconstructor Int
> data AorB = A A | B B
>
> f :: Int -> AorB
> f x
>   | even x     = A (Aconstructor x)
>   | otherwise = B (Bconstructor x)
>
> I just added a Value to each constructor of the data AorB, which happens
> to be
> named the same as the value they store...
>
> What I'm about to do is something I haven't tried, but I don't see why it
> shouldn't
> compile:
>
> > data Try = Int Int
>
> in data Try you have a constructor named Int, and it has a value of type
> Int,
>
> I'm trying to explain it the best I can, but I don't know if I managed to
> do it
> clearly, please let me know if there's something where I wasn't clear
> enough.
>
> Hector
>
>
>  On Tue, Dec 14, 2010 at 4:40 PM, Russ Abbott <russ.abbott at gmail.com>wrote:
>
>> My typo started this most recent confusion. When I wrote
>>
>> data AorB = A | B
>>
>> compiles with error.
>> That raises the question of what it really means!
>>
>>
>> I meant to say
>>
>>  data AorB = A | B
>>
>> compiles *without *error.
>> That raises the question of what it really means!
>>
>> *
>> -- Russ *
>>
>>
>> On Tue, Dec 14, 2010 at 1:04 PM, Tobias Brandt <tob.brandt at googlemail.com
>> > wrote:
>>
>>> On 14 December 2010 22:02, Hector Guilarte <hectorg87 at gmail.com> wrote:
>>> > 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 believe you. As I said, I was talking crap :-)
>>>
>>
>>
>
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