Re: [Haskell-beginners] Type unions

14 Dec 2010

      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 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 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 
...
wrote:
...
On 14 December 2010 22:02, Hector Guilarte  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 :-)