Re: [Haskell-cafe] nooby question on typing

13 Sep 2008

      On Sat, Sep 13, 2008 at 2:49 PM, Han Joosten wrote:
...
Hi,
I have a question about types in Haskell. I feel that I am overlooking some
obvious solution, but I do not manage to get it right. Here is the plot:
I have got 4 different types of 'rules', each with it's own constructor.
 So
i defined:
...
type Rules = [Rule]
 data Rule = RuRule
           | SgRule
           | GcRule
           | FrRule
               deriving (Eq,Show)
This effectively creates an enum type. I.e. each case here doesn't contain
any data other than the "tag". I think you're getting confused because the
constructor is named the same as the type you're expecting to store. Try
something like:
...
type Rules = [Rule]
 data Rule = RuRule
           | MkSgRule SgRule
           | MkGcRule GcRule
           | MkFrRule FrRule
               deriving (Eq,Show)
So MkSgRule is a "tag" or a "label" deciding which version of Rule you're
building, and it also has a value of type SgRule.

Now you can create a list or Rule like so:
...
mylist :: [Rule]
mylist = [ MkSgRule mysgrule, MkGcRule mygcrule ]
where mysgrule :: SgRule and mygcrule :: GcRule.

-- 
Sebastian Sylvan
+44(0)7857-300802
UIN: 44640862