[Haskell-cafe] nooby question on typing

13 Sep 2008

      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)
 data RuRule
    = Ru { rrsrt :: Char
         , rrant :: Expression
         , rrfps :: FilePos
         } deriving (Eq,Show)
 data SgRule
    = Sg { srfps :: FilePos
         , srsig :: Rule
         , srxpl :: String
         , srtyp :: (Concept,Concept)
         } deriving (Eq,Show)
 data GcRule
    = Gc { grfps :: FilePos
         , grspe :: Morphism
         , grgen :: Expression
         } deriving (Eq,Show)
 data FrRule
    = Fr { fraut :: AutType
         , frdec :: Declaration
         , frcmp :: Expression
         , frpat :: String
         } deriving (Eq,Show)
Now I would like to be able to use these rules without knowing what kind
they are, in lists, however i get errors like 
Couldn't match expected type `Rule' against inferred type `SgRule'. 
Is there any other (even trivial?) way to get this done? I know Haskell well
enough to know that it is possible, but I don not know Haskell well enough
to know how to do it :teeth:

Any help is kindly appreciated!

Han Joosten
-- 
View this message in context: http://www.nabble.com/nooby-question-on-typing-tp19470727p19470727.html
Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.