[Haskell-cafe] nooby question on typing

[Han Joosten]

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
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