Newbie question about classes and class parameters

[Dmitry Astapov]

Let's say we have infamous Set class defined this way:

> class Set s a where
>   empty   :: s a
>   isEmpty :: s a -> Bool

Lets also say that we could define sets as lists:

> data SetAsList a = SL [a] deriving (Eq,Ord,Show)
> instance Set SetAsList a where
>   empty = SL []
>   isEmpty (SL []) = True
>   isEmpty _       = False

So far, so good. Lets define type synonim for set of integers:

> type IntegerSet = SetAsList Integer

And set of sets of integers:

> data IntegerSuperSet = ISS (SetAsList IntegerSet)

Testing in Hugs show that everything seems to be working:
Main> :t ISS (SL [ (SL [1,2,3])::IntegerSet, (SL [4,5,6])::IntegerSet ])
ISS (SL [SL [1,2,3],SL [4,5,6]]) :: IntegerSuperSet

However:
Main> isEmpty (ISS (SL [(SL [1,2,3])::IntegerSet, (SL [4,5,6])::IntegerSet]))
ERROR - Type error in application
*** Expression     : isEmpty (ISS (SL [SL [1,2,3],SL [4,5,6]]))
*** Term           : ISS (SL [SL [1,2,3],SL [4,5,6]])
*** Type           : IntegerSuperSet
*** Does not match : a b

This is completely understood - I need to state that IntegerSuperSet is an
instance of Set for isEmpty to be working. But:

> instance Set IntegerSuperSet where
>   empty = ISS (SL [])
>   isEmpty (ISS (SL [])) = True
>   isEmpty _ = False

gives me:
Reading file "/tmp/bug.lhs":
ERROR /tmp/bug.lhs:38 - Wrong number of arguments for class "Set"

Obviously I am missing something very important here. Could someone enlighten me?



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Dmitry Astapov //ADEpt                               E-mail: adept@umc.com.ua
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