Hello,
I am trying to learn haskell , but i am struggling with types , its
been around 7 days , it will be very kind if some explain it why this
error , i think this is the only stumbling block . I am looking for
the comparison on why similar code works , while other code not .
I get this error on ghci :
{-
`a' is not applied to enough type arguments
Expected kind `*', but `a' has kind `* -> *'
In the type `SS a'
In the type `(Monad a) => {Monad (SS a)}'
In the instance declaration for `Monad (SS a)'
-}
Here is the very small code with comments:
data SS a = SS a Int
data Maybe1 a = Nothing1 | Just1 a
instance Monad Maybe1 where
(Just1 x) >>= f = f x
--^^ this loads fine in ghci
-- where as this
instance (Monad a)=> Monad (SS a) where
(SS x y) >>= f = f (x y)
--^^ does not work , so whats the difference , both have type parameters
-- something similar works like this :
instance (Eq a)=>Eq (SS a) where
(SS x y) == (SS b c) = (y == c) && (x == b)
Thanks
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