[Haskell-cafe] Existential Types (I guess)

22 Jan 2010

      Dear Cafe,

I can write and use the following,

data IntHolder = IntHolder Integer

instance Show IntHolder where
    show (IntHolder n) = show n

liftInt :: (Integer -> Integer) -> IntHolder -> IntHolder
liftInt f (IntHolder c) = IntHolder (f c)

But I cannot generalise it to *Num:*

data NumHolder = forall a. Num a => NumHolder a

instance Show NumHolder where
    show (NumHolder n) = show n

liftNum :: (Num a) => (a -> a) -> NumHolder -> NumHolder
liftNum f (NumHolder c) = NumHolder (f c)

The error message I get is the following:

    Couldn't match expected type `a' against inferred type `a1'
      `a' is a rigid type variable bound by
          the type signature for `liftNum' at Lifts.hs:54:16
      `a1' is a rigid type variable bound by
           the constructor `NumHolder' at Lifts.hs:55:11
    In the first argument of `f', namely `c'
    In the first argument of `NumHolder', namely `(f c)'
    In the expression: NumHolder (f c)

Regards,

-- 
Ozgur Akgun