Ashley Yakeley writes:

At 2001-01-16 10:23, Magnus Carlsson wrote:

...

You can use overloading for the definition of theValue instead:

class TheValue a where theValue :: a -> Maybe Int

instance TheValue Base where theValue _ = Nothing instance TheValue Derived where theValue x = Just (x.value)

Doesn't this imply that run-time type information is kept with the structs?

The overloading is resolved statically, so no run-time type information is needed.

Consider:

d :: Derived d = struct value = 3

b :: Base b = d

idb :: Base -> Base idb x = x

f1 = theValue d f2 = theValue b f3 = theValue (idb d) f4 = theValue (idb b)

What are the values of f1, f2, f3 & f4?

f1 = Just 3 f2 = f3 = f4 = Nothing

-- Ashley Yakeley, Seattle WA

/M