On Thu, Oct 4, 2012 at 5:03 PM, Tyson Whitehead
On October 4, 2012 07:59:29 Ganesh Sittampalam wrote:
I have some nebulous concerns about abstraction violation, but since Typeable only lets you find out what the structure is, not actually take apart the contained data, it doesn't seem too much of a problem.
Ummm. Isn't that what cast (in Data.Typeable) does?
cast :: (Typeable a, Typeable b) => a -> Maybe b
For example, the following function
import Data.Typeable import Data.Maybe
magic :: Typeable a => a -> a magic x = case cast x of Just x -> cast $ fromJust $ x ++ " (ta da)" _ -> x
gives you id except for strings.
ITYM fromJust $ cast rather than the other way around. But the ability to cast like this does not give you access to data structure you didn't already have – it does not violate abstraction. On the other hand, were we to do the same with Data, we /would/ violate abstraction, since Data is capable of synthesising constructors and retrieving fields and so forth. Hence manual instances of Data are fairly common, e.g. for Map and Set. Typeable doesn't seem to give you anything like that, so it seems safe.