On October 4, 2012 13:48:42 Ben Millwood wrote:
On Thu, Oct 4, 2012 at 5:03 PM, Tyson Whitehead
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.
Indeed. I did mean those to be swapped. Thanks for picking up on that. I have to confess though, I'm not really following on how this doesn't give you access to data structure you didn't already have? Perhaps we are not thinking of the same thing when you say "access to data structure"? Doesn't the above "a -> a" function (under the proposed system where you don't need a "Typeable a" constraint") take apart some of its argument magic 1 ---> 1 magic 2 ---> 2 magic "hello world" ---> "hello world (ta da)" Doesn't this mean then that whole "theorems for free" stuff goes out the window? Isn't its whole basis the idea that unconstrained types are actually statements that those arguments won't be constructed/deconstructed? Thanks! -Tyson