Re: Newtype wrappers

On Mon, Jan 14, 2013 at 03:28:15PM -0800, Johan Tibell wrote:

On Mon, Jan 14, 2013 at 3:18 PM, Evan Laforge wrote:

...

I assume it would change from "doesn't compile" to "works" if you add the required import. It's the same as the FFI thing, right? If you don't import M (T(..)), then 'foreign ... :: T -> IO ()' gives an error, but import it and coerces T to its underlying type (hopefully that's a C type).

This is what I thought Simon meant. If so, I don't think it's a good idea, as adding the import removes a compiler error in favor of a runtime error. If the programmer really wanted to do something this unsafe, she should use unsafeCoerce.

Simon's proposal would mean that import Data.Set.Internal newtype wrap w :: Set Int -> Set Age would be possible, in the same way that import Data.Set.Internal w :: Set Int -> Set Age w (BinSet x y) = BinSet (MkAge x) (MkAge y) w Empty = Empty would be possible. i.e. it wouldn't let you write anything that you couldn't write anyway (although it would make it easier to write, and it would have better performance). The "adding an import makes it compile" issue is a red herring IMO. Adding the import also makes my second example work for the same reason; it's just more obvious that the constructor is needed in the second example as it's visible in the code. Thanks Ian