On Jul 27, 2010, at 3:02 AM, Brandon S Allbery KF8NH wrote:
As I understand it: 1) carrying [contexts] around complicates Haskell98 (and now Haskell2010) compatibility (also see below);
Like the availability of so many other features, this one could be controlled by a language pragma.
2) GADTs do what you want, since they don't have backward compatibility baggage.
They are also more complex than is needed for the problem at hand.
As to the current proposal, I think nobody's certain what would happen to older programs if data were changed to carry contexts around --- someone might be relying on the current behavior, and changing it might produce runtime oddness instead of a compile-time error --- whereas making contexts illegal will produce an easily-fixed error message in all relevant cases.
Does anyone know why `data' contexts were broken in the first place?