Simon Peyton-Jones wrote:
What you really want is to say is something like this. Suppose my_tree :: Tree String. Then you'd like to say my_tree ::: Tree Foo meaning "please find a way to convert m_tree to type (Tree Foo), using newtype coercions.
The exact syntax is a problem (as usual). We have the technology now. The question is how important it is.
I think extending the syntax for contexts would be sufficient: Write a ~~ b for "a can be converted to b by wrapping / unwrapping newtypes", which is a conservative approximation of "a and b have the same representation". Then we can define safeCoerce :: (a ~~ b) => a -> b safeCoerce = unsafeCoerce and your example would become safeCoerce my_tree :: Tree Foo The feature would add convenience to the language when working with newtypes, and reduce the tension between type safety and performance (where the choice is between using a newtype and unsafeCoerce, and just working with the plain underlying type.) So while the pressure is quite low, I imagine it would become quite a useful feature once we'd have it, but that's of course speculation. As far as I can see, the feature is nontrivial: Care has to be taken to not break abstractions (like "safely"coercing IO to ST), so it's quite possible that the engineering effort outweighs the potential benefits. Best regards, Bertram