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
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