[Haskell-cafe] Are newtypes optimised and how much?

[Bertram Felgenhauer]

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