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

[Bertram Felgenhauer]

> | Then we can define
> | 
> |     safeCoerce :: (a ~~ b) => a -> b
> |     safeCoerce = unsafeCoerce
> 
> Yes, that's right.  When I said "we have the technology" I meant that we (will) have something similar to ~~.  See our paper "Generative Type Abstraction and Type-level Computation" http://www.cis.upenn.edu/~sweirich/newtypes.pdf.  No unsafeCoerce required.

The idea was to put safeCoerce into a library. The syntax extension
would be light-weight because contexts, unlike expressions, still have
plenty of room for extensions. The idea is is based on the assumption
that to the compiler, 'unsafeCoerce' looks like an artificial safe
coercion, so that after inlining safeCoerce, we get exactly the effect
of a safe coercion during type checking and further compilation. Perhaps
that assumption is wrong. I'll look at the paper.

Regards,

Bertram