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

[Simon Peyton-Jones]

| > 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

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.

Simon