On Tue, Nov 21, 2006 at 01:01:46PM +0000, Simon Marlow wrote:
The language doesn't say anything about the runtime representation of newtype. It so happens that the semantics lead to the obvious implementation of a newtype as a type cast (that was the reason for introducing newtype, after all), but there's nothing to say you have to implement it this way. It would be wrong to require that unsafeCoerce let you convert between a newtype and its underlying type across implementations.
indeed, in fact, even though jhc uses the same representation for newtypes as their value inside, unsafeCoerce here is still not valid. The reason being that various core-transformations inside the compiler are type directed, and the explicit knowledge that we are flipping something to its newtype or back is very important to guarenteed the safety of these optimizations. John -- John Meacham - ⑆repetae.net⑆john⑈