Andrew Coppin wrote:

What we're really trying to do here is attach additional information to a value - information which exists only in the type checker's head, but has no effect on runtime behaviour (other than determining whether we *get* to runtime). As far as I can tell, Haskell does not provide any way to take an existing type and attach additional information to it in such a way that code which cares about the new information can make use of it, but existing code which doesn't care continues to work. Any comments on this one?

In defense of Haskell, that's not what the semantics of newtype are. The semantics of newtype declare that the, er, new type is unrelated to the old type--- despite however they may be represented at runtime. Thus, allowing functions to ignore the newtype wrapper would defeat the intentions of using newtypes in the first place. I actually find this to be one of the most powerful things in the type system of H98, and I miss having them in every other language where I'm forced to sacrifice performance or correctness.