Re: RealWorld

This constrains the implementations of IORef and STRef to be the same (obviously). I can't immediately see any benefit from using different implementations for these two types, so maybe it's not an issue.

I could be a bit out of date but I think Hugs uses different implementations because IO has concurrency and exceptions and ST doesn't. Apart from effort required, I don't know if there is any cost in switching. -- Alastair