Proposal: add new forms of unsafePerformIO and unsafeInterleaveIO

[Isaac Dupree]

Stefan O'Rear wrote:
> I propose that unsafeDupablePerformIO be renamed to unsafePerformIO,
> since it satisfies all of the properties guaranteed of unsafePerformIO.
> GHC's unsafePerformIO guarantees more

similar to 'let' denoting sharing normally, and not doing a computation 
more than needed (most of the time).  hmm.