Unboxed singleton tuples

[Don Stewart]

catamorphism:
> While reading some Core code, I noticed a function whose return type
> was (# State# RealWorld #). I was curious why unboxed tuples of arity
> 1 would exist, so I dug around in the GHC sources and found that they
> are used when desugaring foreign calls, as per the comment at the
> beginning of boxResult in deSugar/DsCCall.lhs. But I still don't
> understand -- what's the difference between (# foo #) and foo?
> 
> Just curious.

And is that going to interfere with CPR analysis?

-- Don