So I wonder:
* Is sharing values of type Int (and Bool and similar small values)
always safe?
* If so: does GHC already do that?
* Would it be technically possible?
* Is there an established theory that can tell, for a sharing
candidate, whether it is safe to share it?

Jhc does this for simple types, which are unary constructors with a primitive argument (CPR types). I never actually ran tests to see how much it helps if any.

John