New scope solution

[Liso]

@StefanKarpinski

It satisfies #2 perfectly: the meaning of code no longer depends on any global state—if you evaluate the same sequence of expressions in the REPL multiple times it means the same thing every time.

We could test your “lemma”:

julia> if get(ENV, "debug", "0") != "0"
         macro check(a) :(if !$(esc(a)) throw(ErrorException("ups!")) end) end
       else
         macro check(a) end
       end

       t = 4
       for i in 1:2
         @check ispow2(t)  # I want to check only in debug phase
         t = 16
       end
       t
4

now we could define env variable ENV["debug"]="1" and repeat same sequence.