New scope solution

Internals & Design
35

Error messages: https://discourse.julialang.org/t/improving-error-messages-for-the-scoping-problem/16209.

Remember, the local/global decision for variables does not happen at runtime, it happens at compile-time (I think early in lowering?). I think Stefan’s solution is to follow unconditional @goto and always follow both branches (even for literal if false). So it would fix the following example:

julia> dead=true;
julia> let 
           @show dead
           @goto skip
           dead = false
           @label skip
       end
ERROR: UndefVarError: dead not defined

but not

julia> dead=true;
julia> let 
           @show dead
           if true @goto skip end
           dead = false
           @label skip
       end
ERROR: UndefVarError: dead not defined

The rule would be: Follow all pathes (without evaluating known conditionals). If there exists a write before read path, then the variable defaults to local. Otherwise, it defaults to global.

As a side note: The while gets evaluated in the outer scope, not the inner scope. That is probably confusing for some people as well:

julia> m=4; n=2; i=1; while i>0
       i = n
       @show i, n
       global m -= 1
       global n -= 1
       @show m,n
       m>0 || break
       end; @show m, n, i
(i, n) = (2, 2)
(m, n) = (3, 1)
(i, n) = (1, 1)
(m, n) = (2, 0)
(i, n) = (0, 0)
(m, n) = (1, -1)
(i, n) = (-1, -1)
(m, n) = (0, -2)
(m, n, i) = (0, -2, 1)

So, regardless of this scoping, a minimally invasive (very non-optimizing) @code_semilowered that produces valid julia source code with only let blocks and @goto would be nice for that. It would also teach people about the iterator interface.

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