Macro hygiene only protect you in different modules.

[Ismael-VC]

This was reported at Stack Overflow:

• http://stackoverflow.com/questions/35094986/does-macro-hygiene-only-protect-you-in-different-modules

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julia> macro g(y)
           :((x, $y, $(esc(y))))
       end

       x = 1
       function t()
           x = 2
           println(macroexpand(:(@g(x))))
           println(@g(x))
       end

       t()
       println(x)
(x,x,x)
(2,2,2)
1

julia> workspace()

julia> module A
       macro g(y)
           :((x, $y, $(esc(y))))
       end
       x = 1
       end

       x = 3

       function t()
           x = 2
           println(macroexpand(:(A.@g(x))))
           println(A.@g(x))
       end

       t()
(A.x,A.x,x)
(1,1,2)

I had expected the result to be (1, 1, 2).

However, if I define the macro in a different module, it works as expected.

Seems the hygiene simply prefix the symbols with the module namespace. Therefore it is impossible for the macro expander to distinguish different scopes in the first case.

[JeffBezanson]

Duplicate of #4873