In your two examples, note the difference *when* the macro argument is multiplied by 2.
@ex_timestwo happens at runtime @n_timestwo happens at macro expansion time a = 6 @ex_timestwo(a) will return 12 as expected, whereas: a = 6 @n_timestwo(a) will still result in error because at macro expansion time, it is trying to evaluate (:a) * 2 On Saturday, October 1, 2016 at 7:26:50 PM UTC+10, Lyndon White wrote: > I was generally under the impression that a macro always had the return a > `Expr` > and that doing otherwise would make the compiler yell at me. > > Apparently I was wrong, at least about the second part: > https://github.com/ChrisRackauckas/ParallelDataTransfer.jl/issues/3 > > > > macro ex_timestwo(x) > :($(esc(x))*2) > end > > > > @ex_timestwo(4) > 8 > > macroexpand(:(@ex_timestwo(4))) > :(4 * 2) > > @ex_timestwo(a) > LoadError: UndefVarError: a not defined > > > macroexpand(:(@ex_timestwo(a))) > :(a * 2) > > > > ? > > VS: not returning an expression, and just doing a thing: > > macro n_timestwo(x) > x*2 > end > > > @n_timestwo(4) > 8 > > > macroexpand(:(@n_timestwo(4))) > 8 > > > @n_timestwo(a) > LoadError: MethodError: no method matching *(::Symbol, ::Int64) > > > macroexpand(:(@n_timestwo(a))) > :($(Expr(:error, MethodError(*,(:a,2))))) > > > > > Is this a feature? > Or is it just undefined behavior? > >
