# $(Main.r) vs $(r) -- confusion

**URL:** <https://discourse.julialang.org/t/main-r-vs-r-confusion/58314>\
**Category:** New to Julia\
**Created:** [March 31, 2021, 4:54pm UTC](https://discourse.julialang.org/t/main-r-vs-r-confusion/58314 "2021-03-31T16:54:51Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![healyp](https://avatars.discourse-cdn.com/v4/letter/h/67e7ee/32.png) [@healyp](https://discourse.julialang.org/u/healyp)\
**Post date:** [March 31, 2021, 4:54pm UTC](https://discourse.julialang.org/t/main-r-vs-r-confusion/58314/1 "2021-03-31T16:54:51Z")

</div>

I am trying to write a logging-type macro that will print an informational string depending on a numerical condition. (Yes, I am aware of the `Logging` module :-).) Using the `@assert` macro as a starting point I have essentially made minimal changes to it so that the string is printed using `println()`. Yet I see differences between the expanded version of the two.

For example, my `@trace` macro gets expanded to

```julia
julia> @macroexpand @trace 8 "$r, $c" # 8 < 100 so print string
:(Main.println("$(Main.r), $(Main.c)"))

```

while in a similar situation `@assert` yields

```julia
julia> @macroexpand @assert 9==8 "$r, $c"
:(if 9 == 8
      nothing
  else
      Base.throw(Base.AssertionError((Base.Main).Base.string("$(r), $(c)")))
  end)

```

While `@assert` functions correctly my `@trace` does not with the REPL reporting:

```julia
ERROR: UndefVarError: r not defined
Stacktrace:
 [1] top-level scope at REPL[52]:1

```

There must be something I do not understand because the only difference I detect between the two strings returned is `$(Main.r)` versus `$(r)`. Can this be the cause of failure for my `@trace` when the macros are called from a file? And if so, how can I fix it?

Many thanks.

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**Author:** ![tomerarnon](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tomerarnon/32/3170_2.png) [@tomerarnon](https://discourse.julialang.org/u/tomerarnon)\
**Post date:** [March 31, 2021, 5:59pm UTC](https://discourse.julialang.org/t/main-r-vs-r-confusion/58314/2 "2021-03-31T17:59:14Z")

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If you take a look at the definition of `@assert` from Base, you’ll note it returns an expression. This is how macros that should work “at runtime” are written. Although you didn’t provide the definition of `@trace` (which would be helpful), I expect it performs the “work” of the macro at macro expansion time rather than runtime.

```julia
macro assert(ex, msgs...)
    msg = isempty(msgs) ? ex : msgs[1]
    if isa(msg, AbstractString)
        msg = msg # pass-through
    elseif !isempty(msgs) && (isa(msg, Expr) || isa(msg, Symbol))
        # message is an expression needing evaluating
        msg = :(Main.Base.string($(esc(msg))))
    elseif isdefined(Main, :Base) && isdefined(Main.Base, :string) && applicable(Main.Base.string, msg)
        msg = Main.Base.string(msg)
    else
        # string() might not be defined during bootstrap
        msg = quote
            msg = $(Expr(:quote,msg))
            isdefined(Main, :Base) ? Main.Base.string(msg) :
                (Core.println(msg); "Error during bootstrap. See stdout.")
        end
    end
    return :($(esc(ex)) ? $(nothing) : throw(AssertionError($msg)))
end

```

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<div class="post-metadata">

**Author:** ![healyp](https://avatars.discourse-cdn.com/v4/letter/h/67e7ee/32.png) [@healyp](https://discourse.julialang.org/u/healyp)\
**Post date:** [March 31, 2021, 6:16pm UTC](https://discourse.julialang.org/t/main-r-vs-r-confusion/58314/3 "2021-03-31T18:16:54Z")

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> [@tomerarnon](#):
>
> Although you didn’t provide the definition of `@trace` (which would be helpful), I expect it performs the “work” of the macro at macro expansion time rather than runtime.

Agreed. Sorry, but I feared too much information / noise, which is why I held off. Thank you for the explanation although I am not getting the distinction. Is there a place in the documentation you could point me to, please? FWIW my macro definition is:

```julia
macro trace(tv, msgs...)
    msg = isempty(msgs) ? "empty trace message" : msgs[1]
    if isa(msg, AbstractString)
        msg = msg # pass-through
    elseif !isempty(msgs) && (isa(msg, Expr) || isa(msg, Symbol))
        # message is an expression needing evaluating
        msg = :(Main.Base.string($(esc(msg))))
    elseif isdefined(Main, :Base) && isdefined(Main.Base, :string) && applicable(Main.Base.string, msg)
        msg = Main.Base.string(msg)
    end
    return :($(esc(tv)) > TRACELIMIT ? $(nothing) : println(string($(msg))))
end

```

As you can see, it is a pirated copy of `@assert` 🙂  
Thank you again.

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<div class="post-metadata">

**Author:** ![tomerarnon](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tomerarnon/32/3170_2.png) [@tomerarnon](https://discourse.julialang.org/u/tomerarnon)\
**Post date:** [April 1, 2021, 2:38am UTC](https://discourse.julialang.org/t/main-r-vs-r-confusion/58314/4 "2021-04-01T02:38:09Z")

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I’m not seeing the same thing you are. If I define a global const `TRACELIMIT`, I see:

```julia
julia> @macroexpand @trace 8 "$r, $c"
:(if 8 > Main.TRACELIMIT
      nothing
  else
      Main.println(Main.string((Main.Main).Base.string("$(r), $(c)")))
  end)

julia> let r = 1, c = 2
          @trace 8 "$r, $c"
       end
1, 2

```

Which seems correct. I’m on julia 1.5 and 1.6

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<div class="post-metadata">

**Author:** ![healyp](https://avatars.discourse-cdn.com/v4/letter/h/67e7ee/32.png) [@healyp](https://discourse.julialang.org/u/healyp)\
**Post date:** [April 1, 2021, 8:13am UTC](https://discourse.julialang.org/t/main-r-vs-r-confusion/58314/5 "2021-04-01T08:13:17Z")

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Yes, this is what I am seeing now, too. I’m sorry but I must have been confused yesterday and posted the wrong macro output. Thank you for looking at this anyway.

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<div class="post-metadata">

**Author:** ![tomerarnon](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tomerarnon/32/3170_2.png) [@tomerarnon](https://discourse.julialang.org/u/tomerarnon)\
**Post date:** [April 1, 2021, 3:07pm UTC](https://discourse.julialang.org/t/main-r-vs-r-confusion/58314/6 "2021-04-01T15:07:44Z")

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No problem 🙂  
Glad it works the way you expected after all.