How to pass an arbitrary number of macro arguments from one macro to another? (a bit like splatting)

General Usage
,
1

I have a macro which does stuff depending on how many inputs there are

macro my_macro(arg1)
    println("We got one argument.")
    sum = 1
    if arg1 isa Int64
        sum = sum + arg1
    end
    sum
end
macro my_macro(arg1, arg2)
    println("We got two argument.")
    sum = 1
    if arg1 isa Int64
        sum = sum + arg1
    end
    if arg2 isa Int64
        sum = sum + arg2
    end
    sum
end
macro my_macro(arg1, arg2, arg3)
    println("We got three argument.")
    sum = 1
    if arg1 isa Int64
        sum = sum + arg1
    end
    if arg2 isa Int64
        sum = sum + arg2
    end
    if arg3 isa Int64
        sum = sum + arg3
    end
    sum
end

(just an example to demonstrate what problem I have)

I then have another macro, which basically is the first macro + some modification. If my macros were functions I could just have done:

macro my_macor_expanded(args...)
    sum = my_macor_expanded(args...)
    return sum + 1
end

Now we are dealing with macros, so that do not work. However, I am not sure how to. E.g.

macro my_macro_expanded(args...)
    sum = :(@my_macro $args... )
    return sum + 1
end
@my_macro_expanded 1 2

even if I have two arguments, the internal @my_macro only gets a single one.

2

I believe you just need another set of parenthesis:

macro my_macro_expanded(args...)
    sum = :(@my_macro $(args... ))
    return sum + 1
end
1 Like
3

That worked, tons of thanks!

4

You can, in fact, call a macro with parenthesis, like a function. It does need the @ sign.

1 Like
5

You can also call a macro like this:

var"@my_macro"(__source__, __module__, args...)

For your example:

julia> macro my_macro_expanded(args...)
           sum = var"@my_macro"(__source__, __module__, args...)
           return sum + 1
       end
@my_macro_expanded (macro with 1 method)

julia> @my_macro_expanded 1 2
We got two argument.
5
1 Like
6

Thanks for the additional comments with more info, that is really useful to know!

7

Incidentally, there are some known pitfalls for passing arguments from one macro to another, if you “escape” an argument:

2 Likes
8

Thanks, for mentioning that.

I think i have actually run into this problem in my application. My macro requires me to escape at times, however, this feature fails, which is unfortunate.

Right now I am working around it by implementing my two macros twice (basically identically, but with a small change in the second set, which I initially mostly wanted to re-use the code from the first set).

1 Like