how to know if a macro exists in `execute R`?

Hi the content is this:

- need to pass values as macros, since that seems to be the only way for web service

- use `execute R` to parse the macros, since it's more flexible to handle strings and lots of if...then...

First question is how to refer to macro in `execute R`. I did some tests and figured it out myself:

Suppose you set macro m1 to be x, then whenever your write %{m1} in `execute R`, the software will just literally replace %{m1} with x

For example:

rm_main = function()
{
        x = 3
        z = %{m1} # interpreted as z = x
        print(z) # 3

        xy = 3
        z = %{m1}y # interpreted as z = xy
        print(z) # 3

}

If want to pass a string 'x', either set macro m1 to be 'x',

or do z = '%{m1}' in `execute R`.

Now the second question is, if some macros are optional, how to test if they exist in `execute R`?

One solution is, fix it at the web service side - Always set macro to be NA if it's not used.

But can we handle omitted macro in `exexute R`? (So that the web service side can pass as many macros as they want)

This doesn't seem to work

exists('%{m1}')  # interpreted as exists('x'), can't test if the macro exists

Use `try`

rm_main = function()
{
	try( z <- '%{m1}' )
	print(exists('z'))
}

when there's no macro m1, it stills throws error (even when using `try`)

How do I test the existance of a macro?

Any help's appreciated. Thank you~