Macro to assign default values, avoid collision of variable names - sas

Question

Here's a small macro I want to call at the start of my macro functions.
It is the equivalent of %if not %length(&x) %then %let x = &val.
I want to use it as it's more readable and will help me to parse my code to build documentation (Which I need to do with regex as I can't install external softwares).
%macro def
/*---------------------
Assign default value
---------------------*/
(_x_data_x_ /* variable name (to be passed raw without &) */
,value /* default value */
);
%if not %length(&&&_x_data_x_) %then %let &_x_data_x_ = &value;
%mend;
Here's how it works:
%macro test(a,b);
%def(b,&a)
%put "&b";
%mend;
%test(%str(,)); /* prints "," */
I chose the unusual parameter name _x_data_x_ because the macro fails when it's fed a value equal to the parameter's name (i.e. if my b parameter was named _x_data_x_).
Can I make it really safe, apart from choosing obscure names ?

Answer 1

That is the correct approach.
You could make the macro more robust by adding checks:
%* Prevent self reference because
%* assignment will not bubble to containing scope;
%if %upcase(%superq(x_data_x)) eq X_DATA_X %then %do;
%put ERROR: x_data_x=%superq(x_data_x) is not allowed;
%abort cancel;
%end;
%* Prevent reference to non-existent because
%* assignment will automatically localize
%* and not bubble to containing scope;
%if not %symexist (%superq(x_data_x)) %then %do;
%put ERROR: x_data_x=%superq(x_data_x) is invalid;
%put ERROR: %superq(x_data_x) does not exist in callee scope;
%abort cancel;
%end;