macro madness

[Karl Berry]

    thought about improving macro facilities of texinfo, so they become
    a bit closer to TeX?

Thought about, yes.  Discussed, yes.  Specified (let alone implemented), no.
In any case, your particular example:
    @macro foo{a,b}
    @ifset b
    ..some code..
    @else
    ..other code..
    @end ifset   
    @end macro

Although it would be "logical" to be able to use @ifset with macro
arguments, my mind simply boggles at implementing it (especially in
TeX).  Sorry.

Also, BTW, @else would be problematic.  Conditionals, in general, have
been the cause of many problems, especially in TeX.  The interaction of
line endings inside @macro and line endings of conditionals and line
endings in some commands is especially painful.  I really don't want to
complicate it even further.

Although @else would be convenient, I don't think it's really necessary,
since @ifset ... @end ifset @ifnotset ... @end ifnotset achieves the
same thing (I believe).

Again, can I ask where you need these complicated features in the tar
manual?  Maybe there is a simpler way ...

Overall, Texinfo's @macro should probably never have been invented.  I
rather rue the day that I gave in to rms about it.

karl