Re: [sv-bc] Mantis 1571: Add default values for macro arguments

>From: Greg Jaxon <Greg.Jaxon@synopsys.com>

>Just noting that despite any other merits, this design is not
>backward compatible with existing uses of macro expansion.
>
>In  the expansion of `ABC(.arg1(arg1), ...), existing products
>will use the token sequences .arg1(arg1) and this may have been
>the user's intent.

Agreed.  Argument connection by name cannot be added with the
existing syntax while maintaining backward compatibility.


>In the expansion of `ABC #(.arg1(arg1),...), `ABC has no arguments
>so it's expansion will be followed by #(.arg1(arg1),...).

But if ABC is a macro with arguments, then this usage would be illegal.
So we could give it a different meaning without creating any backward
compatibility issues.

I believe that what Shalom is suggesting is the following extension to
the language:

If ABC is a macro with arguments, and it is invoked with `ABC#(...)
instead of `ABC(...), then the argument list in parentheses should be
treated as a list of named connections for argument values, instead of
as a list of argument values.

The syntax `ABC#(...) would be illegal in the existing language, since
it is missing the required argument list for ABC.  So there cannot be a
backward compatibility issue.  You might need some constraints on the
way this was defined.  For example, you might want to require empty
parentheses or hash-parentheses after a macro with arguments, even if
you were using all default values.  Otherwise you would be unable to
distinguish whether `ABC#(...) was a macro with all default arguments
followed by #(...), or a macro with a named parameter list.  If you
require empty parens, then the first case would look like `ABC()#(...),
which does not have that ambiguity.

I am not sure that this kind of hack is a good idea, but it does seem
possible.

I am sympathetic to Dave's viewpoint.  If someone is making such complex
use of macros that they need named connections to keep the arguments
straight and default values for arguments, then they are probably abusing
them.  I am not strongly swayed by a consistency argument.  Macros are
text processing that occurs before the text is actually compiled, and
their behavior is distinctive.  Users already have trouble making that
distinction.  Keeping the syntax distinct may help with that.

Steven Sharp
sharp@cadence.com