''Please stop me if you've heard this one before...''


What would it take to add a ''macro'' facility to Tcl? What use would such a facility be?

Well, for starters, it would make  syntatical sugar (attrocities) such as:

 proc first {l} {
   lindex $l 0
 }

 proc rest {l} {
   lrange $l 1 end
 }

cost less in terms of performance. Yes, yes, I know -- if performance really mattered I shouldn't be doing this in Tcl. But the difference between calling my own procs vs internal ones (especially in deep loops) count, right?

Well, what about ''immediate'' macros (or macros that have a compile time behavior). Common Lisp should come to mind, but I can't help thinking about Forth ''immediate'' words. So, I could do something like:

 mac log {msg} {
    global debug
    if {$debug} {
	return "puts stderr $msg"
    } else {
	return "\;"
    }
 }

and use it thusly:

 set debug 0 
 ...
 proc something {} {
   set stuff "contents not available for introspection by macro yet." 
   log "Some noise I may not want to see depending on the value of debug"
   log $stuff; # if debug was 1, then the macro would expand to: puts stderr $stuff
   ...
 }

The above assumes that I have redefined ''proc'' to point to a macro evaluator to execute all macros before actually defining the ''real'' proc. The return value of the macro evaluation is what replaces the macro in the procedure. In this example, I can do conditional compilation!

This could all be done in plain Tcl if I gave Tcl access to the Tcl C parser code (to properly locate the arguments for the expanding macro). Note: You are limited by what context is available when executing the macro (you can for instance look into the surrounding proc's variables since we are in ''compile'' mode --- there aren't any values for the variables yet so the arguments to the macros are not eval'd!).

Another use for an ''immediate'' macro facility:

 mac  mloop {idx cnt cmd} {
    return "for {set $idx 0} {\$[set $idx] < $cnt} {incr $idx} {$cmd}"
 }
 
 proc something {} {
   mloop i 5 {puts "hello $i"}
   # above expands to:  for {set i 0} {$i < 5} {incr i} {puts "hello $i"}
 }

My mind is often polluted by such sickness...

-- [Todd Coram]