A short question in [comp.lang.tcl] got me several useful answers that I'm now putting here for later reference:

The (generalized) question:
   * How do I embed comments into an [expr] (or the first argument to [if],[while])
The specific context:
   * an expression was structured as an "&&"ed sequence of separate conditions, each of which should be associated with a simple text (e.g. someone's name). 
   * It was considered a valid assumption, that the comment text does not contain "nasty characters".
   * Readability weighs more than performance.

The answers:

my own: (provided in the posting as an example of what I meant)
   * use a proc that always returns true, and ignores its arguments:

======
proc noop args {return 1}
if {   <cond1>  && [noop  John Doe  ]
    && <cond2>  && [noop  Fu  Bar   ]
    ...
} { do something ... }
======


by Roy Terry:
   * use a proc that expr-evaluates its first argument and ignores the rest.

======
proc ? {cond args} { return [uplevel expr $cond]}
if { [? {<cond1>}  John Doe ]
    && [? {<cond2>}  Fu   Bar ]
    ...
} ...
======

A previous simpler version returned the first argument directly (without uplevel&expr), to which [RS] added: Note that this is another use for the [K] combinator... 

   * use [Tmac - a Tcl macro processor package%|%Tmac], which allows embedding comments everywhere: [http://www.tclbuzz.com/v0/tmac]
   if { cond1 (* John Doe *)
      && cond2 (*  Fu   Bar *)
      ...
   } ...

by Ulrich Schoebel
   * build up the expression condition by condition:

======
set cond [list]
lappend cond $cond1 ;# John Doe                                                 
lappend cond $cond2 ;# Fu Bar                                                   
...
if {[expr [join $cond &&]]} { ...                                                                             
======

by Michael Barth (per email):
   * make use of short-cut boolean evaluation. (slightly simplified)

======
if {  <cond1>  && (1 || " John Doe ")
   && <cond2>  && (1 || " Fu  Bar  ")
   ...
} { do something ... }
======

This is conceptually similar to my own solution, but better insofar, as no special procedure is needed and the "comment-ness" is immediately visible.



[Andreas Leitgeb]: Thanks for all the hints!


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