Note that 

 eval exec $program $args
is not an adequate substitute for 
   eval [linsert $args 0 exec $program]
or
  exec $program {}$args
since it breaks when $program contains spaces.


''Option 4: leading @'':
[Larry Smith]  If you favor a leading character then "@" is easy to
see and read, much more so than `.

   * So... this means [Larry Smith] has no need for the [text] widget?


''Option 4: $(<whatever you want expanded>)'':
[Larry Smith] Personally, I prefer marking a particular string or set
of strings for an additional parsing/substitution pass using something that is
reminicent of the current syntax.  $([[cmd]]) or $([[cmd1]] [[cmd2]]) marks things
clearly as being expanded.  After all, it is the $ that signifies substitution,
not the {}.

   * $([[cmd]]) is already legal syntax within variable substituition and shouldn't be mixed up with expansion.

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[Jacob Levy] July 28, 2003: What the example above tries to achieve seems to be possible without any new syntax, using a procedure called
apply, which behaves as follows:

 set l {a b c d e}
 apply list $l $l
 ==> {a b c d e a b c d e}

The definition of apply is that its first argument is a function that is applied to rest of the arguments which are evaluated and their values
are collected into a list.

The definition of apply is probably something like:

 proc apply {fn args} {
     set l ""
     foreach a $args {
         set l [lconcat $l $a]
     }
     return [eval $fn $l]
 }

 proc lconcat {pre el} {
     set l $pre
     foreach e $el {lappend l $e}
     return $l
 }

Now there may be more to TIP #144 that is not covered by this; however, at least for the given example, no new syntax is needed for implementing the functionality in Tcl.

[DGP] For this specific example, [[apply]] would be enough, yes.

When you extend things to get a command that can solve '''all''' of
the problems addressed by the proposed syntax, you end up with the
[[expand]] command proposed in TIP 103.

TIP 103 was considered and rejected.

Can we move on now please?

[DGP] For that matter, [[apply]] will not even work for the [[exec]] example
above (when ''$program'' contains spaces).

[Jacob Levy] Fixed the $args handling, above. To handle $prog with spaces in it, you'd need to pass [[list $prog]] to the invocation of apply.

[TP] I like the '''apply''' option best.  I would suggest that the apply command could have it's own ''mini-language''
(ala format, regexp, etc) using the {} or ` formatting.  E.g.

   apply {somecommand {}$expand_this $but_not_this}

[DGP] The "special command with its own mini-language" approach
was TIP 103.  [TP] I went back to read TIP 103 and the wandering discussion in the TCT mail archive.  I'd still vote against
including some special syntax in Tcl 8.5, 9.0, 10.0,  ..., where some command or modified ''eval'' could do the job.  
I just don't favor including some hack that was previously illegal syntax ( {}$foo  or `[[bar]] ) in the base language, or
any other non-backward compatible syntax feature.  Just call me an OF.
 

----
[KBK] (2003-07-28) I ''know'' that I have code that will break with
the backquote syntax.

----

[Setok] I know I probably bore people already with my ramblings on the caret expansion syntax but I find it pleasing. Note that [expr] does not allow double-carets (no logical XOR). So:

  file join ^^$FilePath

or

  exec $program ^^$args

The reason I find it nice is because it actually ''looks'' like the operation it is doing. It, to me, feels like real syntax instead of a hack.

The other option I suggested a couple of times was the ability for a command to affect its calling command line. This is somewhat akin to [upvar] and [uplevel] in how dangerous it can be, but allows macro-style functionality into Tcl. Other benefits are that no new syntax is added -- only a new capability provided to commands and procedures. I do realise it's somewhat complex.

----

(Continuing from a thread on comp.lang.tcl)

I think that paired backquotes is a good marker for argument expansion. The desired operation is to unlist something, so having a paired operator seems fitting. In sh, a backquoted string is not a grouping mechanism, but an expansion mechanism. In fact, to be completely analogous to shell programming, a backquoted string would mean to evaluate the backqoted string and retokenize the result.  Hmm...I like this. Essentially, the backquotes would be markers more like square braces than like a dollar sign.   

Fleshing this out, the examples from Tip #144 would become:

   destroy `winfo children .`
   button .b `subst $stdargs` -text $mytext -bd $border
   exec $prog `set opts1` `getMoreopts` $file1 $file2

(Note that there's a couple of ways to get an expanded variable dereferences.  Good thing or bad thing?)

I think this would be very understandable for anybody with a shell programming background, and tcl certainly has a lot of shell heritage.  And IMO, it's *a lot* cleaner. Given that tcl is built around the notion of command evaluation, extending that concept seems like a good thing.  I mean, the language doesn't even have infix math operators, and I see proposals for extending the {} syntax with indexes, ranges, etc.  That's so far removed from my conception of "the Tcl Way" that it's hard to fathom.  After all, wasn't even the dollar sign variable dereference syntax controversial back in the day?  "Why do you need a special syntax? You should just use the set command!"

Scott Gargash

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[DKF]: My opinion is that adding a rule to the Endekalog is the way forward (it adds a new class of capability and #144 is about right on the semantic behaviour) but I'm not 100% happy with either {} (too easy to generate from a typo) or backtick (not compatable.)

----

[Joe Mistachkin]: For the record, I think adding new global syntax to Tcl for this type of feature is a big mistake. I favor something along the lines of [lconvert], [[apply]], or [[return -code expand]].