Special name for an argument to a proc or lambda term - if it's last in the argument list, it will contain a list (possibly empty) of all the remaining arguments. The use of a magical args parameter is common in many other procedure-like methods (e.g. those created by various OO extensions).
proc demo {first {second "none"} args} { puts "first = $first" puts "second = $second" puts "args = $args" } demo one demo one two demo one two three four five
results in:
first = one second = none args = first = one second = two args = first = one second = two args = three four five
example args (using XOTcl)
Class Club -parameter {id {name unknown}} Class Player -superclass Club -parameter {{name unknown} {position unknown}} Player proc show_players args { if { [string length $args] == 0} { puts "Show players DB" foreach p [my info instances] { puts "[$p name] [$p position]" } return } set pos [string toupper $args] puts "Players with postition $pos:" foreach p [my info instances] { foreach char [split $pos {}] { if {[string first $char [$p position]] == 1} { puts "[$p name] [$p position]" break } } } }
Is it just me, or is that a pretty poor example of args? You do [string length $args] and [string toupper $args] but you firmly established that args is a list, not a string. It is generally accepted as bad practice to perform string operations on lists.
LES But, if everything is a string, aren't lists strings too? On the other hand, there has been quite some debate on whether everything is a list rather than a string...
LV I don't know that I would go so far as saying that it is bad practice to perform string operations on lists.
I would say that it is generally bad practice to perform list operations on variables known only to have strings. And I would say that some string operations on lists may result in results that , at first blush, one might not expect.
Lars H: The problem with applying string operations on known pure lists like $args is basically shimmering. Applying a string operation will require generation of a string representation, and the cost for that (in memory and processing time) is often better avoided whenever possible. If you pass multi-megabyte lists around (something Tcl 8 handles beautifully well), you probably don't want to double the memory footprint by also generating their string representations.
An extreme example:
set val x for {set n 1} {$n<=64} {incr n} {set val [list $val $val]} puts "Tcl will get this far."; flush stdout string length $val puts "But it runs out of memory before it gets this far.!"
LV So then the example isn't a poor example of args - but a poor example of good performance.
MG offers a quick example off the top of his head (and therefore untested), on his way through...
proc randomCmd {args} { set error {wrong # args: should be "randomCmd ?-arg value ...? string"} set num [llength $args] if { $num == "0" || ($num%2) == 0 } { error $error; } array set opts [list -width 5 -height 5 -fg [list] -bg [list] -foreground blue -background red] if { $num > 1 } { foreach {x y} [lrange $args 0 end-1] { if { ![info exists opts($x)] } { error "unknown option \"$x\""; } set opts($x) $y } } foreach {x y} [list -fg -foreground -bg -background] { if { [llength $opts($x)] > 0 } { set opts($y) $opts($x) } unset opts($x) } echo "String is \"[lindex $args end]\". Apply these options: [array get opts]" }
LV Here's where I am trying to use args - maybe someone has a suggestion. My code is this:
% proc d {name args} { puts "DEBUG: $name: $args" } % d mainline "this is the first spot in the mainline" DEBUG: mainline: {this is the first spot in the mainline}
That is to say, it is intended as a place to control debug statements. I can just return rather than do the puts, to turn off the debugging.
HOWEVER, the debugging output looks peculiar. Not a problem - it is just for debugging. But if I were going to use this for something else, I wouldn't want the extra {} in there.
RS: try join $args to get rid of braces :)
dzach 2018-01-23: args seems not to follow the rule the other arguments do:
% proc test {a {b 1} {args 2}} { puts "$a $b $args" } % test 0 0 1 ; # <- no default value for 'args'
Is there a reason for this, or I just hit a bug?
I would expect something like this (workaround):
% proc test {a {b 1} {args 2}} { info default [lindex [info level [info level]] 0] args arg if {$args eq {}} { set args $arg } puts "$a $b $args" } % test 0 0 1 2
args doesn't seem to follow the same rules because it is never valueless and thus never eligible to receive a default value. Its value is the (possibly empty) list of arguments that overmatch the named parameters, and it always gets that value.
See also magic names.