Richard Suchenwirth 2004-01-15 - Arguments to commands are mostly by position. But it's very easy to add the behavior known from Python or Ada (?), that arguments can be named in function calls, which documents the code a bit better, and allows any order of arguments.
The idea (as found in Welch's book) is to use an array (here called opt) keyed by argument names. Initially, you can set some default values, and possibly override them with the args of the proc (which has to be paired, i.e. contain an even number of elements):
proc replace {s args} {
array set opt [concat {-from 0 -to end -with ""} $args]
string replace $s $opt(-from) $opt(-to) $opt(-with)
}#--- Testing:
% replace abcdefg -from 3 -to 4 -with xx abcxxfg
Keywords can come in any order or be omitted (in which case the defaults apply). Flaw: "Undefined" keywords, e.g. typos (-wiht), are not detected...
Here's one which catches (but does not identify) typo's -jcw
proc replace {s args} {
array set opt {-from 0 -to end -with ""}
set n [array size opt]
array set opt $args
if {$n != [array size opt]} { error "unknown option(s)" }
string replace $s $opt(-from) $opt(-to) $opt(-with)
}RS feels prompted to reply with an error-identifying version, changed to using the "anonymous array" "" instead of opt, which makes access to such arguments look better:
proc replace {s args} {
array set "" {-from 0 -to end -with ""}
foreach {key value} $args {
if {![info exists ($key)]} {error "bad option '$key'"}
set ($key) $value
}
string replace $s $(-from) $(-to) $(-with)
}As the boilerplate is getting a bit lengthy, yet looks like it might be used more than once, here it is factored out (and with improved error message):
proc named {args defaults} {
upvar 1 "" ""
array set "" $defaults
foreach {key value} $args {
if {![info exists ($key)]} {
error "bad option '$key', should be one of: [lsort [array names {}]]"
}
set ($key) $value
}
}#--- The use case now looks pretty slick again:
proc replace {s args} {
named $args {-from 0 -to end -with ""}
string replace $s $(-from) $(-to) $(-with)
}#--- Testing:
% replace suchenwirth -from 4 -to 6 -with xx suchxxirth % replace suchenwirth -from 4 -to 6 -witha xx bad option '-witha', should be one of: -from -to -with
KPV Nice, but I like to abbreviate my options. So here's a slightly tweaked version that allows you to use shorter versions of options names (as long as they're unique). It also uses DGP's better error handling if possible.
proc named {args defaults} {
upvar 1 "" ""
array set "" $defaults
set aname [lsort [array names {}]]
foreach {key value} $args {
if {! [info exists ($key)]} { ;# Check for unique prefix
set possible {}
foreach keyname $aname {
if {[string first $key $keyname] == 0} {
lappend possible $keyname
}
}
if {[llength $possible] != 1} { ;# Not a unique prefix
set emsg [expr {[llength $possible] ? "ambiguous" : "bad"}]
append emsg " option \"$key\": must be [join $aname {, }]"
regsub {(.*),} $emsg {\1 or} emsg
if {$::tcl_version >= 8.5} {
return -code error -level 2 $emsg
}
error $emsg
}
set key $possible
}
set ($key) $value
}
}DGP Can be a little slicker using a Tcl 8.5 feature. Where you have
error "bad option '$key', should be one of: [lsort [array names {}]]"in [named], replace it with
return -code error -level 2 "bad option '$key', should be one of: [lsort [array names {}]]"for a cleaner stack trace.
jcw - Neat. FWIW, my personal usage style pref would be:
proc replace {s args} {
defargs -from 0 -to end -with ""
string replace $s $(-from) $(-to) $(-with)
}Note that it's quite nice now, but not the same as Python's idiom, which lets you mix required, optional, default, and named args. Required args etc can also be passed as named args. And named args can also be new ones, not listed in the definition.
procargs is a proc that I wrote some time ago that allows one off switchified procs, existing procs to be switchified and the overloading of proc itself. I believe that it addresses the above complaints about not being as general as python (but I know nothing about python). It defines the defaults in the usual proc way and extracts the switch names and values using tcl's info introspection JBR.
procargs replace { str { from 0 } { to end } { with {} } {
string replace $str $from $to $with
}I like this better since it overloads the existing proc API. It is easy to overload proc, source a package and call procs in the package with named args.
Can you give an example of use? Does procargs affect error tracebacks? -jcw
Hmmm... I thought that the above was an example of its use. The code there creates a proc named replace that can be called as the other replace procs on this page:
puts [replace abcdefg -from 3 -to 4 -with xx] abcxxfg
There was a small typo in the error handling which I fixed, and an error in switch specification reports a coherent error message but adds a level to the error stack.
Larry Smith Take a look at init. Small and simple, but powerful. Also, see this one: Matthias Hoffmann - Tcl-Code-Snippets - misc routines - command line parsing.
MNP - 6/11/05 - Rather than write my own, I just use tcllib's cmdline to process named arguments for me. For example,
package require cmdline
proc foobar {args} {
set myoptions {
{bar.arg 5}
}
array set arg [::cmdline::getoptions args $myoptions]
### your code using named arguments here ###
} ;# end proc foobarExample usage: foobar -bar 99
Arts and crafts of Tcl-Tk programming
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