Is there any Tcl package/add-on that handles named arguments? - tcl

Question

In Python, Ruby 2.0, Perl 6, and some hardware description languages, one can use named arguments. See this example. This makes the code more readable, easy to maintain, etc. Is there a way of getting it done/extension, in TCL 8.6, other than using a dictionary as a workaround?

Answer 1

In 8.6, use a dictionary parsed from args. The dict merge command can help:
proc example args {
if {[llength $args] % 2} {
return -code error "wrong # args: should be \"example ?-abc abc? ?-def def?\""
}
set defaults {
-abc 123
-def 456
}
set args [dict merge $defaults $args]
set abc [dict get $args -abc]
set def [dict get $args -def]
puts "abc=$abc, def=$def"
}
example; # abc=123, def=456
example -abc 789; # abc=789, def=456
example -def 789; # abc=123, def=789
example -def 246 -abc 135; # abc=135, def=246
You can go further than that with verifying (the tcl::prefix command can help) but it's a lot more work and doesn't buy you a lot more in production code. Not that that has stopped people from trying.
There are two proposals to add full named argument handling
(TIP #457, TIP #479) to 8.7 at the moment, but I'm not sure that either have really gained traction. (The problem from my perspective is the extra runtime cost that has to be borne by code that doesn't volunteer to support named arguments. There might be other issues too, such as disagreement over preferred syntax; I've not paid so much attention to that as I'm still fretting over the performance implications in a pretty hot piece of code.)

Answer 2

There is an entire page on the tcler's wiki that discusses named arguments: http://wiki.tcl.tk/10702
You can do it yourself with a little creativity. There are several mechanisms that allow you to do this:
procs can define other procs
proc behave just like a proc (the function definition system is not a syntax, it is a function call)
procs can use the args argument instead of positional parameter and manually process the list of arguments
you can execute code in any parent stack frame using uplevel
you can pull variables from any parent stack frame using upvar
everything is a string!
etc.
I'm not sure I've listed all the possible mechanisms.
My personal implementation of this idea is optproc: http://wiki.tcl.tk/20066
The implementation itself is quite simple:
proc optproc {name args script} {
proc $name args [
string map [list ARGS $args SCRIPT $script] {
foreach var {ARGS} {
set [lindex $var 0] [lindex $var 1]
}
foreach {var val} $args {
set [string trim $var -] $val
}
SCRIPT
}
]
}
I basically used string manipulation (string map) to directly insert the function body ($script) into the defined function without any substitutions etc. This is to avoid any $ or [] from being processed. There are many ways to do this but my go-to tool is string map.
This is similar to Donald's answer except for two things:
I don't transform args into a dict instead I manually process args and declare each local variable in a loop.
This is a meta solution. Instead of processing args I created another function (syntax) to create a function that processes args.
Usage (stolen from Donald's answer) would be:
optproc example {abc def} {
puts "abc=$abc, def=$def"
}
But note that my solution is not the only solution. There are many ways to do this limited only by creativity.