'''info default''' ''procname arg varname''

''Procname'' must be the name of a [Tcl procedure] and ''arg'' must be the name of an argument to that procedure. If ''arg'' doesn't have a default value then the command returns '''0'''. Otherwise it returns '''1''' and places the default value of ''arg'' into variable ''varname''.

[Zarutian] 3. july 2005: why does this sub-procedure require an varname? It realy should return an list with two elements. The first one is the boolean (predicate: this arg of this proc has default value?). The second one is
the default value (Empty if the boolean is "0" (false)).

[MG] July 3rd 2005 - You can get the effect yourself easily enough yourself with a wrapper proc:

 proc defaultList {proc arg} {
  set hasDef [info default $proc $arg valDef]
  return [list $hasDef $valDef]
 }

The only problem in that is that the scoping of $proc may differ, between where [[defaultList]] is called and where the [[info default]] inside it is run. An [uplevel] would sort that, but then the var would be created in the wrong scope, and we'd have to [upvar] or [uplevel] to gain access to it, so it could still obliterate a variable in use already. But, if you wanted it...

 proc defaultList2 {proc arg} {
  set hasDef [uplevel info default $proc $arg infoDefaultVariable]
  set valDef [uplevel set infoDefaultVariable]
  uplevel unset -nocomplain infoDefaultVariable
  return [list $hasDef $valDef]
 }

And to test:
 % proc test {a {b defB}} {puts "a -> $a, b -> $b"}
 % defaultList test a
 0 {}
 % defaultList2 test b
 1 defB

[Zarutian] 3. july 2005: It would be easier the other way around, wouldnt it ;)

Then if you wanted to set an variable with the default value of an arg of an proc:

 proc defaultArg {proc arg var} {
   set res [info default $proc $arg]
   upvar $var tmp
   set tmp [lindex $res 1]
   return [lindex $res 0]
 }

[MG] Indeed it would. That was really just to show that you could achieve what you wanted, even with [info default] working as it does now. Perhaps the built-in implementation could be altered to work how you want, if no ''variable'' argument is passed (which currently results in an error), but still work as it does now if the variable is given.

[Zarutian]: If that you are proposing becomes real then one could use it to get the args of an procedure in an
slave interp without triggering a trace (if one has been defined for the ''variable'') in that variable.

This gets me speculating which other built-in commmands in Tcl could be pure-function (binary scan comes to mind).

[Lars H], 3 July 2005:
A better solution to capturing the proper scope of the proc is to get a fully-qualified name for it, by uplevelling a [namespace which]:
 proc defaultList3 {proc arg} {
    set proc [uplevel 1 [list namespace which -command $proc]]
    list [info default $proc $arg val] $val
 }
This won't help when it comes to inspecting a slave interpreter, though.

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See also:

   * [info]

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[Category Command] - [Tcl syntax help]