'''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] 03. 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

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See also:

   * [info]

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[Category Command] - [Tcl syntax help]