Question

How do you provide default arguments to a Tcl procedure that are evaluated at call-time?

Here’s an example of what I’ve tried:

> tclsh
% proc test { {def [expr 4 + 3]} } { puts $def }
too many fields in argument specifier "def [expr 4 + 3]"
% proc test { {def {[expr 4 + 3]}} } {puts $def}
% test
[expr 4 + 3]
% test 5
5
% proc test { {def {[expr 4 + 3]}} } {puts [$def]}
% test
invalid command name "[expr 4 + 3]"
% proc test { {def {[expr 4 + 3]}} } {puts [eval $def]}
% test
invalid command name "7"

The example is just to simplify code. Of course in this simple example one would just use {def 7} to set the proc’s default value.

However, the goal is to be able to call some more complex function that delivers a good default value whenever the procedure test is being called. So the defaults can vary.

My current solution is to default to the empty string and check that:

% proc test { {def {}} }  {  if {$def == {}} { set def [expr 4 + 3] } ; puts $def }
% test
7
% test 5
5

However I consider this not elegant enough: There ought to be a way to put declarations where they belong: In the header.

Also, possibly, the empty string might be a perfectly fine value given by a caller that is not to be replaced with the default call.

Another workaround could be to just use args as a parameter and then inspect that one. But that provides even less explicit declarative style.

Any ideas how I can incorporate the eval into the declarative proc header?

(I’m on Tcl8.4 with no way to upgrade because of use in a commercial tool environment. But for the sake of this site I’d also encourage answers for more modern tcl versions)

Answer 1

What about this one (which is just slightly different from your tries):

% proc test {{def {[expr 4+3]}}} {eval puts "$def"}
% test
7
% test 5
5