CODEWITHSUNDEEP
tcl
my_question
my_question

Reputation: 3245

What does it mean proc argument is variable

I wonder if it is right semantics to have a variable as an argument, something like this:

proc p1 {$aa} {}

I tried it on tclsh, there is no complaint, but the following experiment fails:

% set aa bb
bb
% set bb 200
200
% proc p1 {$aa} {puts $bb}
% p1 bb
can't read "bb": no such variable

Do you see what is wrong?

[UPDATE - after seeing Peter's answer] I know the upvar semantic, thanks.

My main curiosity is still around using variable as proc argument. I know it is not common, but just cannot help musing what it really can do if the language syntax allows it.

Yes, your upvar example is exactly what I want to explore using a variable as proc argument, but my exploration so far tells me, really, there is no way we can do this because "$" is interpreted as a plain char.

Do debunk me please if I am wrong.

Upvotes: 0

Views: 68

Answers (2)

Donal Fellows
Donal Fellows

Reputation: 137767

There are cases where it makes sense for the arguments in a procedure creation call to be supplied from a variable. The main example is where you are creating procedures dynamically, calculating the arguments you want to use as you go along.

That's actually a use-case that isn't done very frequently! It's not particularly easy to use well. But I have done it. (OK, that's a method call, but the syntax of formal arguments is shared.)

The main reason that the capability is there is that it's part of Tcl's general syntax. Tcl tries very hard to not have special cases in how it parses things (other than in how a command parses the strings passed into it) and this includes in things that would be very special cases in the enormous majority of other programming languages, such as formal parameter lists to procedures. In Tcl, these are just ordinary values and can be produced using any technique that gives ordinary values.

The usual thing with putting them in braces is just how you do it reliably and is easy to teach. It's also overwhelmingly what people want to do.

All this is independent of the facts that Tcl's commands (including its procedures) can handle variable numbers of arguments (check out the special args parameter and how to specify default values) and that $aa is a legal (but strange) name for a local variable.

Upvotes: 0

Peter Lewerin
Peter Lewerin

Reputation: 13282

Tcl does not support reference arguments as such: the usual pass-by-reference semantics is too static for Tcl. Instead, the logic of the command can, by use of upvar, dynamically create reference parameters including indirect reference parameters and calculated reference parameters, and also retarget the local name to another external variable. The upvar mechanism may look ungainly, but is very powerful indeed.

(The (edited) remains of my original answer follows:)

The usual idiom for doing this is

proc p varName {
    upvar 1 $varName var
    puts $var
}

The upvar command looks into another stack frame (in this case 1, which is the caller's stack frame) and makes a variable named $varName (i.e. the variable's name is the value of varName) in that stack frame and a variable named "var" in the command's stack frame refer to the same data object.

I won't explain this further since this is not useful to the asker.

Documentation: proc, puts, upvar

Upvotes: 1

Related Questions