CODEWITHSUNDEEP
tcl
David Sligar
David Sligar

Reputation: 3

List become a nested list when passed through a procedure

I'm a beginner who just recently learned some TCL and recently came across a puzzling TCL internal process that I cannot explain. I have the code below:

proc print_items{args} {
    foreach item $args {
        puts $item
    }
}

set arr "A B C D"
print_items $arr

I was expecting the result to be:

A
B
C
D

but it was:

A B C D

Upon closer inspection. I noticed that $args was not the flat list I thought but instead was

> puts $args
{A B C D}
> llength $args
1

That means $args itself must have been wrapped in another list i.e $args must've been like this when received by the procedure: {{A B C D}}

So my question is: Why does TCL wrapped an already formed list in another list when passed inside print_items? This is very different from other programming languages that I've done. In which circumstances does the argument gets wrapped in another data structure (i.e: in another list, dictionary, array, etc.)?

Upvotes: 0

Views: 126

Answers (1)

Chpock
Chpock

Reputation: 682

The keyword "args" in procedure arguments has a special meaning. It collects all parameters to a tcl list. For example:

proc foo { param1 param2 args } {
    # here:
    #     $param1 = alice
    #     $param2 = bob
    #     $args = {alice1 bob1 alice2 bob2}
}
foo alice bob alice1 bob1 alice2 bob2

Thus, the answer for your question is: don't use the special word "args" as a prodecure argument because it is processes in a special way. Try to use any other name for procedure agrument.

Upvotes: 3

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