When in the course of Perl topicalizers

piezocuttlefish has asked for the wisdom of the Perl Monks concerning the following question:

I'm solving exercise 15-1 in the Llama, and I've run across a peculiarity I don't understand. In my program, I use the following loop:

for (
        print "Please enter your guess from " . MINGUESS . " to " . MA
+XGUESS . ":  ";
        <>;
        print "Try again.  Please enter your new guess from " . MINGUE
+SS . " to " . MAXGUESS . ":  ") {
    chomp;
    when ( '' ) { last; }
    when ( 'quit' ) { last; }
    when ( $iTarget ) { last; }
    default { print "\tYour guess of $_ was too " . ($_ < $iTarget ? "
+small." : "large.") . "\n"; }
    }
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The first time through the loop, the code executes as expected. If a second time through is necessary, the program immediately dies with "Can't use when() outside a topicalizer at (line number corresponding to 'default' line).", before the third section of the for loop conditions is able to execute. Is a for loop not a sufficiently good topicalizer? What gives?

Comment on When in the course of Perl topicalizers Download Code

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Re: When in the course of Perl topicalizers by ikegami (Patriarch) on Jun 28, 2010 at 23:39 UTC
Is a for loop not a sufficiently good topicalizer? It doesn't use or change any variables, so it doesn't make a good topicaliser at all. The documentation says you can use a foreach loop instead of `given`, and it doesn't mention you can use a C-style for loop (which is closer to a `while` loop than anything else) instead of `given`.	[reply] [d/l] [select]
Re^2: When in the course of Perl topicalizers by piezocuttlefish (Initiate) on Jul 07, 2010 at 01:13 UTC
Bummer. I think I get it. `foreach` and `given` always change the topic, whereas `for` and `while` aren't topicalizers; Perl just is nice and replaces `<>` with `defined ($_ = <>)`.	[reply] [d/l] [select]