Re: "last expression" quiz

[Aristotle]

Check Re^8: Unhappy returns (just updated) for some tests as to what gets returned when. Apparently:

• If the last thing that happened was an iteration of a for loop, you will get back !1, alias scalar false (it is 0 and an empty string at the same time).
• If the last thing that happened was an iteration of a while loop, you get back nothing at all.
• If the last thing that happened was an if block, you get back the value of the last expression evaluated – that would be either the condition, if it evaluated to false, or the last expression from the body of the conditional block.

In other words, what perlsub says is deceptive and incomplete.

Update: oh goodness, it gets weirder still:

• If the last thing that happened was an iteration of the do {} while $cond or do {} until $cond pseudo-loop, you get back a code reference (whether the condition turned out true of false).

It’s probably safe to say that Perl simply has no rule here, and that what you get back from a sub depends on implementation details of perl.

[Anonymous Monk]

It’s probably safe to say that Perl simply has no rule here, and that what you get back from a sub depends on implementation details of perl.

Wouldn't it be more accurate to say that returning the value from the last expression in subs works fine. But statements (things like if, for, while) don't have values and return pretty much garbage?

[Aristotle]

Maybe, but how do you reconcile that with the behaviour with if blocks, which work precisely as documented?

[Anonymous Monk]

My point is mostly that the problem resides with how "for" is implemented, and not a problem with "sub" returning an incorrect value.

[sauoq]

But statements (things like if, for, while) don't have values and return pretty much garbage?

Well, if works in that it returns the result of the last expression evaluated. It seems that's what looping constructs should do too. Wouldn't it make sense for this sub x { $_ for 10 } to return 10?

[Anonymous Monk]

Well, if works in that it returns the result of the last expression evaluated. It seems that's what looping constructs should do too. Wouldn't it make sense for this sub x { $_ for 10 } to return 10?

Not really. If you're asking my opinion I'd say it should be an error if the sub is called in anything other than a void context. Note the following are also compilation errors...

my $what = (if(1){print "true"}else{print "false"});

sub foo
{
    return (for (1..3) { print $_ });
}

You're not arguing that they shouldn't be, are you?

[robin]

If the last thing that happened was an iteration of the do {} while $cond or do {} until $cond pseudo-loop, you get back a code reference (whether the condition turned out true of false).

That sounds really strange, but sadly I can't reproduce it. I tried

perl -e 'print sub {my $x=5; do {print "."} while $x--}->()'

but the sub just seems to return 0. Can you give an example? (Also, what version of Perl are you using?)

[Aristotle]

As the node you’re replying to already says: check Re^8: Unhappy returns for code.

$ perl -v

This is perl, v5.8.7 built for i486-linux

[robin]

There is a bug in your code. You've missed out the ->() in the last two cases, so it's printing the sub rather than the result of calling the sub.