On Thu, 2002-04-11 at 14:34, Larry Wall wrote:
> Miko O'Sullivan writes:
> : Well, I had been hoping to appeal to the mathematical mindset of the list,
> : but there is a second reason for = in addition to / /=: it's simpler to
> : understand. I just think that the potential Perl hackers will understand =
> : right away but will have to spin a lot of cycles to get / /=, and will
> : meanwhile be wondering why not just =. I'm hoping to point out that = is
> : both logically precise AND more marketable.
>
> Leaving marketability aside for the moment, I don't buy the argument
> that it's a feature for the user to be able to pass an undef past the
> default. The primary purpose of the default is to guarantee the
> semantics of the function, not to make life easier for the caller.
> If that also happens, it's a nice side effect.
Hmmm... I have to disagree with you there.
Consider this bit of Perl5 which I've done in various forms:
sub nukeuser {
my $user = shift;
my $pwf = shift;
$pwf = '/etc/passwd' unless defined $pwf;
my $backup = @_ ? shift @_ : "$pwf.bak";
my $do_backup = defined($backup);
...
}
Notice that we have two different types of defaulting here. The second
argument is the file to work on, and we set it to a reasonable default
if it is undefined for whatever reason. However, the third argument is
sensitive to undef vs no parameter. In the case of not getting a third
arguement, a reasonable default is set. If an argument is given, but it
is undef, no backup will be performed.
We're not just makeing life easier for the caller. What we're doing is
extracting as much information from the arguments as possible (just as
many Perl functions do). With //=, there's simply some data thrown away.
In Perl6, this might be:
sub nukeuser ($user, $pwf //= 'passwd', $backup = "$pwf.bak") {
my $do_backup = defined $backup;
...
}
[I don't know if you can use a parameter to default another parameter.
It would be nice, but I can see why you wouldn't.]
Without an "=" operator here, you would need to add another argument to
flag doing backups or not (not really the Perl Tao) or you would have to
do the argument processing manually, which sort of defeats the whole
point.
