Re: Safe list assignment

On Tue, May 27, 2008 at 6:00 AM, Ed Avis <[email protected]> wrote:
> Perl's list assignment feature is very useful.
>
>    sub returns_three_things { return (1, 2, 3) }
>    my ($x, $y, $z) = returns_three_things();
>
> Often, though, you want to add some error checking.  Particularly when providing
> an interface that others can call.
>
>    # Parameters: first name, last name, birthday.
>    sub print_person_details {
>       croak 'pass three arguments' if @_ != 3;
>       my ($first_name, $last_name, $birthday) = @_;
>       say "$first_name $last_name was born on $birthday";
>    }
>
> The extra 'croak' test gives a more friendly error message than 'use of
> uninitialized value in ...', which would require the user of the subroutine to
> go digging in its code to find what he passed wrongly.
>
> Even in code that's not part of a public interface, you may want to check that
> the list has the right size before you do the assignment, just to catch
> programmer mistakes early rather than waiting for an uninitialized value warning
> later on.
>
> But it's rather a pain to keep checking the list size each time.  When you
> 'unpack a tuple' in Python the size is checked and an exception is thrown if
> it's too big or too small.  Is there an equivalent in Perl?  Something like
>
>     safe_assign ($a, $b, $c) = @array;    # will die if scalar(@array) != 3
snip

Not that I am aware of, but it is a breeze to build one:

#!/usrbin/perl

use strict;
use warnings;

sub safe_assign {
	unless (@_) {
		my ($f, $l) = (caller)[1,2];
		die "bad number of arguments to safe_assign at $f line $l.\n";
	}
	my $targets = shift;

	my $expected = @$targets;
	my $got      = @_;
	unless ($expected == $got) {
		my ($f, $l) = (caller(1))[1,2];
		die "bad number of arguments, exected $expected got $got at $f line $l.\n";
	}

	for my $target (@$targets) {
		$target = shift;
	}

	return @$targets;
}

sub test {
	safe_assign([my ($one, $two, $three)], @_);
}

sub test2 {
	safe_assign();
}

for my $i (0 .. 4) {
	my @args = 1 .. $i;
	eval { test(@args) };
	print "args were: (@args)\nerror was: $@\n" if $@;
}
test2();

}
-- 
Chas. Owens
wonkden.net
The most important skill a programmer can have is the ability to read.