Why do I get an error when I try to use the reptition assignment operator with an array?

Question

#!/usr/bin/perl

use strict;
use warnings;

my @a = qw/a b c/;
(@a) x= 3;
print join(", ", @a), "\n";

I would expect the code above to print "a, b, c, a, b, c, a, b, c\n", but instead it dies with the message:

Can't modify private array in repeat (x) at z.pl line 7, near "3;"

This seems odd because the X <op>= Y are documented as being equivalent to X = X <op> Y, and the following code works as I expect it to:

#!/usr/bin/perl

use strict;
use warnings;

my @a = qw/a b c/;
(@a) = (@a) x 3;
print join(", ", @a), "\n";

Is this a bug in Perl or am I misunderstanding what should happen here?

Answer 1

The problem is that you're trying to modify @a in place, which Perl evidently doesn't allow you to do. Your second example is doing something subtly different, which is to create a new array that consists of @a repeated three times, then overwriting @a with that value.

Arguably the first form should be transparently translated to the second form, but that isn't what actually happens. You could consider this a bug... file it in the appropriate places and see what happens.

Answer 2

My guess is that Perl is not a language with full symbolic transformations. It tries to figure out what you mean. If you "list-ify" @a, by putting it in parens, it sort of loses what you wanted to assign it to.

Notice that this does not do what we want:

 my @b = @a x 3; # we'll get scalar( @a ) --> '3' x 3 --> '333'

But, this does:

my @b = ( @a ) x 3;

As does:

( @a ) = ( @a ) x 3;

So it seems that when the expression actally appears on both sides Perl interprets them in different contexts. It knows that we're assigning something, so it tries to find out what we're assigning to.

I'd chalk it up to a bug, from a very seldom used syntax.

Answer 3

My first thought was that it was a misunderstanding of some subtlety on Perl's part, namely that the parens around @a made it parse as an attempt to assign to a list. (The list itself, not normal list assignment.) That conclusion seems to be supported by perldiag:

Can't modify %s in %s

(F) You aren't allowed to assign to the item indicated, or otherwise try to change it, such as with an auto-increment.

Apparently that's not the case, though. If it were this should have the same error:

($x) x= 3;  # ok

More conclusively, this gives the same error:

@a x= 3;  # Can't modify private array in repeat...

Ergo, definitely a bug. File it.