CODEWITHSUNDEEP
perl
Thrust
Thrust

Reputation: 13

Perl argument parsing into functions when no parentheses are used

I have the following test code:

sub one_argument {
     ($a) = @_;
     print "In one argument: \$a = $a\n";
     return "one_argument";
}
sub mul_arguments {
     (@a) = @_;
     return "mul_argument";
}
print &one_argument &mul_arguments "something", "\n";

My goal is to be able to understand a bit better how perl decides which arguments to go into each function, and to possibly clear up any misunderstandings that I might have. I would've expected the above code to output:

In one argument: mul_argument
one_argument

However, the below is output:

Use of uninitialized value $a in concatenation (.) or string at ./test.pl line 5.
In one argument: $a =
mdd_argument

I don't understand where 'mdd_argument' comes from (Is it a sort of reference to a function?), and why one_argument receives no arguments.

I would appreciate any insight as to how perl parses arguments into functions when they are called in a similar fashion to above.

Please note that this is purely a learning exercise, I don't need the above code to perform as I expected, and in my own code I wouldn't call a function in such a way.

Upvotes: 1

Views: 160

Answers (1)

ysth
ysth

Reputation: 98398

perldoc perlsub:

If a subroutine is called using the & form, the argument list is optional, and if omitted, no @_ array is set up for the subroutine: the @_ array at the time of the call is visible to subroutine instead. This is an efficiency mechanism that new users may wish to avoid.

In other words, in normal usage, if you use the &, you must use parentheses. Otherwise, the subroutine will be passed the caller's @_.

The mysterious "mdd" is caused because &one_argument doesn't have any arguments and perl is expecting an operator to follow it, not an expression. So the & of &mul_arguments is actually interpreted as the stringwise bit and operator:

$ perl -MO=Deparse,-p -e 'sub mul_arguments; print &one_argument &mul_arguments "something", "\n"'
print((&one_argument & mul_arguments('something', "\n")));

and "one_argument" & "mul_arguments" produces "mdd_argument".

Upvotes: 6

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