CODEWITHSUNDEEP
perlfunction-prototypes
Ole Tange
Ole Tange

Reputation: 33740

Make perl look ahead for sub prototypes

Perl is a bit too forgiving: If you pass extra arguments to subs they are simply ignored.

To avoid this I would like to use prototypes to make sure each sub is given the correct amount of arguments.

This works OK as long as I declare the prototype before using it:

sub mysub($);
sub mysub2($);

mysub(8);
mysub(8,2); # Complain here                                                            

sub mysub($) {
    mysub2($@);
}

sub mysub2($) {
    if($_[0] == 1) {
        mysub(2);
    }
    print $@;
}

But I really hate splitting this up. I would much rather that Perl read the full file to see if there are declarations further down. So I would like to write something like:

use prototypes_further_down; # This does not work

mysub(8);
mysub(8,2); # Complain here                                                            

sub mysub($) {
    mysub2($@);
}

sub mysub2($) {
    if($_[0] == 1) {
        mysub(2);
    }
    print $@;
}

Can I somehow ask Perl to do that?

Upvotes: 4

Views: 241

Answers (1)

Dave Sherohman
Dave Sherohman

Reputation: 46225

To avoid this I would like to use prototypes to make sure each sub is given the correct amount of arguments.

No, you would not. Despite the similarity in name, Perl prototypes are not your father's function prototypes. Quoting The Problem with Prototypes (emphasis mine),

Perl 5's prototypes serve two purposes. First, they're hints to the parser to change the way it parses subroutines and their arguments. Second, they change the way Perl 5 handles arguments to those subroutines when it executes them. A common novice mistake is to assume that they serve the same language purpose as subroutine signatures in other languages. This is not true.

In addition to them not having the same intended purpose, bypassing prototypes is trivial, so they provide no actual protection against someone who deliberately wishes to call your code in (what you believe to be) the "wrong" way. As perldoc perlsub tells us,

The function declaration must be visible at compile time. The prototype affects only interpretation of new-style calls to the function, where new-style is defined as not using the & character. In other words, if you call it like a built-in function, then it behaves like a built-in function. If you call it like an old-fashioned subroutine, then it behaves like an old-fashioned subroutine. It naturally falls out from this rule that prototypes have no influence on subroutine references like \&foo or on indirect subroutine calls like &{$subref} or $subref->().

Method calls are not influenced by prototypes either, because the function to be called is indeterminate at compile time, since the exact code called depends on inheritance.

Even if you could get it to complain about mysub(8,2), &mysub(8,2) or $subref = \&mysub; $subref->(8,2) or (if mysub were an object method inside package MyModule) $o = MyModule->new; $o->mysub(8,2) would work without complaint.

If you want to validate how your subs are called using core Perl (prior to 5.20), then you need to perform the validation yourself within the body of the sub. Perl 5.20 and newer have a ("experimental" at the time of this writing) Signatures extension to sub declarations which may work for your purposes, but I've never used it myself, so I can't speak to its effectiveness or limitations. There are also many CPAN modules available for handling this sort of thing, which you can find by doing searches for things like "signature" or "prototype".

Regardless of your chosen approach, you will not be able to get compile-time errors about incorrect function signatures unless you define those signatures before they are used. In cases such as your example, where two subs mutually call each other, this can be accomplished by using a forward declaration to establish its signature in advance:

sub mysub($foo);                               # Forward declaration
sub mysub2 { mysub(8) }
sub mysub  { mysub2('infinite loops ftw!') }   # Complete version of the code

Upvotes: 4

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