Programming Perl, 4th Edition

Care with Prototypes

It’s probably best to put prototypes on new functions, not retrofit prototypes onto older ones. These are context templates, not ANSI C prototypes, so you must be especially careful about silently imposing a different context. Suppose, for example, you decide that a function should take just one parameter, like this:

sub func ($) {
    my $n = shift;
    print "you gave me $n\n";
}

That makes it a unary operator (like the rand built-in) and changes how the compiler determines the function’s arguments. With the new prototype, the function consumes just one scalar-context argument instead of many arguments in list context. If someone has been calling it with an array or list expression, even if that array or list contained just a single element, where before it worked, now you’ve got something completely different:

func @foo;                  # counts @foo elements
func split /:/;             # counts number of fields returned
func "a", "b", "c";         # passes "a" only, discards "b" and "c"
func("a", "b", "c");        # suddenly, a compiler error!

You’ve just supplied an implicit scalar in front of the argument list, which can be more than a bit surprising. The old @foo that used to hold one thing doesn’t get passed in. Instead, 1 (the number of elements in @foo) is now passed to func. And the split, being called in scalar context, scribbles all over your @_ parameter list. In the third example, because func has been prototyped as a unary operator, only “a” is passed in; then the return value from func is discarded ...