Perl 6 and Parrot Essentials, Second Edition

Subroutine Scope

Just like variables, subroutine names are simply entries in a symbol table or lexical scratchpad. So, all subroutines live in a particular scope, whether it’s lexical, package, or global scope.

Package-Scoped Subroutines

Package scope is the default scope for subs. A sub that is declared without any scope marking is accessible within the module or class where it’s defined with an unqualified call, like subname( ), and accessible elsewhere with a fully qualified call using the Package::Name::subname( ) syntax.^[13]

module My::Module {
  sub firstsub ($param) { . . . }

  sub secondsub {
    mysub('arg'); # call the subroutine
  }
}

module Other::Module {
  use My::Module;

  sub thirdsub {
     My::Module::firstsub('arg');
  }
}

This example declares two modules, My::Module and Other::Module. My::Module declares a subroutine firstsub and calls it from within secondsub. Other::Module declares a subroutine thirdsub that calls firstsub using its fully qualified name.

Lexically Scoped Subroutines

Subroutines can also be lexically scoped, just like variables. A myed subroutine makes an entry in the current lexical scratchpad with a & sigil. Lexically scoped subs are called just like a normal subroutine:

if $dining {
    my sub dine ($who, $where) {
         . . . 
    }

    dine($zaphod, "Milliways");
}

# dine($arthur, "Nutri-Matic");  # error

The first call to the lexically scoped dine is fine, but the second would be a compile-time error because dine doesn’t exist in the outer scope.

The our keyword declares a lexically ...