Learning Perl 6

Chapter 11. Subroutines

Now it’s time for more sophisticated subroutines. You were introduced to them in Chapter 5 but you only saw enough to support the upcoming chapters. Now that you’ve seen Arrays and Hashes, there’s much more you can do with subroutine signatures.

A Basic Subroutine

When you run a subroutine you get some sort of result: the last evaluated expression. That’s the return value. This sets basic routines apart from the simpler Blocks you saw in Chapter 5. A Routine knows how to send a value back to the code that called it. This subroutine returns a different Str if the argument is odd or even:

sub odd-or-even {
    if ( @_[0] %% 2 ) { 'Even' }
    else              { 'Odd'  }
    }

odd-or-even( 2 );    # Even
odd-or-even( 137 );  # Odd

Without a signature the arguments show up in @_. Each subroutine has its own version of that variable so it doesn’t conflict with any other subroutine’s @_. This code calls one subroutine that calls another. top-call shows its @_ before and after show-args:

top-call( <Hamadryas perlicus> );

sub show-args { say @_ }
sub top-call {
    put "Top: @_[]";
    show-args( <a b c> );
    put "Top: @_[]";
    }

Even though both use @_ they are separate. The @_ in top-call isn’t disturbed by show-args:

Top: Hamadryas perlicus
[a b c]
Top: Hamadryas perlicus

The subroutine definition is lexically scoped. If you need it for only part of the code you can hide it in a Block. Outside the Block that subroutine is not visible:

{ put odd-or-even( 137 ); sub odd-or-even { ... } # only defined in this block ...