I Am Greet

Let's revisit our greeting script and see how we can do much better than bash-Perl.

$ cat -n greet2.pl6
     1    #!/usr/bin/env perl6
     2
     3    sub MAIN ($greeting, $name='Stranger') {
     4        put "$greeting, $name";
     5    }
$ ./greet2.pl6
Usage:
  ./greet2.pl6 <greeting> [<name>]
$ ./greet2.pl6 --help
Usage:
  ./greet2.pl6 <greeting> [<name>]
$ ./greet2.pl6 Hello
Hello, Stranger
$ ./greet2.pl6 Hello Kenny
Hello, Kenny

Just look at that script. Just look at it! Do you see how much disappeared, and yet we still have all the functionality that we wrote before?! We have a usage statement, we have required and optional arguments, we have variable assignment, we have sane defaults, we have clean syntax.

So what is this MAIN thing? It's a subroutine that gets run automatically with the arguments to the script (https://docs.perl6.org/language/functions#index-entry-MAIN). The great masters of old passed down the wisdom that a good starting point for programs should be a function called "main," and so we do.

Now, let's add named arguments:

$ cat -n greet3.pl6
     1     #!/usr/bin/env perl6
     2
     3     sub MAIN (:$greeting!, :$name='Stranger') {
     4         put "$greeting, $name";
     5     }
$ ./greet3.pl6
Usage:
  ./greet3.pl6 --greeting=<Any> [--name=<Any>]
$ ./greet3.pl6 --greeting=Salutations
Salutations, Stranger
$ ./greet3.pl6 --name=Wilbur --greeting=Salutations
Salutations, Wilbur

By putting a colon : in front of the argument names, we used a shorthand to create a Pair type (https://docs.perl6.org/type/Pair). This was much easier than all that getopt business in bash, and naming the arguments lets the user specify them in any order desired. The ! at the end of :$greeting! indicates that this is a required argument, while the = after $name indicates a default value for an optional argument. You can learn more about the signatures of subroutines https://docs.perl6.org/type/Signature.

Did you notice also that the generated help included a hint as the type of data wanted by each argument? The (Any) is a data type (https://docs.perl6.org/type.html) in Perl 6 at the top of the type hierarchy (https://docs.perl6.org/type/Any). We could indicate that we want strings by adding a type to the signature:

$ cat -n greet4.pl6
     1     #!/usr/bin/env perl6
     2
     3     sub MAIN (Str :$greeting!, Str :$name='Stranger') {
     4         put "$greeting, $name";
     5     }
$ ./greet4.pl6
Usage:
  ./greet4.pl6 --greeting=<Str> [--name=<Str>]
$ ./greet4.pl6 --greeting="Top o' the morning"
Top o' the morning, Stranger
$ ./greet4.pl6 --greeting="Top o' the morning" --name=11
Top o' the morning, 11

We got off to a nice start there with the USAGE telling us to provide strings, so why didn't it complain about "11"? The problem is that everything coming from the command line looks like a string. There are ways to handle this, but they can't be so easily fixed. We'll come back to this later.