Perl5 -> Perl 6: slurpy things

Perl 5 is famous for having a slurpy nature, especially when dealing with method arguments. In Perl 6 this behavior has changed, so the developer must be aware and instrument the compiler for when it should expect a slurpy argument.

Being Slurpy: the Perl 5 way

Perl 5 passes all arguments to a function or a method via the topic array @_. When I was approaching Perl 5 coming from other typed languages (like C, Java, etc.) I thought this behavior was horrible. Doing more and more Perl 5 development, I found it really smart and compact: you name parameters only when (and where) you need them, otherwise just consume them as a flat list. This slurpy behavior means that you have to use references to structured data as arguments or you will not be able to distinguish parameters within the same flat list. With that in mind, the following program illustrates the Perl 5 slurpy behavior:
use v5.20;

sub hello {
    my ( @args ) = @_;
    say 'hello got ' . scalar @args . ' arguments';
    say join ' -> ', @args;

sub hello_2 {
    say 'hello_2 got '. scalar @_ . ' arguments';
    say join ' ->> ', @{ $_ } for @_;


hello 1, 2, 3;
hello( (1, 2), qw( a b ) );

hello_2( [1, 2], ['a', 'b'] );
producing the following output
hello got 3 arguments
1 -> 2 -> 3
hello got 4 arguments
1 -> 2 -> a -> b
hello_2 got 2 arguments
1 ->> 2
a ->> b
In order to recap it is possible to see that:
  • hello got a flat list even when two lists are passed as arguments, what the function “sees” is a single list made up by all the elements;
  • hello_2 again sees a single flat list, this time made up by two references to arrays and therefore it is able to elaborate such references in order to get the array identities.

Being not slurpy: the Perl 6 way

In Perl 6 you declare exactly what type of arguments your method is going to receive, and this will prevent the generation of the flat catch-all list. That means that the above program simply becomes:
sub hello( @args ) {
    @args.join( ' -> ' ).say;

sub hello_2( @arg1, @arg2 ) {
    @arg1.join( ' ->> ' ).say;
    @arg2.join( ' ->> ' ).say;

hello <1, 2, 3>;
hello_2 (1,2,3), ('a', 'b');
producing the output
1, -> 2, -> 3
1 ->> 2 ->> 3
a ->> b
What happens if we pass two arrays to the hello method that accepts only one?
hello (1,2,3), ('a', 'b');
the complier will claim the mistake since it is not able to understand what is going on:
Too many positionals passed; expected 1 argument but got 2
  in sub hello at test.p6 line 4
  in block <unit> at test.p6 line 14
So Perl 6 is saying quite loudly we declared the method in a specific way, and we cannot call in a more general way.

Being slurpy, the Perl 6 way

It is possible to instrument Perl 6 to accept a slurpy parameter, of course not a scalar, using a * in front of the parameter type. Once Perl 6 encounters a slurpy parameter, nothing else will be considered (of course), so slurpy parameters must be at the end of the declaration:
sub hello( *@args ) {
    @args.join( ' -> ' ).say;

hello (1,2,3), ('a', 'b');
that produces the following output:
1 -> 2 -> 3 -> a -> b
It is important to note that all parameters are printed within the same line, that is within the same say context, as the array is effectively a flat list of arguments. But Perl 6 allows also for another slurpy mode: the ** slurpy mode. Modifying the function declaration the program changes its behavior:
sub hello( **@args ) {
    @args.join( ' -> ' ).say;

hello (1,2,3), ('a', 'b');
producing the following output:
1 2 3 -> a b
What has changed? With the double-star the argument has kept its slurpy behavior, but it has also preserved the identifies of the single lists within the array. In other words *@ provides a slurpy flat behavior, while **@ provides a slurpy structured behavior.

The article Perl5 -> Perl 6: slurpy things has been posted by Luca Ferrari on December 13, 2017