What is precedence dropping call syntax?

Perl 6 introduces a lot of syntax changes from its predecessor (or cousin?), and one of it is the so called precedence dropping in method call. The syntax follows the following rules:

no need for brackets;
a comma : before the argument list;
all things after the comma are managed as a list.

Please note that this syntax applies only to method calls, since for subroutine calls it does suffice to avoid parenthesis at all. So let’s see this in action:

class Foo {
    method foo( *@v ) {
        say @v;
        return 1;
    }
}

Now imagine you are going to call as the following:

my $f = Foo.new;
say $f.foo( 'a', 'b', 'c').^name;

which in turn prints:

[a b c]
Int

What happened? Well, what we were expecting: the called the foo method and then chained the name on its return value, that is an integer. Now let’s rewrite this with precedence dropping:

$f.foo: 'a', 'b', 'c'.^name;

which produces the following:

[a b Str]

Here the arguments are managed as a whole huge list, so name is not applied to the method return value but to the last argument. In other words precedence dropping means that what is following the method call has the precedence on the call itself, and therefore is evaluated before it is passed to the method itself. So the following are equivalent:

$f.foo: 'a', 'b', 'c'.^name;
$f.foo( 'a', 'b', 'c'.^name );

that is what compose the argument list must be evaluated before (i.e., has the precedence) on the method call itself. Having said that, it is quite clear that the following does not work as expected too:

$f.foo: ( 'a', 'b', 'c' ).^name; # [List]

since again the argument is converted to a whole list, on which is evaluated name before and then the method is called. So when is precedence dropping useful? Well, in my opinion when you are hitting the last method call of a chain call you can quite safely apply precedence dropping, and this avoid parenthesis balance problems:

$f.baz.bar.map.sort.reverse.foo: 'a','b','c';
 #                           ^^^ end of method chain!

$f.bar.baz.sort.foo: 'a', 'b', 'c' .reverse  # wrong!

The article What is precedence dropping call syntax? has been posted by Luca Ferrari on September 20, 2017

Tags: raku , programming