Perl 6 whatever star

Naming a whatever star is quite simple, since it is a simple * in the code. Be aware that not all * in code are whatever stars, in particular if the * appears within a list or a range chances are it is not a whatever star rather a limit or boundary. But what does a whatever star actually do?

The magic behind whatever star

A whatever star is automatically translated to Whatever class by the compiler, and in turn into a WhateverCode. In short a whatever star will produce a closure. Let’s see this in action: imagine a simple piece of code that requires a closure to upper case a list of words:
my @words = < blah foo bar baz >;
say { $_.uc } );
quite clear: on each element of the @words array, the map method assign $_ to the current element and the uc method is called on it. How does the whatever star translates applies to the above code? The code became really shorter:
my @words = < blah foo bar baz >;
say *.uc );
the whole closure became *.uc, that can be read as:
  1. create a closure with one argument;
  2. apply the uc method to each element passed to the closure.

Improving signatures with whatever stars

Every * in the block of code for the closure will be considered as a separate argument. That means that:
my @words = < blah foo bar baz >;
say * ~ *.uc );
having two * will be as the closure has been defined as:
say -> $a, $b { $a ~ $b.uc } );
Please note that every * is managed as a different argument, so in the case you need to backreference a specific argument you cannot use the whatever star. On the other hand, the code is cleaner and simpler without having the details of the closure explicitly printed out.

Whatever star for methods-like

Assuming a code block can be used as a method with a signature, the following two pieces of code are the same:
my $func = -> $a, $b { $a + $b };
say $func( 10, 20 );
as well as using a couple of stars:
my $f = * + *;
say $f( 10, 20 );


When using a whatever star be aware of the following simple rule of thumbs:
  • it is possible to use whatever stars only where a block of code is allowed;
  • you don’t need the braces to limit the block of code, the whatever star will create a block for you (or better, the compiler will do);
  • each * is a separate argument on the block signature.

The article Perl6 whatever star has been posted by Luca Ferrari on December 12, 2017