Course of Raku / Advanced / Subroutines

The Whatever star 🆕

You may have seen the * used in expressions like * * 2 or * %% 2 when calling map and grep. That * is the Whatever star, and the expression around it builds a small one-argument function on the fly. This section explains what it really is, how it relates to a block, and where the bare star means simply “whatever you want”.

A star makes a function

When * appears in an expression, Raku turns the whole expression into a function — a WhateverCode — in which the star stands for the argument:

my $double = * * 2;

say $double.^name; # WhateverCode
say $double(21);   # 42

* * 2 is a function that multiplies its one argument by two. This is why (1..5).map(* * 2) works: map is handed exactly such a one-argument function.

Whatever versus a block

A Whatever expression is a shorter way of writing a function you could also write as a block. These three are equivalent:

* * 2
{ $_ * 2 }
-> $x { $x * 2 }

The Whatever form names no parameter; the block form uses the topic variable $_; the pointy form names $x. That last one, -> $x { ... }, is a small function in its own right — you will meet it properly in Anonymous subroutines; here it is enough to read it as a third way to spell the same one-argument function. For a simple expression the star is the most compact, which is why it is so common with map, grep, and sort:

say (1..5).map(* * 2);            # (2 4 6 8 10)
say (1..5).map({ $_ * 2 });       # (2 4 6 8 10)
say (1..5).map(-> $x { $x * 2 }); # (2 4 6 8 10)

Reach for a block when the logic needs more than one expression, or when a clearer named parameter helps. Reach for the star when a short expression says it all.

More than one star

Each * in the expression is a separate argument, in order. So two stars make a two-argument function:

my $add = * + *;

say $add(3, 4); # 7

Here * + * is a two-argument function that adds its arguments — for example a running total and the next element.

The bare Whatever

On its own, * means “whatever” — as much as there is, or no limit. Two everyday uses:

my @a = 10, 20, 30;
say @a[*-1];        # 30  — * is the array length, so *-1 is the last index

say (1..*).head(3); # (1 2 3)  — 1..* is an open-ended range

In @a[*-1] the star stands for the number of elements, and in 1..* it stands for “no upper bound”. Whether it builds a function or means “whatever you want”, the star is one of Raku’s most useful pieces of shorthand.

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