Course of Raku / Advanced / Subroutines

Anonymous subroutines

A subroutine does not have to have a name. An anonymous subroutine is written just like a normal one, but without a name, and is usually stored in a variable so that you can call it later.

my $double = sub ($x) {
    $x * 2
};

say $double(5); # 10

The variable $double holds the subroutine, and $double(5) calls it with the argument 5.

Pointy blocks

Raku offers a shorter notation for the same thing — the pointy block. It uses the arrow -> to introduce the parameters, and needs neither the word sub nor parentheses around them:

my $double = -> $x {
    $x * 2
};

say $double(5); # 10

This is the most common way to write a small anonymous subroutine.

The & sigil

If you would rather call the subroutine by a name than through a $ variable, declare the variable with the & sigil. Then you can call it without the sigil, exactly like an ordinary subroutine:

my &triple = -> $x {
    $x * 3
};

say triple(4); # 12

Passing one to another function

You do not have to store an anonymous subroutine in a variable at all. Because it is just a value, you can hand it straight to a function that expects one — such as map, grep, or sort:

say (1, 2, 3).map(-> $x { $x * 2 }); # (2 4 6)

The pointy block is the anonymous subroutine here, and map calls it once for each element. This is, in fact, what you have been doing all along: every { ... } block and -> $x { ... } you have passed to map and grep is a small anonymous subroutine, written right where it is needed instead of being named first.

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