Course of Raku / Advanced / Operators / Meta-operators

Reduction metaoperators

A reduction meta-operator is written as a regular operator wrapped in square brackets, such as [+] or [*]. It takes a list of values and inserts the operator between every pair of them, reducing the whole list to a single value.

For example, [+] adds up all the elements of a list:

my @data = 3, 5, 7, 9, 11;
say [+] @data; # 35

The construct [+] @data is equivalent to writing the operator out by hand:

say 3 + 5 + 7 + 9 + 11; # 35

Any suitable infix operator works the same way. With [*] you get the product of the list, so applying it to the range 1..$n is a handy way to compute a factorial:

my $n = 5;
say [*] 1..$n; # 120

Here, the range 1..$n produces the numbers from 1 to 5, and [*] multiplies them: 1 * 2 * 3 * 4 * 5, which is 5!.

String concatenation works too. The [~] reduction joins a list of strings into one:

my @strings = <neun hundert fünf und zwanzig>;
say [~] @strings; # neunhundertfünfundzwanzig

Even comparison operators can be reduced. [<] reports whether the values are in strictly increasing order:

say [<] 1, 2, 3; # True

Triangular reduction

If you put a backslash inside the brackets, you get a triangular reduction, which keeps all the intermediate results instead of only the final one:

say [\+] 1..5; # (1 3 6 10 15)

Each element of the result is a partial sum: 1, then 1+2, then 1+2+3, and so on up to the sum of the whole list.

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