Course of Raku / Functional, concurrent, reactive, and web programming / Functional programming / Reduction

The reduce routine

When you want to reduce with a custom operation rather than a single named operator, use the reduce routine. It takes a block of two arguments and applies it across the list, carrying the running result forward:

say (1..5).reduce(* + *); # 15

The block * + * has two Whatever stars: the first is the result so far, the second is the next element. So reduce computes ((((1 + 2) + 3) + 4) + 5), that is 15 — the same as [+], but written as a block.

To see those two arguments at work, give them names and print each step. A pointy block hands back its last expression, so $x + $y is both what the block prints about and the value it returns to reduce:

say (1..5).reduce(
    -> $x, $y {
        say "$x + $y";
        $x + $y;
    }
);

This prints:

1 + 2
3 + 3
6 + 4
10 + 5
15

Each of the first four lines is one call to the block. $x is the result accumulated so far and $y is the next element from the list. The very first call gets the list’s first two values, 1 and 2; every later call receives the previous step’s total back as $x — 3, then 6, then 10 — paired with the next number as $y. The block’s value becomes the $x of the following call, and the final total, 15, is what reduce returns and say prints on the last line.

Because the block can be anything, reduce is more general. For example, build a value step by step:

say (1..4).reduce(-> $acc, $x { $acc * 10 + $x }); # 1234

Each step multiplies the accumulator by ten and adds the next digit, turning the separate digits 1, 2, 3, 4 into the number 1234.

Like the other block-taking methods, reduce reads well with the colon-call form, which drops the parentheses around the block:

say (1..5).reduce: * + *; # 15

In short, [op] is the quick form for a named operator, and reduce is the flexible form for an arbitrary combining block. Both fold a list down to a single value.

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