Course of Raku / Functional, concurrent, reactive, and web programming / Functional programming / Data feeds

Chaining feeds

The real value of feeds appears when you have several stages. Each ==> passes its result to the next operation, so a whole pipeline reads top to bottom in the order the work happens:

(1..10)
    ==> grep(* %% 2)
    ==> map(* ** 2)
    ==> my @result;

say @result; # [4 16 36 64 100]

Follow the data down the page: start with 1..10, keep the even numbers, square each of them, and collect the result. The even numbers are 2, 4, 6, 8, 10, and their squares are 4, 16, 36, 64, 100.

Written as a method chain, the same pipeline is (1..10).grep(* %% 2).map(* ** 2) — and that reads left to right just as the feed does. A method chain is not inside-out; it runs its stages in the very same order. The form that genuinely reads inside-out is nesting the operations as plain functions, map(* ** 2, grep(* %% 2, 1..10)), where you have to start at the innermost call and work outwards.

A feed, then, is not a way to escape inside-out code — it is another spelling of a left-to-right pipeline. What it adds is layout: each ==> stage can sit on its own line, and the one operator works the same for method-style and function-style steps alike, which keeps a long pipeline easy to scan. Feeds and method chains ultimately do the same job; choose whichever makes a particular transformation clearest.

Course navigation

← Quiz — The feed operator   |   Quiz — Feeds →

💪 Or jump directly to the exercises in this section.