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

The backward feed

Every feed so far has flowed forward with ==>, from a source on the left into a target on the right. Raku also has the mirror operator, <==, the backward feed. It runs the other way round: the target sits on the left, and the data is pulled in from the source on the right.

Here is the even-numbers pipeline from before, written backward:

my @evens <== grep(* %% 2) <== (1..10);
say @evens; # [2 4 6 8 10]

Read it right to left: take 1..10, keep the even numbers, and collect them into @evens. The result is exactly the same as (1..10) ==> grep(* %% 2) ==> my @evens — only the direction in which you write the stages has changed.

Backward feeds chain in the same way, and again the target leads:

my @result <== map(* ** 2) <== grep(* %% 2) <== (1..10);
say @result; # [4 16 36 64 100]

The work still happens source-first — start with 1..10, keep the evens, square them — but on the page the stages are listed from the destination back towards the origin. The order of the elements is untouched; <== changes only the reading direction, never the data.

Because code usually reads left to right, the forward ==> tends to read more naturally: you meet the data first and follow it to where it lands. The backward <== is there for the occasions when leading with the destination tells the story better — naming what you are building before what it is built from. The two are exact mirrors, so choose whichever direction reads best.

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