Course of Raku / Functional, concurrent, reactive, and web programming / Functional programming / Data feeds
The feed operator
The feed operator ==> sends a list from the left into
the operation on the right. The result then flows on to wherever the
next ==> points, ending in a variable that collects
it:
(1..10) ==> grep(* %% 2) ==> my @evens;
say @evens; # [2 4 6 8 10]Read it left to right: take 1..10, keep the even
numbers, and store the result in @evens. The
%% operator means “is divisible by”, so * %% 2
keeps the even numbers.
The important rule is that a feed must end in a
target — usually my @array (or an existing
variable). The data flows forward into it. Writing the assignment the
other way round, with =, does not do what you want, because
the feed and the assignment compete; always let the feed finish into its
variable.
A feed is just another way to spell a chain of list operations. The
same result could be written
my @evens = (1..10).grep(* %% 2). The feed form comes into
its own when there are several stages, as the next topic shows.
Practice
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