Course of Raku / Functional, concurrent, reactive, and web programming / Reactive programming / Supplies

Transforming a supply

A supply can be transformed before you tap it, much like a list. Methods such as map and grep produce a new supply whose values are the transformed ones:

my @out;
Supply.from-list(1, 2, 3).map(* * 10).tap(-> $v { @out.push($v) });
say @out; # [10 20 30]

The map(* * 10) builds a new supply that emits ten times each original value, and the tap collects those into @out.

grep filters a supply, passing on only the values for which its block is true:

my @out;
Supply.from-list(1, 2, 3, 4, 5, 6).grep(* %% 2).tap(-> $v { @out.push($v) });
say @out; # [2 4 6]

Because each transformation returns another supply, you can chain them, building a small pipeline that reacts to the stream:

my @out;
Supply.from-list(1..6).grep(* %% 2).map(* ** 2).tap(-> $v { @out.push($v) });
say @out; # [4 16 36]

This is the same map/grep vocabulary you know from lists, applied to values arriving over time. The supply does the pushing; your pipeline shapes what comes out.

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