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

whenever vs when

The shape of a react block may look familiar. An outer block holding several inner blocks, each acting on the topic $_, is exactly how given/when is written. If you had a single temperature reading instead of a whole stream of them, you would branch on it like this:

given $temperature {
    when 18 { say 'cool' }
    when 21 { say 'warm' }
}

The resemblance to react/whenever is real, but the two do very different jobs:

given/when handles one value, right now. given sets $_ to a single value; each when tests it ($_ ~~ 18), the first match runs, and the block is finished. It is an ordinary, synchronous conditional.
react/whenever handles many values, over time. A whenever does not test $_ against a pattern — it subscribes to a supply, and its body runs for every value that supply emits, whenever that value arrives. All the whenevers are live at once, and react keeps blocking until each of their supplies is finished.

So when asks “does this one value match?”, while whenever says “for each value this stream ever produces, do this”. Same skeleton, very different lives: a branch taken a single time, versus a reaction that runs again and again as events arrive.

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whenever vs when

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