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

Solution: Two whenevers

Here is a possible solution to the task.

Code

my $deposits    = Supply.from-list(100, 50, 200);
my $withdrawals = Supply.from-list(30, 120);

my $balance = 0;

react {
    whenever $deposits    { $balance += $_ }
    whenever $withdrawals { $balance -= $_ }
}

say $balance;

🦋 You can find the source code in the file two-whenevers.raku.

Output

200

Comments

1. The two whenever blocks watch the two named supplies and react differently: deposits are added, withdrawals subtracted. One react block, two streams, two distinct reactions — that is the point of having a whenever per source.

2. The react block finishes only when both supplies are done, so the balance is final before it is printed: 100 + 50 + 200 - 30 - 120 gives 200. Since addition and subtraction commute, the interleaving of the two streams does not affect the result.

3. Here both supplies deliver their values instantly, one stream after the other. In real life the events of a bank account arrive spread out over time — a deposit now, a withdrawal an hour later — and the two streams interlace, as in the Two timers demo. The code stays exactly the same: the react block runs whichever whenever body matches each event as it comes in.

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