Course of Raku / Addendum 🆕 / Flow and functions / Subroutines and functional style / Exercises / A running accumulator

Solution: A running accumulator

Here is a possible solution to the task.

Code

sub make-accumulator($start) {
    my $total = $start;
    return -> $amount { $total += $amount };
}

my $acc = make-accumulator(100);

say $acc(10);
say $acc(25);
say $acc(-5);

🦋 You can find the source code in the file accumulator.raku.

Output

110
135
130

Comments

1. The returned block closes over $total: that variable lives on between calls, so each call remembers the total from the previous one.

2. $total += $amount both updates the running sum and returns it, which is what each say prints: 110, then 135, then 130.

3. An alternative keeps the total inside the block itself, in a state variable. Unlike an ordinary my, a state variable is initialised only once — the first time the block runs — and then keeps its value across later calls:

   sub make-accumulator($start) {
       return -> $amount { state $total = $start; $total += $amount };
   }
   
   my $acc = make-accumulator(100);
   say $acc(10);
   say $acc(25);
   say $acc(-5);

   Each call to make-accumulator produces a fresh block with its own state $total, so separate accumulators stay independent — and the closure over $start still supplies each one its own starting value.

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