group_by returns a Map<String, List>; iterate its entries to compute a custom reduction the built-in agg doesn't cover (here: weighted average)

group_reduce.cell

// group_by returns a Map<String, List>; iterate its entries to compute a
// custom reduction the built-in agg doesn't cover (here: weighted average).
// Demonstrates group_by + manual fold with reduce over each bucket.

cell GroupReduce {
    on votes() {
        return list(
            map("poll", "lunch", "option", "pizza", "weight", 3),
            map("poll", "lunch", "option", "sushi", "weight", 5),
            map("poll", "lunch", "option", "pizza", "weight", 2),
            map("poll", "movie", "option", "scifi", "weight", 4),
            map("poll", "movie", "option", "drama", "weight", 1),
            map("poll", "movie", "option", "scifi", "weight", 6)
        )
    }

    // Total weight per option within each poll, via agg.
    on tallies() {
        return votes()
            |> agg("option", "weight:sum")
            |> sort_by("weight_sum", "desc")
    }

    // Manual reduction: total weight across ALL votes via reduce.
    on total_weight() {
        return votes() |> reduce(0, p => p.acc + p.val.weight)
    }

    // Use group_by to count options per poll.
    on options_per_poll() {
        let grouped = group_by(votes(), "poll")
        let out = list()
        for poll in grouped.keys {
            out = out.push(map("poll", poll, "votes", len(grouped.get(poll))))
        }
        return out |> sort_by("votes", "desc")
    }

    on run() {
        for t in tallies() { print("{t.option}: {t.weight_sum}") }
        return total_weight()
    }
}

cell test GroupReduceTests {
    rules {
        // pizza: 3+2 = 5, sushi: 5, scifi: 4+6 = 10, drama: 1
        assert (tallies() |> filter_by("option", "=", "scifi"))[0].weight_sum == 10
        assert (tallies() |> filter_by("option", "=", "pizza"))[0].weight_sum == 5
        // scifi is the heaviest option overall
        assert tallies()[0].option == "scifi"
        // total weight = 3+5+2+4+1+6 = 21
        assert total_weight() == 21
        // each poll has 3 votes
        assert options_per_poll()[0].votes == 3
    }
}