Conversion funnel: count distinct users reaching each stage, then compute step-to-step conversion rates

funnel.cell

// Conversion funnel: count distinct users reaching each stage, then compute
// step-to-step conversion rates. Demonstrates count over a lambda, distinct
// per stage, and building a derived report list with map().

cell Funnel {
    on hits() {
        return list(
            map("user", "u1", "stage", "visit"),
            map("user", "u2", "stage", "visit"),
            map("user", "u3", "stage", "visit"),
            map("user", "u4", "stage", "visit"),
            map("user", "u1", "stage", "signup"),
            map("user", "u2", "stage", "signup"),
            map("user", "u3", "stage", "signup"),
            map("user", "u1", "stage", "purchase"),
            map("user", "u2", "stage", "purchase")
        )
    }

    // Unique users at a given stage.
    on reached(stage: String) {
        return hits() |> filter_by("stage", "=", stage) |> distinct("user") |> len
    }

    // visit -> signup -> purchase counts and conversion %.
    on report() {
        let stages = list("visit", "signup", "purchase")
        let out = list()
        let prev = 0
        let i = 0
        while i < len(stages) {
            let n = reached(stages[i])
            let rate = if prev == 0 { 100 } else { floor(n * 100 / prev) }
            out = out.push(map("stage", stages[i], "users", n, "rate", rate))
            prev = n
            i += 1
        }
        return out
    }

    on run() {
        for r in report() { print("{r.stage}: {r.users} users ({r.rate}%)") }
        return report()
    }
}

cell test FunnelTests {
    rules {
        // 4 visited, 3 signed up, 2 purchased
        assert reached("visit") == 4
        assert reached("signup") == 3
        assert reached("purchase") == 2
        // signup conversion = floor(3*100/4) = 75
        assert report()[1].rate == 75
        // purchase conversion = floor(2*100/3) = 66
        assert report()[2].rate == 66
    }
}