Kanban board: Card records stored by id in a Map slot, moved between columns by mutating a field and writing back

kanban.cell

// Kanban board: Card records stored by id in a Map slot, moved between
// columns by mutating a field and writing back. Demonstrates records in a
// keyed store, a column roll-up, and counting by a record field.

cell Kanban {
    face {
        signal add(title: String, col: String) -> String
        signal move(id: String, col: String) -> Map
        signal column(col: String) -> List
        signal counts() -> Map
    }

    memory {
        cards: Map<String, Map> [persistent, consistent]
    }

    on add(title: String, col: String) {
        let id = "card-{next_id()}"
        cards.set(id, Card { id: id, title: title, col: col })
        return id
    }

    on move(id: String, col: String) {
        let c = cards.get(id)
        if c == () { return map("error", "no card") }
        c.col = col
        cards.set(id, c)
        return map("id", id, "col", col)
    }

    on column(col: String) {
        return cards.values |> filter(c => c.col == col) |> pluck("title")
    }

    on counts() {
        let vals = cards.values
        return map(
            "todo", count(vals, c => c.col == "todo"),
            "doing", count(vals, c => c.col == "doing"),
            "done", count(vals, c => c.col == "done")
        )
    }
}

cell test KanbanTest {
    rules {
        assert starts_with(add("Spec", "todo"), "card-")
        assert add("Build", "doing") != ""
        assert move(add("Ship", "todo"), "done").col == "done"
        assert counts().done == 1
        assert len(column("todo")) == 1
        assert move("card-999", "done").error == "no card"
    }
}