Inventory: a list of Item records with in-place quantity mutation, restock/sell operations, and aggregate reporting via pipelines

inventory.cell

// Inventory: a list of Item records with in-place quantity mutation,
// restock/sell operations, and aggregate reporting via pipelines.
// Demonstrates building a list of records and mutating xs[i].field.

cell Inventory {
    face {
        signal stock() -> List
        signal value_of(sku: String) -> Int
        signal total_value() -> Int
        signal low_stock(threshold: Int) -> List
        signal sell(sku: String, n: Int) -> Map
    }

    on stock() {
        return list(
            Item { sku: "WIDGET", qty: 12, price: 4 },
            Item { sku: "GADGET", qty: 3,  price: 25 },
            Item { sku: "GIZMO",  qty: 40, price: 1 }
        )
    }

    on value_of(sku: String) {
        let it = stock() |> find(i => i.sku == sku)
        if it == () { return 0 }
        return it.qty * it.price
    }

    on total_value() {
        return sum(stock() |> map(i => i.qty * i.price))
    }

    on low_stock(threshold: Int) {
        return stock() |> filter(i => i.qty < threshold) |> pluck("sku")
    }

    on sell(sku: String, n: Int) {
        let items = stock()
        let idx = -1
        for e in enumerate(items) {
            if e.value.sku == sku { idx = e.index }
        }
        if idx == -1 { return map("error", "no such sku") }
        if items[idx].qty < n { return map("error", "insufficient stock") }
        items[idx].qty = items[idx].qty - n      // mutate the record in the list
        return map("sku", sku, "remaining", items[idx].qty)
    }
}

cell test InventoryTest {
    rules {
        assert value_of("GADGET") == 75
        assert total_value() == 163
        assert len(low_stock(10)) == 1
        assert low_stock(10)[0] == "GADGET"
        assert sell("WIDGET", 5).remaining == 7
        assert sell("GADGET", 99).error == "insufficient stock"
    }
}