Trade settlement as a sum type

trade_settlement.cell

// Trade settlement as a sum type. A settlement attempt yields one of three
// tagged variants; `match` over them is exhaustive (drop an arm and
// `soma check` refuses). Demonstrates struct / tuple / unit variants and
// exhaustive matching for money outcomes.

cell type Settlement {
    variants {
        Settled { trade_id: String, net: Int }   // struct variant
        Failed(String)                            // tuple variant
        Pending                                   // unit variant
    }
}

cell TradeSettlement {
    face {
        signal settle(trade_id: String, qty: Int, price: Int, cash: Int) -> Map
        signal describe(r: Map) -> String
        signal is_terminal(r: Map) -> Bool
    }

    // net = qty*price; fails if the account can't cover it; pending if qty 0
    on settle(trade_id: String, qty: Int, price: Int, cash: Int) {
        if qty == 0 { return Pending }
        let net = qty * price
        if net > cash { return Failed("insufficient cash") }
        return Settled { trade_id: trade_id, net: net }
    }

    on describe(r: Map) {
        return match r {
            Settled { trade_id, net } -> "settled {trade_id} for {net}"
            Failed(reason)            -> "failed: {reason}"
            Pending                   -> "pending"
        }
    }

    on is_terminal(r: Map) {
        return match r {
            Settled { trade_id, net } -> true
            Failed(reason)            -> true
            Pending                   -> false
        }
    }
}

cell test SettlementTests {
    rules {
        // a funded trade settles
        assert describe(settle("t1", 10, 50, 1000)) == "settled t1 for 500"
        assert is_terminal(settle("t1", 10, 50, 1000)) == true
        // underfunded → failed
        assert describe(settle("t2", 10, 50, 100)) == "failed: insufficient cash"
        // zero qty → pending, not terminal
        assert describe(settle("t3", 0, 50, 1000)) == "pending"
        assert is_terminal(settle("t3", 0, 50, 1000)) == false
    }
}