Position-limit risk gate. Each symbol's net position must stay within

position_limit.cell

// Position-limit risk gate. Each symbol's net position must stay within
// [-MAX, +MAX]. The memory invariant enforces it on every write, so an
// order that would breach the limit is refused and the book is unchanged.

cell PositionBook {
    face {
        signal buy(symbol: String, qty: Int) -> Map
        signal sell(symbol: String, qty: Int) -> Map
        signal position(symbol: String) -> Int
        promise all_persistent
    }

    memory {
        positions: Map<String, Int> [persistent, consistent]
        // net position per symbol bounded to +/- 100 lots
        invariant positions >= -100 && positions <= 100
    }

    on position(symbol: String) {
        let p = positions.get(symbol)
        if p == () { return 0 }
        return p
    }

    on buy(symbol: String, qty: Int) {
        let cur = position(symbol)
        let r = try { positions.set(symbol, cur + qty) }
        if r.error != () {
            return map("filled", false, "reason", "long limit", "position", cur)
        }
        return map("filled", true, "position", cur + qty)
    }

    on sell(symbol: String, qty: Int) {
        let cur = position(symbol)
        let r = try { positions.set(symbol, cur - qty) }
        if r.error != () {
            return map("filled", false, "reason", "short limit", "position", cur)
        }
        return map("filled", true, "position", cur - qty)
    }
}

cell test PositionTests {
    rules {
        assert buy("ES", 60).filled == true
        assert position("ES") == 60
        // adding 60 more would hit 120 > 100 → refused
        assert buy("ES", 60).filled == false
        assert position("ES") == 60
        // can still trade within bounds
        assert buy("ES", 40).position == 100
        // shorting past -100 is refused too
        assert sell("NQ", 200).filled == false
        // direct breach is a runtime error
        assert_fails positions.set("X", 500)
    }
}