FX converter with a rate table in memory and a non-negative rate invariant

currency_converter.cell

// FX converter with a rate table in memory and a non-negative rate
// invariant. Cross rates are derived via a USD base. Demonstrates a
// Map<String,Float> slot, an invariant on stored values, and triangular
// cross-rate arithmetic.

cell FxConverter {
    face {
        signal set_rate(ccy: String, per_usd: Float) -> Map
        signal to_usd(ccy: String, amount: Float) -> Float
        signal convert(from_ccy: String, to_ccy: String, amount: Float) -> Float
        promise all_persistent
    }

    memory {
        // units of CCY per 1 USD; must be strictly sensible (non-negative)
        rates: Map<String, Float> [persistent, consistent]
        invariant rates >= 0.0
    }

    on set_rate(ccy: String, per_usd: Float) {
        let r = try { rates.set(ccy, per_usd) }
        if r.error != () {
            return map("ok", false, "reason", "{r.error}")
        }
        return map("ok", true, "ccy", ccy, "per_usd", per_usd)
    }

    on to_usd(ccy: String, amount: Float) {
        if ccy == "USD" { return amount }
        let rate = rates.get(ccy)
        if rate == () { return 0.0 }
        return amount / rate
    }

    // cross rate via USD: from -> USD -> to
    on convert(from_ccy: String, to_ccy: String, amount: Float) {
        let usd = to_usd(from_ccy, amount)
        if to_ccy == "USD" { return usd }
        let rate = rates.get(to_ccy)
        if rate == () { return 0.0 }
        return usd * rate
    }
}

cell test FxTests {
    rules {
        assert set_rate("EUR", 0.92).ok == true
        assert set_rate("JPY", 156.0).ok == true
        // 100 EUR -> USD = 100/0.92 ≈ 108.70
        assert abs(to_usd("EUR", 100.0) - 108.695652) < 0.001
        // 100 EUR -> JPY via USD
        assert abs(convert("EUR", "JPY", 100.0) - (100.0 / 0.92 * 156.0)) < 0.001
        // USD passthrough
        assert abs(to_usd("USD", 50.0) - 50.0) < 0.0001
        // a negative rate is rejected by the invariant
        assert set_rate("BAD", -1.0).ok == false
        assert_fails rates.set("X", -0.5)
    }
}