Monthly budget tracker with a spend cap invariant

budget_tracker.cell

// Monthly budget tracker with a spend cap invariant. Total spend per
// category must never exceed its allotted budget; an over-budget write
// is rejected by memory. Shows group accounting and a remaining-budget query.

cell BudgetTracker {
    face {
        signal spend(category: String, amount: Int) -> Map
        signal spent(category: String) -> Int
        signal remaining(category: String) -> Int
        promise all_persistent
    }

    memory {
        // cents spent per category; capped so no category overspends its 50_000 cap
        outlays: Map<String, Int> [persistent, consistent]
        invariant outlays >= 0 && outlays <= 50000
    }

    on spent(category: String) {
        let s = outlays.get(category)
        if s == () { return 0 }
        return s
    }

    on remaining(category: String) {
        return 50000 - spent(category)
    }

    on spend(category: String, amount: Int) {
        let cur = spent(category)
        let r = try { outlays.set(category, cur + amount) }
        if r.error != () {
            return map("ok", false, "reason", "over budget", "remaining", remaining(category))
        }
        return map("ok", true, "spent", cur + amount, "remaining", 50000 - (cur + amount))
    }
}

cell test BudgetTests {
    rules {
        assert spend("groceries", 12000).ok == true
        assert spent("groceries") == 12000
        assert remaining("groceries") == 38000
        // a charge that blows the cap is refused
        assert spend("groceries", 40000).ok == false
        // and nothing was recorded
        assert spent("groceries") == 12000
        // hitting the cap exactly is fine
        assert spend("rent", 50000).ok == true
        assert remaining("rent") == 0
        assert_fails outlays.set("x", 60000)
    }
}