はてなブックマーク

/// A state variable to store the balance of each account var balances: str -> int pure val ADDRESSES = Set("alice", "bob", "charlie") action withdraw(account, amount) = { // Decrement balance of account by amount // Whoops, we forgot to check for enough balance balances' = balances.setBy(account, curr => curr - amount) } // ... /// Invariant: Account balances should never be negative val no_negat