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Update Balance

Textio needs a new way to update user's account balance.

Assignment

Implement the updateBalance function. It should take a customer pointer and a transaction, and return an error. Depending on the transactionType, it should either add or subtract the amount from the customer's balance. If the customer does not have enough money, it should return the error insufficient funds and leave the balance unchanged. If the transactionType isn't a withdrawal or deposit, it should return the error unknown transaction type. Otherwise, it should process the transaction and return nil.

alice := customer{id: 1, balance: 100.0}
deposit := transaction{customerID: 1, amount: 50, transactionType: transactionDeposit}

updateBalance(&alice, deposit)
// id: 1 balance: 150