The constructor should be payable if you send value

Question

I have written a contract as follows:

pragma solidity ^0.4.0;

/*
Simple contract that mediates disputes using a trusted arbiter
*/
contract Taxi {

    enum State {IDLE, FINDING_DRIVER, AWAITING_PAYMENT, AWAITING_DESTINATION, COMPLETE}
    State public currentState;
    mapping (address => uint) public balances;
    bytes32 STRING_COMPLETE = "Complete";
    bytes32 STRING_PARTIAL = "Partial";

    modifier customerOnly() { require(msg.sender == customer); _; }
    modifier driverOnly() { require(msg.sender == driver); _; }
    modifier arbiterOnly() { require(msg.sender == arbiter); _; }
    modifier inState(State expectedState) { require(currentState == expectedState); _; }

    address public customer;
    address public driver;
    address public arbiter;

    function Taxi(address _customer, address _driver, address _arbiter) public {
        customer = _customer;
        driver = _driver;
        arbiter = _arbiter;
    }

    function assignDriver() customerOnly inState(State.IDLE) public {
        //arbiter.transfer(this.balance);
        currentState = State.FINDING_DRIVER;
    }

    function confirmAmount(uint amount) driverOnly inState(State.FINDING_DRIVER) public returns (bool ) {
        if(amount < 0) {
            amount = 20;
        }
        //driver has set this amount and user has accepted this fare, return true if customer has sufficient balance
        if(balances[customer] < amount) {
            return false;
        }
        currentState = State.AWAITING_DESTINATION;
        balances[arbiter] += amount;
        balances[customer] -= amount;
        return true;
    }
}

First I execute the assignDriver function which runs correctly. Later, when I execute the confirmAmount function, I get the error:

transact to Taxi.confirmAmount errored: VM error: revert.
revert The transaction has been reverted to the initial state.
Note: The constructor should be payable if you send value.  Debug the transaction to get more information.

I tried to add the payable keyword to the constructor, but still I got the same error. What can be the problem?

Answer 1

It is unclear what the comfirmAmount does:

function confirmAmount(uint amount) driverOnly inState(State.FINDING_DRIVER) public returns (bool ) {
        if(amount < 0) {
            amount = 20;
        }

This will never enter the if as numbers below zero are not possible in solidity.

//driver has set this amount and user has accepted this fare, return true if customer has sufficient balance
if(balances[customer] < amount) {
    return false;
}
currentState = State.AWAITING_DESTINATION;
balances[arbiter] += amount;
    balances[customer] -= amount;
    return true;
}

The balances are never changed for custumer, there is no function that does that since the beginning so this should always return false.

But none of the issues above will revert. The only possibility is that the transaction to execute confirmAmount is not done by the account "driver" and the modifier throws.