Design by contract

solidity这门语言设计思路是什么？
什么是COP？

COP

面向条件的编程（COP）是面向合约编程的一个子域，作为一种面向函数和命令式编程的混合模式。COP解决了这个问题，通过需要程序员显示地枚举所有的条件。逻辑变得扁平，没有条件的状态变化。条件片段可以被正确的文档化，复用，可以根据需求和实现来推断。重要的是，COP在编程中把预先条件当作为一等公民。这样的模式规范能保证合约的安全。

post-condition

contract PostCheck {

    uint public data = 0;

    // Check that the 'data' field was set to the value of '_data'.
    modifier data_is_valid(uint _data) {
        _
        if (_data != data)
            throw;
    }

    function setData(uint _data) data_is_valid(_data) {
        data = _data;
    }

}

pre- and post-conditions

“_”

contract PrePostCheck {

    uint public data = 0;

    // Check that the input '_data' value is not the same as the value
    // already stored in 'data'.
    modifier data_is_valid(uint _data) {
        if (_data == data)
           throw;
        _
    }

    // Check that the 'data' field was set to the value of '_data'.
    modifier data_was_updated(uint _data) {
        _
        if (_data != data)
            throw;
    }

    function setData(uint _data) data_is_valid(_data) data_was_updated(_data) {
        data = _data;
    }

}

FEATURES

• 函数主体没有条件判断

例子：

contract Token {
    // The balance of everyone
    mapping (address => uint) public balances;
    // Constructor - we're a millionaire!
    function Token() {
        balances[msg.sender] = 1000000;
    }
    // Transfer `_amount` tokens of ours to `_dest`.
    function transfer(uint _amount, address _dest) {
        balances[msg.sender] -= _amount;
        balances[_dest] += _amount;
    }
}

改进后：

function transfer(uint _amount, address _dest) {
    if (balances[msg.sender] < _amount)
        return;
    balances[msg.sender] -= _amount;
    balances[_dest] += _amount;
}

COP的风格

modifier only_with_at_least(uint x) {
    if (balances[msg.sender] >= x) _;
}

function transfer(uint _amount, address _dest)
only_with_at_least(_amount) {
    balances[msg.sender] -= _amount;
    balances[_dest] += _amount;
}

扩展阅读：

• Condition-Orientated Programming