Allow mapping storage parameters for library functions

[chriseth]

Since library functions allow storage struct parameters, storage mapping parameters should also be allowed. This would enable us to modularize some things:

library Balances {
    function send(mapping(address => uint256) storage balances, address from, address to, uint amount) internal {
        require(balances[from] >= amount);
        require(balances[to] + amount >= balances[to]);
        balances[from] -= amount;
        balances[to] += amount;
    }
}

contract Token {
    mapping(address => uint256) balances;
    using Balances for *;
    mapping(address => mapping (address => uint256)) allowed;

    event Transfer(address from, address to, uint amount);
    event Approval(address owner, address spender, uint amount);

    function balanceOf(address tokenOwner) public constant returns (uint balance) {
        return balances[tokenOwner];
    }
    function transfer(address to, uint amount) public returns (bool success) {
        balances.send(msg.sender, to, amount);
        emit Transfer(msg.sender, to, amount);
        return true;

    }

    function transferFrom(address from, address to, uint amount) public returns (bool success) {
        require(allowed[from][msg.sender] >= amount);
        allowed[from][msg.sender] -= amount;
        balances.send(from, to, amount);
        emit Transfer(from, to, amount);
        return true;
    }

    function approve(address spender, uint tokens) public returns (bool success) {
        require(allowed[msg.sender][spender] == 0, "");
        allowed[msg.sender][spender] = tokens;
        emit Approval(msg.sender, spender, tokens);
        return true;
    }
}

[axic]

I think this a good idea, extends the usability of libraries.

[ekpyron]

Should this only be the case for internal library functions?

Currently the following does not work either, and I don't see a good reason, why not:

contract C {
  function get(mapping(uint => uint) storage m, uint key) internal view returns (uint) {
    return m[key];
  }
}

This yields TypeError: Type is required to live outside storage.

Is there any good reason to require a type to live outside storage for the arguments of internal functions at all?

[ekpyron]

Somewhat related: #4670

[chriseth]

This requires defining a type string for mappings for the function selector (as we did with strings).

[chriseth]

(extended scope to public library functions)

[chriseth]

Moved to 0.5.1