Solidity mapping access with parentheses instead of square brackets?

Question

I am wondering about a particular statement in the Moloch v2.1 smart contract code:

https://github.com/Moloch-Mystics/Molochv2.1/blob/main/Flat_Moloch_v2.1.sol

In the Moloch contract, there is a contract level mapping called members

mapping(address => Member) public members;

As expected in almost every line of code that reads or writes to this mapping, square brackets are used. However there is one line of code involving a tuple that uses parentheses instead, wrapping a pair of them around msg.sender:

(,,,bool exists,,) = moloch.members(msg.sender);

Why isn't this line of code using square brackets after the moloch.members reference, like this?:

(,,,bool exists,,) = moloch.members[msg.sender];

I don't understand why that line is using parentheses and what kind of different behavior might result from this. Can someone explain this to me?

For reference purposes, this occurs inside the function below, which is a member of the MolochSummoner contract, which inherits from CloneFactory:

function registerDao(
    address _daoAdress,
    string memory _daoTitle,
    string memory _http,
    uint _version
  ) public returns (bool) {
      
  moloch = Moloch(_daoAdress);
  (,,,bool exists,,) = moloch.members(msg.sender);

  require(exists == true, "must be a member");
  require(daos[_daoAdress] == false, "dao metadata already registered");

  daos[_daoAdress] = true;
  
  daoIdx = daoIdx + 1;
  emit Register(daoIdx, _daoAdress, _daoTitle, _http, _version);
  return true;
  
}

Answer 1

You can only use brackets if you are accessing the variable inside the same contract.

Outside the contract, when accessing a public variable you are calling a getter function members(). Solidity automatically generates this function for you. There is no get prefix.

More about Solidity getter functions here.

This is a bit confusing and somewhat bad design decision, so I apologize.