contract reference another contract, what ABI information needed?

Question

To reference another contract, the ABI is easily included like this. What information needs to be there? The function name, I guess, what about accessability such as public/private/internal/external/pure/view, or if it returns a type and what it returns? Asking for what is needed, not so much what looks good or makes it more human readable.

contract A { function foo() public returns (uint) {} }

contract B {

    A a = A();

Answer 1

You just need an interface for the contract that you want to reference. Just add the functions prototype and you're good to go.

Example

contract ERC20 {
  function allowance(address owner, address spender) constant returns (uint);
  function transferFrom(address from, address to, uint value);
  function approve(address spender, uint value);
  event Approval(address indexed owner, address indexed spender, uint value);
}

contract test {
    ERC20 token = ERC20(address); // to reference 
    ERC20 token = new ERC20(); //creates new contract

    token.transferFrom(from, to, value); //call the transferFrom function of the reference contract or newly created one if thats the case.
}

Answer 2

In order to construct proper call code, Solidity need the following information about the function being called:

1. Name
2. Argument types
3. Return types

Call code generated by Solidity basically does no following:

1. Packs 4-byte function selector (derived from function name and argument types)
2. Pack arguments (needs to know argument types here)
3. Parse returned value (needs to know return types here)