What is the purpose of interface contracts?

Question

Reading The DAO and DigixDAO source code I notice that in contract systems there are often interface contracts where functions are declared without any body.

For example,

contract TokenInterface {
    function balanceOf(address _owner) constant returns (uint256 balance);
    function transfer(address _to, uint256 _amount) returns (bool success);
}

What is the point of interface contracts? The above contract in The DAO is followed by a contract Token is TokenInterface {} which repeats the exact functions declared in the interface contract except followed by the full function logic. Not sure what the point of this is- it seems redundant.

Answer 1

Interface contracts work like other interface agreements in traditional object oriented languages. In your example, the interface specifies the functions a contract of type TokenInterface must implement to fit into the overall application framework.

This allows you to customize code without having to implement everything from scratch. For example, in the case of TokenInterface you could change/customize the logic for the 2 functions balanceOf and transfer and still use the rest of the framework.

The main point of using interfaces ( or Abstract Contracts ) is to provide a customizable and re-usable approach for your contracts.