Solidity: Call an unimplemented function?

Question

I am trying to understand some solidity code, and I see what appears to be the calling of an unimplemented function.

There is an interface defined as:

interface IOneInchCaller is ISafeERC20Extension, IGasDiscountExtension {
    struct CallDescription {
        uint256 targetWithMandatory;
        uint256 gasLimit;
        uint256 value;
        bytes data;
    }

    function makeCall(CallDescription memory desc) external;
    function makeCalls(CallDescription[] memory desc) external payable;
}

And then elsewhere in the codebase I see a call to this function:

function swap(
        IOneInchCaller caller,
        SwapDescription calldata desc,
        IOneInchCaller.CallDescription[] calldata calls
    )
        external
        payable
        whenNotPaused
        returns (uint256 returnAmount)
    {

        caller.makeCalls{value: msg.value}(calls);

It is calling the IOneInchCaller.makeCalls function, but nowhere is the function implementation defined. What is going on here?

---

By the way, the code is pulled from here.

Answer 1

For making calls to an external contract you don't need the implementation, having the interface is enough.

As example all ERC20 compliant tokens use the same interface defined in EIP-20. Any contract can interact with them through the same interface without having to worry about particular implementations details.