By using the type system. You define your type constraints at the module interface point and when you try to link the third-party module into that interface the compiler ensures that the constraints are satisfied. Same thing the compiler is already doing in simpler cases. If you specify that a third-party library function must return an integer, the compiler will ensure that function won't unexpectedly return a string. Just like that, except the type system is expanded to enable describing more complex behaviours.