Chapter: Basic modules

[rauschma]

[nielsbom]

In the intro to "Abstract types: hiding internals" there's the sentence: "You can use interfaces to hide the details of types."

I think it's good to start with a (non-exhaustive) overview of advantages to hiding modules' internals.

• not unnecessarily exposing implementation details
• decoupling internal implementation from module signature (somewhat overlapping with 1)
• module signatures being more helpful

I think it may help being a little bit more explicit here. Or including an example of not-hiding-module-internals leading to potential problems.