[Haskell-beginners] How to design functions so they can be part of other larger systems?

[Costello, Roger L.]

Hi Folks,

I read this statement by Tim Berners-Lee [1]:

    It is not only necessary to make sure your own system 
    is designed to be made of modular parts. It is also 
    necessary to realize that your own system, no matter 
    how big and wonderful it seems now, should always be 
    designed to be a part of another larger system.

Recently I have been working hard to learn how to better modularize. But now TBL says that I must do more - I must not only modularize well, but I must also build the modules so that they can be part of other larger systems. 

How do I design modules so that they may be part of other larger systems? Are there any articles that give guidelines on how to do this? What are your thoughts on how to do this?

/Roger

[1] http://www.w3.org/DesignIssues/Principles.html