This post is part of the F# Advent Calendar in English 2015. Thanks to Sergey Tihon for organizing and letting me be a part of it this year! Some short time after implementing MembershipProvider for the first time I realized that simply relying on abstractions to make your code extendable wasn't enough. To embrace and encourage change I had to also make those abstractions as small as possible. While the class makes it possible to change implementations, its 27 abstract methods all but guarantee you'[...]