Happstack is a great library that has a rich ecosystem of tools and libraries. It has a good development environment. It's not as user friendly as some other libraries, but the developers are very responsive. It's a great library to use for web application development. The documentation is a bit sparse, so it can be a bit difficult to get answers to questions. I like using Happstack because it is very powerful and has a rich ecosystem of tools and libraries. It also has a good development environment. We are building a web application using Happstack.