Yesod uses Persistent for its (you guessed it) persistence layer.
This tool will build in either SQLite or PostgreSQL support for you. If you
want to use a different backend, you'll have to make changes manually.
If you're not sure, stick with SQLite: it has no dependencies.

We also have a new option: a mini project. This is a site with minimal
dependencies. In particular: no database, no authentication.

So, what'll it be? s for sqlite, p for postgresql, m for mini: