I am all for increasing the M in the MVC of Yesod. In a simple REST app where resources closely map to the data modeling, this can be less of an issue. Basically you need the M once you would have duplicate model code between any 2 requests. Chris is advocating a fairly strict separation. I think it can still be
simpler to put a read query in a controller. Once that query (or a part
of it) is needed elsewhere it should definitely be named and reused.
Updates do need to maintain invariants and thus should generally stay out of the
controller. I would like to come up with a good system for maintaining
invariants and propagating violations to forms, and other useful systems
for doing more than just taking the query out of the controller.
Let us know if there is anything stopping you from separating out code into a Models directory right now.
Greg Weber