All, Since I'm stuck on a coding problem and can't figure out how to proceed, I decided to call for help. I'm unable to get some code to typecheck, so maybe I'm missing something obvious on the implementation side, or more likely the design is completely wrong. Here's the idea: I have some code which uses some log of "Entry a" values (for some polymorphic type 'a'), and provides actions which output "Command a" values which should be interpreted by some wrapper code, e.g. adding some new entries to the log. These actions are implemented in a custom WriterT transformer (in this demo code, the original is more complex) to output Commands, while the inner monad should give access to the log entries (this could be in-memory like in the example, or stored on disk so using IO). You can find a trimmed-down version at https://gist.github.com/NicolasT/4230251f4f87f110d197 This doesn't type-check, and I'm not sure how to proceed. Am I taking a wrong approach? Do I need a different design? Thanks, Nicolas