Hi, I'm trying to implement mfix for a monad that is nested inside another monad. I came up with the following logic for how to implement this, but I suspect there are some things I'm missing. My conclusion is that I want interpret (MFix f) = mfix (interpret.f) Does this seem right? Has this situation been discussed somewhere? -BenRI P.S. Here's what I mean by the monad being nested in another monad. Let's say that the monad M2 has interpreter i2, with type i2 :: M2 a -> M1 a and then M1 is the other monad, and has interpreter i1: i1 :: M1a -> a I suppose that the nesting is really a nesting of interpreters. P.P.S. I came up with some equational reasoning for how to treat mfix in the i2 interpreter. (a) For some interpreters `i`, it makes sense to require i (mfix f) = let x = (i . f) x in x (b) I want the composition of interpreters (i1.i2) to act like (a) above: (i1 . i2) (mfix f) = let x = (i1 . i2 . f) x in x (c) Rearranging, and then substituting using (a): i1 (i2 (mfix f)) = let x = i1 . (i2 . f) x in x = i1 (mfix (i2 . f)) (d) Therefore, we could set i2 (mfix f) = mfix (i2 . f) I'm probably going to make a `data` declaration for M2 so I could actually write i2 (Mfix f) = mfix (i2 . f) -BenRI