(redirected to haskell-cafe) mfix is "value" recursion, not "effect" recursion. It allows you to "tie-the-knot" with data being constructed recursively even in a monadic context. When you are using the Writer monad like this, the bind operation between statements in a "do" construct is just ++. "simulation" in your message is

simulation:: Writer [String] () simulation = mdo a <- server cr cr <- client $ take 10 a return ()

This is really just the following:

simulation :: Writer [String] () simulation = Writer result where ( a, a_out ) = runWriter (server cr) ( cr, cr_out ) = runWriter (client $ take 10 a) result = ( (), a_out ++ cr_out )

With "mdo" you are allowed to have the values refer to each other; the right hand side of "( a, a_out ) = ..." can refer to "cr" and vice versa. But there's no way to follow the thread of computation between the server and the client with this style. -- ryan