:t runMemLog (runTransitionT $ demo 1) runMemLog (runTransitionT $ demo 1) :: MonadLog (MemLog a) () => Log a -> ((), [Command ()])
That means, that "foo", if you manage to compile it, would have type MonadLog (MemLog a) () => ((), [Command ()]). That means that in each call for foo it would be used as something of the type ((), [Command ()]), therefore giving no indication of which "a" should be used. The fact that there is only one instance of MonadLog (MemLog a) () - namely, the one with a = () - doesn't matter, as there could be others in the future. That's why GHC is complaining. So, you have to provide such indication by youself, replacing IntMap.empty with (IntMap.empty :: Log ()). 20.05.2013, 15:36, "Nicolas Trangez" <nico...@incubaid.com>: > 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 > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
