Miguel, On Mon, 2013-05-20 at 16:38 +0400, Miguel Mitrofanov wrote: > :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 ()).
Thanks for the pointer. I tried using an explicit type-annotation myself before as well, but that failed, and I found the root-cause: in my original code, the type-signature of runTransitionT was all wrong, instead of runTransitionT :: TransitionT a m a -> m (a, [Command a]) this was intended to be runTransitionT :: TransitionT a m r -> m (r, [Command a]) After making this change, and using an explicit type-signature for the empty IntMap as you suggested, things work as expected, see https://gist.github.com/NicolasT/4230251f4f87f110d197/revisions In my original code, this didn't help out somehow (some other things at play there as well), but another suggestion on this list did. Thanks for the help! Nicolas > > 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