[Haskell-cafe] Stuck on design problem

[Nicolas Trangez]

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" <nicolas at 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
> >
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