[Haskell-beginners] cyclic dependency problem

[Konstantin Saveljev]

I believe Francesco has a point here. I do have "deriving (MonadState
MyState)" which is probably causing the problem.

Unfortunately I'm not capable of writing the instance myself. Maybe someone
could help me?

My full declaration looks something like this:

    newtype X a = X (StateT MyState (ExceptT String IO) a)
                    deriving (Functor, Applicative, Monad, MonadIO,
MonadError String, MonadState MyState)

If I was to remove "MonadState MyState" from the "deriving" section, how
would one implement it?


Konstantin

On Thu, Mar 5, 2015 at 4:25 PM, <amindfv at gmail.com> wrote:

>
>
> El Mar 5, 2015, a las 8:43, Konstantin Saveljev <
> konstantin.saveljev at gmail.com> escribió:
>
> > Hello,
> >
> > I'm having some trouble with cyclic dependency.
> >
> > My simplified version of hierarchy:
> >
> >     module A where
> >
> >     import MyState
> >
> >     data A a = A (StateT MyState IO a) deriving (...)
> >
> > Now there is a module MyState:
> >
> >     module MyState where
> >
> >     import SomeType
> >
> >     data MyState = MyState { st :: SomeType, ... }
> >
> > Finally the module that introduces cyclic dependency:
> >
> >     module SomeType where
> >
> >     import A
> >
> >     data SomeType = SomeType { f :: A (), ... }
> >
> > I have been suggested to move the types into one module and then import
> it wherever needed. But the problem is that even if I put them into one
> file those three data types still form a cyclic dependency and compiler
> throws errors saying "i don't know about this"
> >
> > So if I have a single module:
> >
> >     data A a = A (StateT MyState IO a) deriving (...)
> >     data MyState = MyState { st :: SomeType, ... }
> >     data SomeType = SomeType { f :: A (), ... }
> >
> > With this setup the compiler is gonna tell us it doesn't know about
> MyState. And no matter how we shuffle these types within a file we are
> still getting some "unknowns"
> >
>
> Did you try it? It should work fine -- haskell doesn't care about the
> order of data declarations within a file.
>
> Tom
>
>
> > How would one approach such a problem?
> >
> > Best regards,
> > Konstantin
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