[Haskell-beginners] cyclic dependency problem

[Konstantin Saveljev]

Would this be a correct implementation?:

{-# LANGUAGE MultiParamTypeClasses #-}     -- for some reason compiler says
to use it ?

import qualified Control.Monad.State.Lazy as Lazy

instance MonadState MyState X where
    get = X Lazy.get
    put s = X $ Lazy.put s


Konstantin

On Thu, Mar 5, 2015 at 4:37 PM, Konstantin Saveljev <
konstantin.saveljev at gmail.com> wrote:

> 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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