[Haskell-beginners] cyclic dependency problem

[Francesco Ariis]

On Thu, Mar 05, 2015 at 03:43:30PM +0200, Konstantin Saveljev wrote:
> I'm having some trouble with cyclic dependency.
> 
> [..]
> 
> 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"

Hello Konstantin. I just tried to feed this to ghci:

    data StateT a b = StateT a b
    data A a = A (StateT MyState (IO a))
    data MyState = MyState { st :: SomeType }
    data SomeType = SomeType { f :: A () }

and it works. I think the deriving clause is causing problems (not
surprisingly); writing the instance out yourself should solve it.