[Haskell-cafe] need help understanding how to specify constraints on monads

[Tom Ellis]

Your sample code has a few bugs which make it not compile, for example the
following is not valid syntax

    data MyStateData t1 t2 = MyStateData t1 t2
      { theGen :: StdGen
      , theReports :: [StepReport t1 t2]
      }

and you use "StepReport" when I think you mean "ReportData".  Could you post
a version which is completely working besides the error you are trying to
solve? Otherwise it's rather hard to help.

Tom


On Wed, Jun 27, 2018 at 10:22:46PM -0700, Dennis Raddle wrote:
> I'm writing a program with several functions, some of which depend on
> certain fields in a state monad, others of which depend on others, but no
> routine needs all the fields.
> 
> So I thought I would declare a two classes, one for each type of data need
> that a function has:
> 
> -- as an aside, here's an example of data which is parameterized by two
> types.
> 
> data ReportData t1 t2 = ...
> 
> -- this is rolling my own state monad with a random generator
> class Monad m => RandMonad m where
>    getGen :: m StdGen
>    putGen :: StdGen -> ()
> 
> -- this is a class of state monad which logs ReportData:
> 
> class Monad m => LogMonad m where
>    putReport :: ReportData t1 t2 -> m ()
> 
> For a particular use case, I declare a type of State monad:
> 
> data MyStateData t1 t2 = MyStateData t1 t2
>   { theGen :: StdGen
>   , theReports :: [StepReport t1 t2]
>   }
> 
> type MyState t1 t2 = State (MyStateData t1 t2)
> 
> And I try to define my instances:
> 
> instance RandMonad (MyState t1 t2) where
>   getGen = gets theGen
>   putGen g = modify (\s -> s { theGen = g})
> 
> instance LogMonad (MyState t1 t2) where
>   putReport r = modify (\s -> s { theReports = r : theReports s})
> 
> I get an error on the LogMonad instance, saying that there's no instance
> for (MonadState (MyState t1 t2) (StateT (MyState t1 t2) Identity))
> 
> I guess I don't really understand typeclasses once you start using higher
> kinded types, so please enlighten me. Any reading on this subject would be
> helpful, too.

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