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

[Dennis Raddle]

okay, will do. It has a lot of details that aren't really necessary to ask
the question, but now that I think about it, all that's required of you is
to download and try to compile it.

D

On Wed, Jun 27, 2018 at 10:41 PM, Tom Ellis <
tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:

> 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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