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

[David Kraeutmann]

Apart from a bunch of minor errors, the crux here is that

class Monad m => LogMonad m where
   putReport :: ReportData t1 t2 -> m ()

has locally quantified type variables t1, t2, and thus the `r` in
`putReport r` has type `ReportData a b` while the state type expects
`ReportData x y`.


On 06/28/2018 01:54 AM, Dennis Raddle wrote:
> I created a minimal example of what I'm trying to do --- in fact, I
> think this will be better than what I wrote in the first place --- but
> now I'm baffled by a different error entirely, which is some
> identifiers not in scope. I'm going to post this anyway, because I
> suspect the error is related to what the compiler can infer about my
> instance, which is something I need to understand better.
>
> Once you get past that error, either it will be working (yay!) or
> you'll encounter the error about no instance for MonadState which was
> my original problem.
>
> {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances,
> FlexibleContexts #-}
>
>
> import Control.Monad.State
> import System.Random
>
> data ReportData t1 t2 = 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 :: [ReportData 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})   -- ERROR : theGen not in
> scope
>
> instance LogMonad (MyState t1 t2) where
>   putReport r = modify (\s -> s { theReports = r : theReports s}) --
> ERROR: theReports not in scope
>
>
> On Wed, Jun 27, 2018 at 10:43 PM, Dennis Raddle
> <dennis.raddle at gmail.com <mailto:dennis.raddle at gmail.com>> wrote:
>
>     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
>     <mailto: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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