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

[Dennis Raddle]

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>
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@
> 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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>
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