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

You're not trying to do something impossible but in my experience it's really only very, very rare cases where it's good design to introduce your own typeclasses. What problem are you trying to solve where introducing new typeclasses is the solution? (And FYI these are not higher-kinded types, they're multiparameter type classes.) Tom On Thu, Jun 28, 2018 at 02:15:08AM -0700, Dennis Raddle wrote:

So, does that mean I'm trying to do something impossible?

I'm often not clear on what higher-kinded types are doing, and I'm aware that sometimes I'm asking the compiler to do something that is logically impossible.

Or is there a correct way to do this?

On Wed, Jun 27, 2018 at 11:05 PM, David Kraeutmann wrote:

...

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 mailto:dennis.raddle@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 mailto: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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