[Haskell-cafe] need help understanding how to specify constraints on monads
Tom Ellis
tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk
Thu Jun 28 09:33:54 UTC 2018
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 <kane at kane.cx> 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
> > > <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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