[Haskell-cafe] Question about typeclass & constraints

[jean-christophe mincke]

Thank you

The problem here is that I cannot modify the original PP class (In fact it,
in the real life, it is PersistStore).

So I guess I am left with PP' solution.

Regards

J-C

On Tue, Apr 8, 2014 at 10:53 AM, Erik Hesselink <hesselink at gmail.com> wrote:

> I've run into problems like this as well. Often you can redesign a bit
> to fix things, although it depends on the application how to do that.
> Two other technical options in addition to your PP':
>
> You can add a constraint to the 'create' function. It depends on the
> actual semantics if this makes sense, I guess:
>
> class PP2 m where
>   create :: CB a => a -> m a
>
> You can use ConstraintKinds [1] to add a constraint to create
> depending on the type 'm':
>
> type family C m a :: Constraint
>
> class PP3 m where
>   create :: C m a => a -> m a
>
> This way you can choose if you want to add a constraint and which one,
> depending on the 'm'.
>
> If you can solve it without any of these things, I'd probably prefer
> that personally, though. As Tom also said, complicated type class
> constructions often make your program more difficult to understand and
> maintain.
>
> Erik
>
> [1]
> http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/constraint-kind.html
>
> On Tue, Apr 8, 2014 at 10:17 AM, jean-christophe mincke
> <jeanchristophe.mincke at gmail.com> wrote:
> > Tom,
> >
> > Yes of course it is simplified for clarity.
> >
> > Here is a modified version where fb does something (a bit more usefull)
> >
> >
> > class PP m where
> >     create :: a -> m a
> >
> > data A a = A a
> > instance PP A where
> >     create a = A a
> >
> > class CB a where
> >     fb :: a -> a
> >
> > data B m a = B (m a)
> > instance (PP m) => PP (B m) where
> >     create a =  let a' = fb a
> >                 in B (create a')
> >
> > class PP' m a where
> >     create' :: a -> m a
> >
> > instance (PP m) => PP' m a where
> >     create' = create
> >
> > instance (PP m, CB a) => PP' (B m) a where
> >     create' a = let a' = fb a
> >                 in B (create a')
> >
> > Actually I ran into that problem when trying to add a kind of rule engine
> > layer above the Persistent typeclass. Given the complexity of these
> > typeclass, I think it is more practical to reason about a simpler form of
> > the same problem.
> >
> > Thanks
> >
> > J-C
> >
> > On Tue, Apr 8, 2014 at 9:42 AM, Tom Ellis
> > <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:
> >>
> >> On Tue, Apr 08, 2014 at 09:25:17AM +0200, jean-christophe mincke wrote:
> >> > instance (PP m) => PP (B m) where
> >> >     create a =  let _ = fb a
> >> >                 in B (create a)
> >>
> >> Your use of 'fb' here is baffling.  Am I right in thinking you have
> tried
> >> to
> >> simplify your problem for clarity?  If so I think you have simplified
> too
> >> far!
> >>
> >> Could you give an example where the use of 'fb' actually matters?
> >>
> >> Tom
> >> _______________________________________________
> >> Haskell-Cafe mailing list
> >> Haskell-Cafe at haskell.org
> >> http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
> >
> >
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>
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