Re: [Haskell-cafe] Question about typeclass & constraints

8 Apr 2014

      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
 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
 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
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Re: [Haskell-cafe] Question about typeclass & constraints

Erik Hesselink