[Haskell-cafe] Default "type" in type class

[Erik Hesselink]

You can also define a default type family instance to the class declaration:

class HasToList collection where
  type Requirement collection element :: Constraint
  type Requirement collection element = ()

Regards,

Erik

On Mon, 16 Sep 2019 at 04:02, Hilco Wijbenga <hilco.wijbenga at gmail.com>
wrote:

> On Sun, Sep 15, 2019 at 5:28 PM Hilco Wijbenga <hilco.wijbenga at gmail.com>
> wrote:
> >
> > Hi all,
> >
> > I'm trying to create a "HasToList" type class. It starts off really
> simple:
> >
> > class HasToList collection where
> >         toList ∷ collection element → [element]
> >
> > instance HasToList Set where
> >         toList = Data.Set.toList
> >
> > Unfortunately, this breaks down with Data.EnumSet.EnumSet. So
> >
> > instance HasToList EnumSet where
> >         toList = Data.EnumSet.toList
> >
> > doesn't compile and I could not find a way to add "Enum a =>" to
> > either the instance or "toList". (Well, not without making the
> > compiler upset.)
> >
> > With the aid of ConstraintKinds and TypeFamilies (and not much
> > understanding on my end, obviously) I created
> >
> > class HasToList collection where
> >         type Requirement collection element :: Constraint
> >         toList ∷ Requirement collection element => collection element
> > → [element]
> >
> > instance HasToList Set where
> >         type Requirement Set a = ()
> >         toList = Data.Set.toList
> >
> > instance HasToList EnumSet where
> >         type Requirement EnumSet a = Enum a
> >         toList = Data.EnumSet.toList
> >
> > This works but now I have to add the constraint to every instance even
> > though only a tiny minority actually need it. Is there a way to add a
> > default "type implementation"?
> >
> > I tried the obvious
> >
> > type Requirement collection element :: Constraint = ()
> >
> > and
> >
> > type Requirement collection element :: Constraint
> > type Requirement collection element :: Constraint = ()
> >
> > but neither compiles.
> >
> > Another problem is that the compiler can't compile something like this:
> >
> > isteps ∷ HasToList collection ⇒ collection String → [(Int, String)]
> > isteps steps = Data.List.Index.indexed (toList steps)
> >
> > because it doesn't know which constraint to apply ("Could not deduce:
> > Requirement hasToList String"). That seems fair but also irrelevant at
> > this point.
> >
> > I feel like this is all far too complicated for what I'm trying to
> > accomplish. :-) While I would like to have an answer to the questions
> > above, I guess what I really want is a simpler way to implement
> > HasToList.
> >
> > Cheers,
> > Hilco
>
> This:
>
> class HasToList collection element where
>         toList ∷ collection element -> [element]
>
> instance HasToList Set element where
>         toList = Data.Set.toList
>
> instance Enum element => HasToList EnumSet where
>         toList = Data.EnumSet.toList
>
> compiles fine, of course.
>
> Strange, I thought I had tried that.
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