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

[Hilco Wijbenga]

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