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

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 wrote:

On Sun, Sep 15, 2019 at 5:28 PM Hilco Wijbenga 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. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.