In my opinion, such a class should usually have more than one parameter. In the case of Set, I think it makes more sense to use a value type than a constraint type.
class e ~ Elem s => SetC e s where
type Elem s :: Type
type Elem (_ a) = a
singleton :: e -> s
elem :: e -> s -> Bool
union :: s -> s -> s
...
instance Ord a => SetC a (S.Set a) where
singleton = S.singleton
...
instance a ~ Int => SetC a IntSet where
type Elem IntSet = Int
...
For maps, you can do something similar:
class k ~ Key m => MapC k m where
type Key m :: Type
type Key (_ k) = k
lookup :: k -> m a -> Maybe a
...
instance Ord k => MapC k (M.Map k) where
lookup = M.lookup
....
instance k ~ Int => MapC k IM.IntMap where
type Key IntMap = Int
lookup = IM.lookup
If you like, you can add some constraints, like Traversable m. If you want to use MFoldable for sets, you can use its Element type family instead of Elem.
Dear Cafe,
we have Data.Set, Data.IntSet, Data.HashSet,
and they all have similar API, where the only difference
is the constraint on the elements. (Same thing for maps.)
Can we unify this as follows:
{-# language ConstraintKinds, TypeFamilies #-}
class SetC s where
type Con s :: * -> Constraint
singleton :: (Con s a) => a -> s a
foldMap :: (Con s a, Monoid m) => (a -> m) -> s a -> m
...
Then for Data.Set, we write
instance SetC S.Set where type Con S.Set = Ord ; ...
It seems to work, and it allows me to write polymorphic code,
and switch implementations from the top.
Full source:
https://gitlab.imn.htwk-leipzig.de/waldmann/pure-matchbox/tree/master/src/Data/Set
Example use case (switch implementation):
https://gitlab.imn.htwk-leipzig.de/waldmann/pure-matchbox/blob/master/src/Matchbox/Tiling/Working.hs#L48
Still, there are some clumsy corners in this code, perhaps you can help:
* for instance SetC HashSet, there are two constraints. I want to write
type Con HashSet = \ e -> (Hashable e, Eq, e)
but this does not work (there is no "type lambda"?)
* for maps, I want to write
class (forall k . Foldable m k) => MapC m
but this seems impossible now (This is would work
with -XQuantifiedConstraints ?)
* in some other code using the same idea (the class exports the
constraint), I had an instance where the constraint was empty.
Again, I cannot write type Con Foo = \ s -> ()
- J.W.
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