Type constraints on all type family instances

Question

I suppose what I want is impossible without Template Haskell but I\'ll ask anyway.

I have an interface for types like Data.Set and Data.IntSet

Answer 1

I like Daniel Wagner's post, but you can't just write:

test :: (HasEfficientSet a) => EfficientSet a
test = empty

You have to write:

test :: (HasEfficientSet a, SetLike (EfficientSet a)) => EfficientSet a
test = empty

But this can be overcome with a constraint synonym:

class SetLike s where 
    type Elem s :: *
    empty :: s
    insert :: Elem s -> s -> s 
    member :: Elem s -> s -> Bool 

class (Elem (EfficientSet a) ~ a) => HasEfficientSet' a where
    type EfficientSet a :: *

type HasEfficientSet a = (HasEfficientSet' a, SetLike (EfficientSet a))


newtype UnitSet = UnitSet Bool
    deriving (Show, Eq, Ord)

instance SetLike UnitSet where
    type Elem UnitSet = ()
    empty = UnitSet False 
    insert () _ = UnitSet True
    member () u = UnitSet True == u

instance HasEfficientSet' () where
    type EfficientSet () = UnitSet


test :: (HasEfficientSet a) => EfficientSet a
test = empty

test1 :: EfficientSet ()
test1 = empty

test2 :: EfficientSet ()
test2 = test

test3 :: UnitSet
test3 = empty

test4 :: UnitSet
test4 = test