[Haskell-cafe] Closed classes

[Imants Cekusins]

David, thank you very much for the examples. They are very clear.
Would it be difficult to post the first one (working in current setup)
on Wiki?

either https://wiki.haskell.org/GHC/Type_families
or https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/type-families.html
?

expanding your and Clinton's suggestions, here is one more version.
Not sure if this may work however this may be intuitive to C#, Java
users:



-- Names for the instances
data InstanceChoice = ChooseFirst | ChooseSecond | IgnoreBoth


-- Determine which instance should be used
type family Choose a b where
  Choose Int y = 'ChooseFirst
  Choose x Int = 'ChooseSecond
  Choose x y = 'IgnoreBoth


class Closed a b where
  fun :: a -> b -> Int


instance (Choose Int y) => Closed Int y where fun x _ = x
instance (Choose x Int) => Closed x Int where fun _ y = y
instance (Choose x y) => Closed x y where fun _ _ = 0


the differences:
no need to use new keyword "closed"
no need for new language pragma
non-type family instances may still be defined
instances may be defined in other modules

(if possible:) multiple type families may be used with the same class.
Instance constraint would hint, which (if any) type family to apply in
selecting an instance

how does this sound?