The user guide documentation of RequiredTypeArguments highlights a hypothetical alternative to the interface of the `sizeOf` method of the Storable class: https://downloads.haskell.org/ghc/9.12.2/docs/users_guide/exts/required_type... sizeOf :: forall a -> Storable a => Int If sizeOf had this type, we could write sizeOf Bool without passing a dummy value. This is fine in isolation, but how would this actually work in a type class method definition? The below naïve attempt does not work: {-# LANGUAGE AllowAmbiguousTypes #-} {-# LANGUAGE RequiredTypeArguments #-} class MyStorable a where sizeOf :: forall a -> MyStorable a => Int instance MyStorable Bool where sizeOf (type Bool) = 1 --- • Expected kind ‘k’, but ‘Bool’ has kind ‘*’ ‘k’ is a rigid type variable bound by the type signature for: sizeOf :: forall {k}. forall (a1 :: k) -> MyStorable a1 => Int at /tmp/foo.hs:8:5-10 • In the type ‘Bool’ In a type argument: Bool In the pattern: type Bool • Relevant bindings include sizeOf :: forall (a :: k) -> MyStorable a => Int (bound at /tmp/foo.hs:8:5) | 8 | sizeOf (type Bool) = 1 | ^^^^ Is the hypothetical `sizeOf` actually realisable as a type class method? Or can it only be a module-level wrapper? Something like the below, which does work? {-# LANGUAGE RequiredTypeArguments #-} class MyStorable a where _sizeOf :: a -> Int instance MyStorable Bool where _sizeOf _ = 1 instance MyStorable Int where _sizeOf _ = 8 sizeOf :: forall a -> MyStorable a => Int sizeOf a = _sizeOf (undefined :: a) Is it possible to avoid the (undefined :: a) term and somehow use TypeApplications here, with an "ambiguous" `_sizeOf` method? class MyStorable a where _sizeOf :: Int -- Viktor. 🇺🇦 Слава Україні!