[Haskell-cafe] Check a lack of a constraint?

[Viktor Dukhovni]

> On 12 Jul 2021, at 4:24 pm, Viktor Dukhovni <ietf-dane at dukhovni.org> wrote:
> 
> However, it is possible to get something along those lines with
> a closed type family and an explicit list of verboten types:

Somewhat cleaner (no complaints from -Wall, and the Filtered type family
now returns a constraint):

    {-# LANGUAGE ConstraintKinds
               , DataKinds
               , FlexibleContexts
               , TypeFamilies
               , TypeOperators
               , UndecidableInstances
      #-}
    
    import GHC.TypeLits (ErrorMessage(..), TypeError)
    import Data.Kind (Constraint)
    
    type family Filtered a :: Constraint where
      Filtered Int = TypeError ('ShowType Int ':<>: 'Text "s not welcome here")
      Filtered a   = ()
    
    foo :: (Show a, Filtered a) => a -> String
    foo = show

-- 
	Viktor.