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

13 Jul 2021


      Oh, very nice approach Viktor. It really seems easier than having a 
custom typeclass for which the blessed types have an instance, if the 
set of verboten types is considerably smaller than the set of allowed types.

Le 12/07/2021 à 22:59, Viktor Dukhovni a écrit :
...
...
On 12 Jul 2021, at 4:24 pm, Viktor Dukhovni  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
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