gistfile1.txt

{-# LANGUAGE TypeFamilies, UndecidableInstances #-}
data A = A
data B = B
data Found a = Found a
data NotFound = NotFound
type family F x
type instance F A = Found (Int -> Int)
type family FindF x where
  FindF f = CheckFoundF (F f)
type family CheckFoundF res where
  CheckFoundF (Found f) = f
  CheckFoundF notfound = NotFound
test :: FindF B
test = undefined
{-
*Main> test

<interactive>:150:1:
    No instance for (Show (CheckFoundF (F B)))   <==== Should be (Show NotFound)
      arising from a use of ‘print’
    In a stmt of an interactive GHCi command: print it
-}