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{-# LANGUAGE GADTs #-}
 
class Show a => Num' a where fromInteger' :: Integer -> a
 
data X a where
     X :: (Show a, Num' a) => a -> X a
 
instance Show (X a) where
         show (X x) = "X " ++ show x
 
instance Num' (X a) where
         fromInteger' x = X $ fromInteger' x
 
main = return ()

[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:12:27:
    Could not deduce (Num' a) arising from a use of `X'
    from the context (Show (X a))
      bound by the instance declaration at prog.hs:11:10-19
    Possible fix:
      add (Num' a) to the context of the instance declaration
    In the expression: X
    In the expression: X $ fromInteger' x
    In an equation for fromInteger':
        fromInteger' x = X $ fromInteger' x

Standard output is empty