Why?

printTest.hs

class     Show' a       where show' :: a -> String
instance  Show' Integer where show' = show

main = do
    putStrLn $ show  42  -- OK
    putStrLn $ show' 42  -- Error! Why?

stderr

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

printTest.hs:6:16:
    No instance for (Show' a0) arising from a use of show'
    The type variable `a0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Note: there is a potential instance available:
      instance Show' Integer -- Defined at printTest.hs:2:11
    In the second argument of `($)', namely `show' 42'
    In a stmt of a 'do' block: putStrLn $ show' 42
    In the expression:
      do { putStrLn $ show 42;
           putStrLn $ show' 42 }

printTest.hs:6:22:
    No instance for (Num a0) arising from the literal `42'
    The type variable `a0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Note: there are several potential instances:
      instance Num Double -- Defined in `GHC.Float'
      instance Num Float -- Defined in `GHC.Float'
      instance Integral a => Num (GHC.Real.Ratio a)
        -- Defined in `GHC.Real'
      ...plus three others
    In the first argument of show', namely `42'
    In the second argument of `($)', namely `show' 42'
    In a stmt of a 'do' block: putStrLn $ show' 42