factorial' :: Int -> Int
factorial' 0 = 1
factorial' (n+1) = (n+1)*factorial' n
n+k patterns were removed from Haskell 2010. You can re-enable them in GHC with
{-# LANGUAGE NPlusKPatterns #-}
or
{-# LANGUAGE Haskell98 #-}
as the first line of the source file.