asNewtypeIf ::
     (a -> b)    -- Type constructor
  -> (a -> Bool) -- Constraint for newtype
  -> a           -- Unwrapped value
  -> Maybe b     -- Maybe wrapped value
asNewtypeIf constructor predicate input
  | predicate input = Just (constructor input)
  | otherwise       = Nothing

--

newtype ShortString = ShortString String
  deriving Show

makeShortString :: String -> Maybe ShortString
makeShortString = ShortString `asNewtypeIf` isValidShortString
  where
    isValidShortString = (<= 3) . length