error "todo" is special? `in` can contain statements?

gistfile1.hs

hardCoreFunction a b c =
  let z = a + b
      k = b + c
  in 
      error "todo" -- should be removed when the code below is not comment
      z + k        -- these are only experiements, usually explorations into the types of z and k

The real case - compiled! And obvious throws exceptions!

natural ::
  Parser Int
natural =
  -- bindParser :: (a -> Parser b) -> Parser a -> Parser b
  let 
    oparser :: Chars -> Optional (Parser Int)
    oparser cs = (valueParser . fst) <$> (reads cs)
  in 
    error ""
    bindParser (\cs -> (oparser cs) ?? failed) (list digit)