The Nature of Lisp(defmacro.org)

  withFile fileName mode = bracket (openFile fileName mode) hClose

  withFile "/usr/share/dict/words" ReadMode $ \h -> do
    contents <- hGetContents h
    count (lines contents)

  (Handle -> IO a) -> IO a

  data Cont r a = Cont ((a -> r) -> r)

  Cont (IO a) Handle

    from contextlib import contextmanager
    
    @contextmanager
    def temp_assign(obj, attr, val):
        old_val = getattr(obj, attr)
        setattr(obj, attr, val)
        yield
        setattr(obj, attr, old_val)
    
    class X:
        pass
    
    x = X()
    x.a = 1
    with temp_assign(x, "a", 2):
       print x.a # prints 2
    print x.a # prints 1

  from contextlib import contextmanager

  @contextmanager
  def traverse(obj, attr, val):
    cached = getattr(obj, attr)
    setattr(obj, attr, val)
    yield
    setattr(obj, attr, cached)

  with traverse(obj, "traversing", True):
    # ... do stuff ...