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Would that be like having a yield statement AND passing a context (e.g. binding a this that represents "current-continuation")?



Here are two examples in Python syntax, assuming "yield" is no longer a keyword.

  def f():
    def receiver(yield):
      yield(2)
      # yield jumps out of the 'receiver' function and never returns
      print("This never prints")
  
    x = callcc(receiver)
    return x+1
  
  print(f())
  # f returns 2+1
  
  # backtracks stores (continuation,[values]) pairs.  Whenever a computation fails, we pull out another
  # value from the list and feed it to the continuation.  The idea is to perform a depth-first search.
  backtracks = []
  def fail():
    """Aborts the current continuation and backtracks to another alternative stored in backtracks."""
    if not backtracks: raise Exception('amb ran out of alternatives')
    yield, alternatives = backtracks.pop()
    alt = alternatives.pop()
    if alternatives: # this continuation still has alternatives, so put them back on the list in case of a future 'fail'
      backtracks.push((yield, alternatives))
    yield(alt)
  def amb(*alternatives):
    """The "ambivalent operator."  The arguments to amb are alternatives, and the return value of amb
    is an alternative which makes the future computation not fail."""
    
    if not alternatives: fail()
    def receiver(yield):
      backtracks.append((yield, alternatives))
      fail()
    return callcc(receiver)
  def expect(b):
    if not b: fail()
  
  def test_amb():
    """Let's find a Pythagorean triple."""
    x = amb(*range(1,100))
    y = amb(*range(1,100))
    z = amb(*range(1,100))
    expect(x**2 + y**2 == z**2)
    return (x,y,z)
  
  # We can print out all the Pythagorean triples.
  trip = test_amb()
  print("(x,y,z)=(%s,%s,%s)" % trip)
  fail() # fail every time so it keeps backtracking, but the side effect of printing out remains




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