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pythonrubycontinuations
bhadra
bhadra

Reputation: 12971

Python equivalent of continuations with Ruby

What is the Python equivalent of the following code in Ruby?

def loop
  cont=nil
  for i in 1..4
    puts i
    callcc {|continuation| cont=continuation} if i==2
  end
  return cont
end

> c=loop
1
2
3
4
> c.call
3
4

Reference: Secrets of lightweight development success, Part 9: Continuations-based frameworks

Upvotes: 7

Views: 2095

Answers (5)

pts
pts

Reputation: 87361

There are many weak workarounds which work in special cases (see other answers to this question), but there is no Python language construct which is equivalent to callcc or which can be used to build something equivalent to callcc.

You may want to try Stackless Python or the greenlet Python extension, both of which provide coroutines, based on which it is possible to build one-shot continutations, but that's still weaker than Ruby's callcc (which provides full continuations).

Upvotes: 2

jfs
jfs

Reputation: 414695

Using generator_tools (to install: '$ easy_install generator_tools'):

from generator_tools import copy_generator

def _callg(generator, generator_copy=None):
    for _ in generator: # run to the end
        pass
    if generator_copy is not None:
        return lambda: _callg(copy_generator(generator_copy))

def loop(c):
    c.next() # advance to yield's expression
    return _callg(c, copy_generator(c))

if __name__ == '__main__':
    def loop_gen():
        i = 1
        while i <= 4:
            print i
            if i == 2:
                yield
            i += 1

    c = loop(loop_gen())
    print("c:", c)
    for _ in range(2):
        print("c():", c())

Output:

1
2
3
4
('c:', <function <lambda> at 0x00A9AC70>)
3
4
('c():', None)
3
4
('c():', None)

Upvotes: 2

jfs
jfs

Reputation: 414695

def loop():    
    def f(i, cont=[None]):        
        for i in range(i, 5):
            print i
            if i == 2:
                cont[0] = lambda i=i+1: f(i)
        return cont[0]
    return f(1)

if __name__ == '__main__':
    c = loop()
    c()

Upvotes: 0

user40098
user40098

Reputation:

take a look at the yield statement to make generators.

I don't speak any ruby, but it seems like you're looking for this:

def loop():
    for i in xrange(1,5):
        print i
        if i == 2:
            yield


for i in loop():
    print "pass"

Edit: I realize this is basically a specialization of real continuations, but it should be sufficient for most purposes. Use yield to return the continuation and the .next() message on the generator (returned by just calling loop()) to reenter.

Upvotes: 2

Greg Hewgill
Greg Hewgill

Reputation: 994221

The article you quoted contains a link to Continuations Made Simple And Illustrated in the Resources section, which talks about continuations in the Python language.

Upvotes: 5

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