Michael wrote: > vasudevram wrote: > > > > > Hi, > > > > I am Googling and will do more, found some stuff, but interested to get > > viewpoints of list members on: > > > > Continuations in Python. > > > > Saw a few URLs which had some info, some of which I understood. But > > like I said, personal viewpoints are good to have. > > Python doesn't really support continuations. Generators (and co-routines in > general) have similar properties to continuations, however they're not > continuations. Closures are also sometimes considered cousins to > continuations, and python's support for those is pretty good IMO. (again > however, closures are not continuations). > > Since it looks like you're also looking for what continuations are, I think > the following is the simplest way of explaining them. If you've ever > programmed in BASIC, *in a way* if you think of methods/functions as a > named GOSUB, then continuation (in a way) are a way of giving a name to a > goto (it's more subtle than that though since a continuation os often > defined to "remember" state in a similar way to a closure). Like a function > you can pass them round as objects. Like a generator/closure they remember > the state they were in. > > *Personally* , I think python NOT supporting full continuations is a GOOD > thing, since full continuations, whilst powerful, are also a great source > of confusion for people. (That said, I have a usecase I'd find them useful > for - I think they'd be useful for plugin architectures - but IMO that > doesn't outweigh the risk of code obfuscation :-) > > One particular usecase that people seem to like continuations for, > specifically how they're used in seaside, is actually a subset of > functionality that python *can* support (to a large extent). > > Essentially the idea is to be able to take a web application and make it > look linear. CherryFlow allows you for example to write this: > > @expose > @flow > def example_flow(): > yield view.first_page() > if request.args["choice"] == "a": > yield view.choice_a_page() > else: > yield view.choice_b_page() > yield view.last_page() > > (example from: http://tinyurl.com/qzpqu ) > > This causes the user's browser to show a page where they have a choice of > "a" or "b". Depending on what they choose, they then either are presented > with choice_a_page or choice_b_page. Finally, no matter which option they > chose, you are presented with the last page. > > Something similar can be done using Kamaelia, though at present only the > mechanism exists there, without any sugar (http://tinyurl.com/n3bh7 - > specifically websiteSessionExampleComponent). The reason I mention it is to > say that it's relatively simple to do using python :-) > > I have to stress though, this usage of continuations in Seaside, as > emulate-able in python is a subset of the full power of continuations, > which isn't available in python at present. (Though it's possible hacking > greenlets could result in something). (In fact the way seaside uses them as > far as I can tell is more to implement co-routine like behaviour than > anything else (!)) > > Anyway, hope that's interesting/useful - looking at your other comments, > you're just looking for information and usecases at the moment :-) > > Regards, > > > Michael.
Yes, that's quite right - only looking for info and use cases at present - and thanks for the info, it was interesting and useful :-) Need to check all the inputs out ... Vasudev -- http://mail.python.org/mailman/listinfo/python-list
