On Fri, 10 Feb 2006 16:49:13 +1300, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
>Valentino Volonghi aka Dialtone wrote:
>
>> when some_operation_that_results_in_a_deferred() -> result:
>> if result == 'Initial Value':
>> when work_on_result_and_return_a_deferred(result) -> inner_res:
>> print inner_res
>> else:
>> print "No work on result"
>> reactor.stop()
>
>Hmmm. This looks remarkably similar to something I got half
>way through dreaming up a while back, that I was going to
>call "Simple Continuations" (by analogy with "Simple Generators").
>Maybe I should finish working out the details and write it up.
>
>On the other hand, it may turn out that it's subsumed by
>the new enhanced generators plus a trampoline.
This in only partially true. In fact, let's consider again twisted for the example, you can do something like this:
@defgen
def foo():
for url in urls:
page = yield client.getPage(url)
print page
This has 2 disadvantages IMHO. First of all I have to use a function or a method decorated with @defgen to write that. But most important that code, although correct is serializing things that could be parallel. The solution is again simple but not really intuitive:
@defgen
def foo():
for d in map(client.getPage, urls):
page = yield d
print page
Written in this way it will actually work in a parallel way but it is not really an intuitive solution.
Using when instead:
for url in urls:
when client.getPage(url) -> page:
print page
This wouldn't have any problem and is quite readable. A similar construct is used in the E language and here http://www.skyhunter.com/marcs/ewalnut.html#SEC20 is explained how when works for them and their promise object. You can also have multiple things to wait for:
when (client.getPage(url), cursor.execute(query)) -> (page, results):
print page, results
or
l = [list, of, deferreds]
when l -> *results:
print results
and we could catch errors in the following way:
when client.getPage(url) -> page:
print page
except socket.error, e:
print "something bad happened"
HTH
--
Valentino Volonghi aka Dialtone
Now Running MacOSX 10.4
Blog: http://vvolonghi.blogspot.com
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