[plt-scheme] Fun with Unicode and delimited continuations

On 06/03/2010 11:07 AM, Eli Barzilay wrote:
> On Jun  3, Matthias Felleisen wrote:
>    
>> On Jun 3, 2010, at 12:31 PM, John Clements wrote:
>>
>>      
>>>> Sorry, maybe this wasn't clear: I mean e.g. the python 'yield',
>>>> which allows a function to return multiple values in a serialized
>>>> form.
>>>>          
>> If you can write a yield + create a two-element structure in Python
>> that way, you can mimic delimited continuations.
>>
>>
>>      
>>> The only problem with this mechanism is that the yield boundary is
>>> fixed on entry to the function call.
>>>        
>> That's minor. Reset is next to the function entry point too.
>>
>> Go for it. Show us that Python has delimited continuations.
>>      
> Here's some quick code:
>
>    #lang scheme
>    (require scheme/generator)
>    (define (foo x y) (yield x) (yield (+ x y)))
>    (define (bar a b c)
>      (generator             ; v5 will require () after `generator'
>        (for* ([x (in-generator (foo a b))]
>               [y (in-generator (foo x c))])
>          (yield y))))
>    (define g (bar 100 10 1))
>    (g) (g) (g) (g)
>
> where `foo' is the moral equivalent of plus/minus.  Given that this
> uses exactly the feature that scheme/generator has that python doesn't
> (it uses a dynamic `yield' so it can be used in "helper" functions),
> it would be interesting to see if there's a way to do that in python.
>
> (I think that Jon looked for an example that shows the difference.)
>
>    

This python program behaves the same, although it would break if one of 
the `yield's in (foo) were moved to a helper function, like

(define (foo1 x) (yield x))
(define (foo x y) (yield x) (foo1 (+ x y)))

# -- python --
def foo(x, y):
     yield x
     yield x + y

def bar(a, b, c):
     for x in foo(a, b):
         for y in foo(x, c):
             yield y

for n in bar(100, 10, 1):
     print n
# -- end --

I wrote some stuff about delimited continuations in scheme vs python, 
I'll try to dig it up and see if there is anything interesting in it.