[Python-ideas] A Continuations Compromise in Python

[Joel Bender]

Adam Olsen wrote:

> def a():
>     return b
> 
> def b():
>     return a
> 
> def trampoline(func):
>     while True:
>         func = func()

Every once in a while (usually with actor and message passing 
algorithms) I get the urge to have something like this, I didn't realize 
it had a formal name:

     def a(n):
         trampoline b(n-1)
         print "never gets printed"

     def b(n):
         if cond(n):
             return n
         else:
             trampoline a(n * 2)
         print "never gets printed"

So a trampolined call bails out of the current call frame, and whatever 
the trampolined call returns, it returns.

     >>> a(5) == b(4)
     True
     >>>

The same thing would happen with generators and try/except handlers:

     def a(n):
         try:
             yield n
             trampoline b(n-1)
         except Exception, err:
             print "a() error:", err

     def b(n):
         yield n
         raise RuntimeError, "snort"

     >>> for i in a(2):
     ...     print i
     ...
     2
     1
     Traceback (most recent call last):
       File "<stdin>", line 1, in <module>
       File "<stdin>", line 3, in b
     RuntimeError: snort
     >>>


Joel