Python-based monads essay part 2

[Marko Rauhamaa]

Gregory Ewing <greg.ewing at canterbury.ac.nz>:

> Marko Rauhamaa wrote:
>
>>     def main():
>>         try:
>>             guard_it()
>>         except ValueError:
>>             # I'm having none of it!
>>             resume "CarryOn!"
>>
>> The problem is, how can the file f unclose itself when work resumes?
>
> The same thing could happen in Scheme, though. Re-entering a context
> via a continuation doesn't magically reset everything to the way it
> was at some earlier time. A data structure that has been mutated will
> still be mutated, and a file that has been closed will still be
> closed.

Obviously. However, Scheme continuations are (or can be) used to
implement all other single-threaded paradigms (coroutines, generators,
exceptions etc), so the execution context quite normally jumps around
"erratically". There is no automatic method to decide when final is
final.


Marko