Alright, I didn’t see those problem. Though I was suggesting that for functions like map, we just let the used iterator answer, this is interesting. Thanks for this
On Thu 29 Nov 2018 at 17:40, David Mertz <me...@gnosis.cx> wrote: > On Thu, Nov 29, 2018 at 2:29 AM Adrien Ricocotam <ricoco...@gmail.com> > wrote: > >> Some suggested above to change the definition of len in the long term. >> Then I think it could be interesting to define len such as : >> >> - If has a finite length : return that length (the way it works now) >> - If has a length that is infinity : return infinity >> - If has no length : return None >> > > Do you anticipate that the `len()` function will be able to solve the > Halting Problem? > > It is simply not possible to know whether a given iterator will produce > finitely many or infinitely many elements. Even those that will produce > finitely many do not, in general, have a knowable length without running > them until exhaustion. > > Here's a trivial example: > > >>> def seq(): > ... while random() > 0.1: > ... yield 1 > >>> len(seq()) > # What answer do you want here? > > Here's a slightly less trivial one: > > In [1]: from itertools import count > In [2]: def mandelbrot(z): > ...: "Yield each value until escape iteration" > ...: c = z > ...: for n in count(): > ...: if abs(z) > 2: > ...: return n > ...: yield z > ...: z = z*z + c > > What should len(mandelbrot(my_complex_number)) be? Hint, depending on the > complex number chosen, it might be any Natural Number (or it might not > terminate). > > > -- > Keeping medicines from the bloodstreams of the sick; food > from the bellies of the hungry; books from the hands of the > uneducated; technology from the underdeveloped; and putting > advocates of freedom in prisons. Intellectual property is > to the 21st century what the slave trade was to the 16th. >
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