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Date: Wed, 7 Aug 2013 01:16:39 +0100
Subject: Re: Newbie: static typing?
From: Chris Angelico <rosuav@gmail.com>
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Xref: csiph.com comp.lang.python:52093

On Wed, Aug 7, 2013 at 12:02 AM, Terry Reedy <tjreedy@udel.edu> wrote:
> 3) The code falls into an infinite loop or recursion.
>
> The solution is to think before looping or recursing.  This often involves
> value checking (non-negative int or non-fractional float, for instance)
> rather than type checking in the usual static type-checking sense.

Yeah, there aren't many languages that let you declare that the
argument must be a positive integer. That's just something you have to
test for manually.

> One also needs to be careful about passing unbounded iterators to other
> functions and remember that unboundedness is contagious. (filter(pred,
> unbounded_iterator) is an unbounded iterator). Again, this is a 'value' or
> implicit sub-type issue rather than a explicit, visible 'type' issue.

Not quite always; I'd say that unboundedness is as contagious as IEEE
Infinity. Lots of operations on infinity will yield infinity, but a
few won't. itertools.islice can guarantee a finite iterator, and
takewhile may terminate. But yes, with filter() it certainly is.

ChrisA