a break for comprehensions

[Steve Holden]

"Tom Good" <Tom_Good1 at excite.com> wrote in message
news:ac677656.0107301331.5330d4db at posting.google.com...
> "Steven D. Majewski" <sdm7g at Virginia.EDU> wrote in message
news:<mailman.996515638.21022.python-list at python.org>...
> >
> > The 'while' clause seems useful, and I don't see anything inherently
> > ambiguous about it, but sticking a procedural clause into what's
> > otherwise a declarative seens to go against the spirit -- what's
> > nice about list comprehensions is that they are just like set
> > notations. ( well -- the left hand side is often procedural -- it
> > would be even more readable if it were more clearly separated
> > with a "where" clause, but that would be Yet Another Keyword... )
> >
> > -- Steve Majewski
>
> That is a compelling argument.  However, I am still struggling with
> how to best work with infinite sets (which are now easy to create with
> generators).  With set notation, you can refer to an infinite set
> without actually creating each member of the set.  List comprehensions
> try to "fill up" the list, which causes problems if an unbounded
> generator is involved.
>
> If g is a generator that generates all positive integers, I would like
> to write something like:
>
> L = [x * 2 for x in g()]
> print L[:10]
>
> This does not work, of course, but that is how I would tend to think
> about it in my head.  List comprehensions with lazy evaluation?  Is it
> possible?
>
With the introduction of generators there doesn't seem to be anything
inherently impossible about the introduction of such a concept. A major
problem with existing list comprehensions, however, is that they produce a
list! Seems that a "lazy comprehension" would have to encapsulate a
generator somehow, and limit the operations you could perform on it to those
for generators.

I believe a separate syntax would be required to represent such a radically
different object. But you, of course, may know better ... if you're looking
for a syntax, one possibility would be:

    [[x * 2 for x in g()]]

regards
 Steve
--
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