[Python-ideas] Another use case for the 'lazy' (aka 'delayed') keyword

David Mertz mertz at gnosis.cx
Tue Feb 28 11:35:49 EST 2017

Clearly there is SOME semantics that is consistent and coherent since many
languages have either a lazy or eager declaration syntax, with different
defaults between languages but both being built in.  There are a lot of
ways that Python isn't Haskell, obviously.  But both Scheme and OCaml are
eager by default with a lazy declaration (and Haskell or Miranda have an
eager declaration correspondingly).

It might be worth looking at their semantics in the PEP.

On Tue, Feb 28, 2017 at 7:57 AM, M.-A. Lemburg <mal at egenix.com> wrote:

> On 28.02.2017 15:46, Michel Desmoulin wrote:
> > lazy is not only practical, but it's also beautiful. It reads well. It
> > solves a problem we all have on a regular basis.
> The only practical use case I ever ran into for lazy evaluation
> are lazy imports, simply because imports cause a lot of
> startup overhead. It's a case which the new keyword wouldn't
> help with, though.
> For the discussion, it would help if you'd write up a definition
> of where the lazy evaluation should finally happen, which use
> cases would be allowed or not and how the compiler could detect
> these.
> IMO, there are much better ways to write code which only evaluates
> expensive code when really needed.
> I don't see how "lazy" could automate this in a well defined,
> helpful and obvious way, simply because the side effects of moving
> evaluation from the place of definition to an arbitrary other place
> in the code are not easy to manage.
> Even if you do manage to clearly define when to evaluate a lazy
> expression, the context in which it gets evaluated may
> have already changed, giving wrong results, causing exceptions
> due to missing information. Delaying exceptions can also
> break the exception handling in your application.
> PS: You run into the same issues with lazy imports, which is
> why special care has to be taken when using these.
> --
> Marc-Andre Lemburg
> eGenix.com
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