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Thread-Topic: [PHP-DEV] Re: RFC: CachedIterable (rewindable, allows any
 key&repeating keys)
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Date: Thu, 10 Jun 2021 14:16:11 +0000
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Subject: Re: [PHP-DEV] Re: RFC: CachedIterable (rewindable, allows any
 key&repeating keys)
From: tysonandre775@hotmail.com (tyson andre)

Hi Alex,=0A=
=0A=
> > I've created a new RFC https://wiki.php.net/rfc/cachediterable adding C=
achedIterable,=0A=
> > which eagerly evaluates any iterable and contains an immutable copy of =
the keys and values of the iterable it was constructed from=0A=
> >=0A=
> > A heads up - I will probably start voting on https://wiki.php.net/rfc/c=
achediterable this weekend after https://wiki.php.net/rfc/cachediterable_st=
raw_poll is finished.=0A=
> >=0A=
> > Any other feedback on CachedIterable?=0A=
> =0A=
> Thanks for explaining 4 months=A0ago about=A0my=A0concern.=0A=
> I think I understand the main real impact of an eager iterable cache vs a=
 lazy iterable cache from a functional point of view:=0A=
> - exceptions=A0are thrown during construction vs during the first iterati=
on=0A=
> - predictable=A0performance also=A0on the first iteration.=0A=
> =0A=
> How did you gather the information that eager implementation is more valu=
able than lazy one? I'm mostly curious also how to assess this as technical=
ly to me it also looks the other way around. Maybe mention that in the RFC.=
=0A=
> I was even thinking=A0that CachedIterable should be lazy and an EagerCach=
edIterable would be built upon that with more methods. Or have it in the sa=
me class with a constructor parameter.=0A=
=0A=
One of the reasons was size/efficiency. Adding the functionality to support=
 lazy evaluation would require extra properties to track internal state and=
 extra checks at runtime, =0A=
point to the original iterable and the functions being applied to that iter=
able - so an application that creates lots of small/empty cached iterables =
would have a higher memory usage.=0A=
=0A=
Having a data structure that tries to do everything would do other things p=
oorly =0A=
(potentially not support serialization, use more memory than necessary,=0A=
have unintuitive behaviors when attempting to var_export/var_dump it, =0A=
surprisingly throw when being iterated over, etc)=0A=
=0A=
> Also, being able to have a perfect userland implementation, not very comp=
lex, even considering the lower performance, is not that good for positive =
voting from what I remember from history...=0A=
=0A=
1. The userland polyfill included in the RFC is an incomplete implementatio=
n that only supports iteration. =0A=
   It's meant to be as fast as possible at the cost of memory usage.=0A=
   It's not even an IteratorAggregate, doesn't support json encode, createF=
romPairs, and many other functions.=0A=
2. Virtually all of the spl iterables that don't deal with filesystems can =
be reimplemented in userland.=0A=
   (https://en.wikipedia.org/wiki/Turing_completeness)=0A=
=0A=
   Even complicated extensions such as redis or memcached can be reimplemen=
ted in userland on top of sockets,=0A=
   but with higher cpu usage than native extensions (https://github.com/pre=
dis/predis/blob/main/FAQ.md#predis-is-a-pure-php-implementation-it-can-not-=
be-fast-enough)=0A=
=0A=
   The benefit of having data structures internally is the fact that develo=
pers who learn them can use them in any project without adding dependencies=
=0A=
   (even in single file scripts) and that applications using CachedIterable=
 would have much better performance =0A=
=0A=
Also, you and Levi have pointed out that iterable/iterator functionality is=
 traditionally on-demand=0A=
(https://en.wikipedia.org/wiki/Lazy_evaluation) (e.g. iterables such as Cal=
lbackFilterIterator, RecursiveArrayIterator, etc)=0A=
=0A=
As a result, I'm thinking CachedIterable is really not a good name for the =
eagerly evaluated data structure I'm proposing here,=0A=
and that there was confusion about how the data structure behaved when the =
name CachedIterable was suggested.=0A=
If functionality like that described in https://externals.io/message/114805=
#114792 was added, it could use the name CachedIterable instead.=0A=
=0A=
So I'm probably changing this to `ImmutableTraversable` as a short name for=
 the functionality,=0A=
to make it clear arguments are eagerly evaluated when it is created.=0A=
(ImmutableSequence may be expected to only contain values, and would be con=
fused with the ds PECL's https://www.php.net/manual/en/class.ds-sequence.ph=
p)=0A=
=0A=
Thanks,=0A=
Tyson=0A=