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Date: Sat, 05 Jan 2013 12:28:17 +0100
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Subject: Re: [PHP-DEV] Ruby's symbols
From: ivan.enderlin@hoa-project.net ("Ivan Enderlin @ Hoa")

Hi :-),

Nice proposal but I have few questions and remarks.

My first question is how to delete symbols? I don't know if it is useful =

or not. We should discuss about that. Is `unset(:foo)` enough? How are=20
they handled by the GC?

Another question is: where are we able to use symbols? Is it allowed to=20
write `const FOO =3D :bar` for example? Or in a default argument value:=20
`public function f ( $x =3D :foo ) { =E2=80=A6 }`?

What kind of (arithmetical) operations are allowed on symbols?

You proposed to represent symbols as integers. Could it create=20
conflicts? For example: `$foo =3D [0 =3D> 'bar', :baz =3D> 'qux']`. What =

happens if `:baz` is set to 0?

Finally, if my memory is good, a few months ago, some patches were=20
updating =E2=80=9Cconstant strings=E2=80=9D performances by representing =
them as=20
=E2=80=9Cbuffers=E2=80=9D/=E2=80=9Cconstants=E2=80=9D internally, no? May=
be I'm wrong, but it could be a=20
middle-way solution if it is not yet implemented (especially since we=20
have strings dereferencing, implementation could be ease).

My two cents :-).
Best regards.




On 05/01/13 15:07, Nikita Nefedov wrote:
> I know I shouldn't write "Ruby" in the subject of a letter for=20
> php-internals ML, but... Just wanted to ask, is anybody interested in=20
> this feature in PHP?
> You can read about it here:=20
> http://www.randomhacks.net/articles/2007/01/20/13-ways-of-looking-at-a-=
ruby-symbol
>
> It can be implemented in PHP as a HashTable where key would be strings =

> and values would be unique identifiers (just incrementing long, can be =

> tracked with nNextFreeElement).
>
> How it would work in userland?
>
> If it would be proposal, I would divide it on two different=20
> (competitive) proposals:
>
> First, we could implement it so symbol expression would return an=20
> associated integer.
> (note that symbols are most often used as hash keys in ruby so my=20
> examples are relying on it)
> Example:
> $foo =3D array();
> $foo[:bar] =3D "qwerty";
>
> var_dump($foo);
> // array(1) {
> //  [0] =3D>
> //  string(6) "qwerty"
> //}
>
> var_dump(:bar); // int(0)
>
> var_dump(:some_new_symbol); // int(1)
>
> This is the easiest way, and I can implement it. But, it would be hard =

> to debug.
>
> The second way - we can implement it as a new variable type, and the=20
> main thing is that arrays will be able to take variables of symbol=20
> type as keys, I know it sounds scary but it's not so hard actually.
> I will drop the part about new variable type, the main thing is how=20
> HashTables could work with symbols as keys.
> We could change the type of nKeyLength (in Bucket) to signed integer=20
> (now it's unsigned, I don't think we will miss something from that)=20
> and use it as a flag: if it's -1 (less than zero) - then we know the=20
> symbol was used as a key, there will be filled bucket.h member with a=20
> symbol identifier.
> That way the above code will evaluate so:
> $foo =3D array();
> $foo[:bar] =3D "qwerty";
>
> var_dump($foo);
> // array(1) {
> //  [:bar] =3D>
> //  string(6) "qwerty"
> //}
>
> var_dump(:bar); // symbol(:bar)
>
> var_dump(:some_new_symbol); // symbol(:some_new_symbol)
>
> To make it clear, when retrieving an element by symbol from array,=20
> these steps will be needed:
> 1. Get symbol identifier (ulong)
> 2. Get elements from array where bucket.h equals to this identifier
> 3. Iterate over those elements and find the one that has nKeyLength =3D=
=3D -1
> (actually we will iterate over pNext/pLast elements)
>
>
> What symbols can give:
> 1. More convenient way to use it almost everywhere as a replacement=20
> for strings and sometimes for constants. There's a lot of code that=20
> uses arrays as a parameter-stores. For example, here's how you usually =

> define a form in Symfony2:
>         $builder
>             ->add('title', 'text', array(
>                 'label' =3D> 'Album title'
>             ))
>             ->add('title_alias', 'text', array(
>                 'label' =3D> 'Album alias',
>                 'required' =3D> false,
>                 'property_path' =3D> 'properties.titleAlias'
>             ))
>             ->add('comment', 'text', array(
>                 'label' =3D> 'Comment',
>                 'required' =3D> false,
>                 'property_path' =3D> 'properties.comment'
>             ))
>             ->add('labels', 'text', array(
>                 'label' =3D> 'Musical labels',
>                 'required' =3D> false,
>                 'property_path' =3D> 'properties.labels'
>             ))
>             ->add('language', 'text', array(
>                 'required' =3D> false,
>                 'property_path' =3D> 'properties.language'
>             ))
> It could be improved this way:
>         $builder
>             ->add('title', :text, array(
>                 :label =3D> 'Album title'
>             ))
>             ->add('title_alias', :text, array(
>                 :label =3D> 'Album alias',
>                 :required =3D> false,
>                 :property_path =3D> 'properties.titleAlias'
>             ))
>             ->add('comment', :text, array(
>                 :label =3D> 'Comment',
>                 :required =3D> false,
>                 :property_path =3D> 'properties.comment'
>             ))
>             ->add('labels', :text, array(
>                 :label =3D> 'Musical labels',
>                 :required =3D> false,
>                 :property_path =3D> 'properties.labels'
>             ))
>             ->add('language', :text, array(
>                 :required =3D> false,
>                 :property_path =3D> 'properties.language'
>             ))
> 2. Memory usage reduction. AFAIK, there's a lot of cases like the=20
> above and the use of symbols would affect on memory usage significantly=
=2E
> 3. Memory leaks. Symbols can't be just garbage collected as, for=20
> example zvals, it's not possible. But we can do it for every request,=20
> so I don't think it would be problem.
> 4. Autocompletion from IDEs.
>
> PS I don't want to call it "proposal", despite the fact that this is=20
> actually a proposal, I'm just not sure it can go as a proposal.
>

--=20
Ivan Enderlin
Developer of Hoa
http://hoa.42/ or http://hoa-project.net/

PhD. student at DISC/Femto-ST (Vesontio) and INRIA (Cassis)
http://disc.univ-fcomte.fr/ and http://www.inria.fr/

Member of HTML and WebApps Working Group of W3C
http://w3.org/