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Genuine question: What do virtual objects enable that cannot be done through the Trait system?

by iherbig 11y ago

shmerl 11y ago
Problems that Rust can't now address are listed in that RFC. Such as sharing of fields between definitions for instance. Traits don't address that. See also these posts: * http://smallcultfollowing.com/babysteps/blog/2015/05/05/where-rusts-enum-shines/ http://smallcultfollowing.com/babysteps/blog/2015/05/05/wher... * http://smallcultfollowing.com/babysteps/blog/2015/05/29/classes-strike-back/ http://smallcultfollowing.com/babysteps/blog/2015/05/29/clas... * http://smallcultfollowing.com/babysteps/blog/2015/08/20/virtual-structs-part-3-bringing-enums-and-structs-together/ http://smallcultfollowing.com/babysteps/blog/2015/08/20/virt... * http://smallcultfollowing.com/babysteps/blog/2015/10/08/virtual-structs-part-4-extended-enums-and-thin-traits/ http://smallcultfollowing.com/babysteps/blog/2015/10/08/virt...
- Manishearth 11y ago
  Note that while many OOP patterns are hard to express in Rust, there are equivalent patterns that handle the use cases. To give some context on those blog posts; they exist to figure out how to add single inheritance to Rust. One of the strongest arguments for single inheritance in Rust is "Servo and similar things need it to model the DOM". I.e; the strongest argument out there is that modelling cross-language things with languages that _do_ have single inheritance is hard. That's not a very strong argument. That's a pretty niche use case (which Servo itself has moved past; we have a setup emulating inheritance that works pretty ok now). So while I do agree that Rust can't model these patterns easily; I'm skeptical that that can be an issue for the large majority of Rust users.
  - shmerl 11y ago
    > while many OOP patterns are hard to express in Rust, there are equivalent patterns that handle the use cases. May be there are, but they far from trivial, and with the current level of documentation they are in the realm of some obscure hacks rather than straightforward part of the language. Which is a disadvantage. May be when The Book will cover OOP topics in depth and explain such patterns, or there will be some other dedicated documentation on this topic, things could become more clear.
    - Manishearth 11y ago
      > current level of documentation they are in the realm of some obscure hacks I don't think so. Rust encourages a certain style of designing code, using enums and traits. Almost all the time this suffices. My comment was at the macro level; I'm not saying that an OOP pattern will have an equivalent in Rust, I'm saying that if you look at the larger use case you can design your code in idiomatic Rust to avoid it.
      - shmerl 11y ago
        > I'm saying that if you look at the larger use case you can design your code in idiomatic Rust to avoid it. As I said, may be you can, but it's far from trivial and unless you know how to do it already, coming up with such approaches isn't something that one who just learns the language would be focusing on. You yourself said above, that it's hard. And unlike languages like C++ where OOP approaches are very well documented, Rust documentation lacks such kind of information at present.
        Manishearth 11y ago
        > You yourself said above, that it's hard. I said modeling single inheritance is hard. That's a very niche use case; basically only crops up when writing an interpreter for a language with single inheritance (and in cases like Servo's DOM) > unless you know how to do it already, coming up with such approaches isn't something that one who just learns the language would be focusing on Again, it's not a thing that you do as a drop-in for inheritance. I'm saying that the regular approach (i.e. using traits and enums idiomatically) to designing your application in Rust will cover the use cases for inheritance, differently. Basically, don't try to write Rust code as if it's C++ or Java and stuff should work out.
        shmerl 11y ago
        I don't mean one has to copy ideas from C++ or Java to Rust verbatim, but having some explanation on how to use Rust OOP abstractions for solving common programming problems would be useful. I don't think current Rust documentation covers that well.
Rusky 11y ago
Sharing data layout between implementations of a trait is much more manual and tedious in Rust than it is in something with inheritance. You can often change your design to avoid that issue, but Servo, for example, needs to implement the DOM, and that's just the best way to do it.
dbaupp 11y ago
I believe they're equally expressive, but the existing trait approach has a different run-time/memory profile. The issue linked in the parent comment has some info/links, and, http://smallcultfollowing.com/babysteps/blog/2015/10/08/virtual-structs-part-4-extended-enums-and-thin-traits/ http://smallcultfollowing.com/babysteps/blog/2015/10/08/virt... is the latest word on the subject (including various links).

Genuine question: What do virtual objects enable that cannot be done through the Trait system?

by iherbig 11y ago

Genuine question: What do virtual objects enable that cannot be done through the Trait system?

Problems that Rust can't now address are listed in that RFC. Such as sharing of fields between definitions for instance. Traits don't address that. See also these posts: * http://smallcultfollowing.com/babysteps/blog/2015/05/05/where-rusts-enum-shines/ http://smallcultfollowing.com/babysteps/blog/2015/05/05/wher... * http://smallcultfollowing.com/babysteps/blog/2015/05/29/classes-strike-back/ http://smallcultfollowing.com/babysteps/blog/2015/05/29/clas... * http://smallcultfollowing.com/babysteps/blog/2015/08/20/virtual-structs-part-3-bringing-enums-and-structs-together/ http://smallcultfollowing.com/babysteps/blog/2015/08/20/virt... * http://smallcultfollowing.com/babysteps/blog/2015/10/08/virtual-structs-part-4-extended-enums-and-thin-traits/ http://smallcultfollowing.com/babysteps/blog/2015/10/08/virt...

Manishearth 11y ago
Note that while many OOP patterns are hard to express in Rust, there are equivalent patterns that handle the use cases. To give some context on those blog posts; they exist to figure out how to add single inheritance to Rust. One of the strongest arguments for single inheritance in Rust is "Servo and similar things need it to model the DOM". I.e; the strongest argument out there is that modelling cross-language things with languages that _do_ have single inheritance is hard. That's not a very strong argument. That's a pretty niche use case (which Servo itself has moved past; we have a setup emulating inheritance that works pretty ok now). So while I do agree that Rust can't model these patterns easily; I'm skeptical that that can be an issue for the large majority of Rust users.
- shmerl 11y ago
  > while many OOP patterns are hard to express in Rust, there are equivalent patterns that handle the use cases. May be there are, but they far from trivial, and with the current level of documentation they are in the realm of some obscure hacks rather than straightforward part of the language. Which is a disadvantage. May be when The Book will cover OOP topics in depth and explain such patterns, or there will be some other dedicated documentation on this topic, things could become more clear.
  - Manishearth 11y ago
    > current level of documentation they are in the realm of some obscure hacks I don't think so. Rust encourages a certain style of designing code, using enums and traits. Almost all the time this suffices. My comment was at the macro level; I'm not saying that an OOP pattern will have an equivalent in Rust, I'm saying that if you look at the larger use case you can design your code in idiomatic Rust to avoid it.
    - shmerl 11y ago
      > I'm saying that if you look at the larger use case you can design your code in idiomatic Rust to avoid it. As I said, may be you can, but it's far from trivial and unless you know how to do it already, coming up with such approaches isn't something that one who just learns the language would be focusing on. You yourself said above, that it's hard. And unlike languages like C++ where OOP approaches are very well documented, Rust documentation lacks such kind of information at present.
      - Manishearth 11y ago
        > You yourself said above, that it's hard. I said modeling single inheritance is hard. That's a very niche use case; basically only crops up when writing an interpreter for a language with single inheritance (and in cases like Servo's DOM) > unless you know how to do it already, coming up with such approaches isn't something that one who just learns the language would be focusing on Again, it's not a thing that you do as a drop-in for inheritance. I'm saying that the regular approach (i.e. using traits and enums idiomatically) to designing your application in Rust will cover the use cases for inheritance, differently. Basically, don't try to write Rust code as if it's C++ or Java and stuff should work out.
        shmerl 11y ago
        I don't mean one has to copy ideas from C++ or Java to Rust verbatim, but having some explanation on how to use Rust OOP abstractions for solving common programming problems would be useful. I don't think current Rust documentation covers that well.

Rusky 11y ago

Sharing data layout between implementations of a trait is much more manual and tedious in Rust than it is in something with inheritance. You can often change your design to avoid that issue, but Servo, for example, needs to implement the DOM, and that's just the best way to do it.

dbaupp 11y ago

I believe they're equally expressive, but the existing trait approach has a different run-time/memory profile. The issue linked in the parent comment has some info/links, and, http://smallcultfollowing.com/babysteps/blog/2015/10/08/virtual-structs-part-4-extended-enums-and-thin-traits/ http://smallcultfollowing.com/babysteps/blog/2015/10/08/virt... is the latest word on the subject (including various links).