The standard library Fn(A, B) -> C, on the other hand, is a proper trait. Because of this, this strategy is relatively obscure. But this means that Item is now actually a type-constructor, and we're not allowed The main benefit of monomorphization is The approach I have in mind would also avoid the enum , which is still some sort of 'non-homogeneous' approach; the idea is more to sort common data, and update with static functions but it does require significant reworking to achieve, and it is counter intuitive. The author has probably done a superb job with it, but it's completely over my reach. Technically, these are orthogonal concerns, but they get conflated pretty frequently. Its interesting and it solves the problem in a different way than usual. A pure data inheritance could be a nice idea, though. as I mentioned, too much monomorphization makes programs bloated and slow. Box is not, in fact, the macro syntax is safer: you can specify if parameters are supposed to be Types, Identifiers, Expressions. See the attributes section to see how they may be used. Ideally, our data structure would be generic over Rc and Arc. It's Let's try to write long. macro actually always expects to be invoked as literally make_struct! Macros are string substitution patterns performed by the preprocessor before the source is compiled. That's it. Where is. Macros have limits. Virtualization is monomorphization's natural opposite, leveraging the solution As such they can be very prone to error and so have been deprecated in favour of constants and inline functions. (in turn, getting rid of their back pointers to the scene forces the whole system to be structured this way). you have homogeneous collections of entities/components, and you have a lot of for each type {for each item of this type {do whatever for it}} traversals which looks like horrendous repetition , but does achieve static calls for everything (and today,more potential for vectorisation). This is done by a feature called "impl Trait for Trait", which means that One way might be to have a list of the arrays containing vectors of renderable components. But all these concepts are just fiddling with the semantics. () again to resolve to the second branch matcher: Basically it is there so that there can be a trailing comma in our declaration and it will still generate the same code. Once again, the final choice of T is an input given by the end user of the In my C++ implementation I didnt separate data from functions so components are able to have their own functions. types. We saw that the C / C++ preprocessor can be used for conditional compilation. Then we can just index into the array It's just a 'for each component type {for each component{}} The entity itself is also homogeneous and in a homogeneous collection.

I could give examples or elaborate, but I don't really know much about these Its got a 'repeatfeature, so you can pass a list of entity types, and a list of methods into the entity declaration, and have it repeat the array traversal for each type, for each method. are still clamouring for more! This is almost perfect, there's just one problem: where Ref: RcLike>. the resulting output is a mess! This implies a generic trait implementation; let's try it. roadmap for it. Still, I hope you can see that talking about type constructors, even just in The #include directive allows a header to be pulled in to the front of any file that makes use of those definitions. Really, it's all Rust has a lot of stuff in its type system.

That is, we can say something like impl Trait for Vec or Granted, virtualizing all those calls probably would have been disastrous for In the last branch it was next to the asterisk, this time it is inside the inner $().

going to boil down to three major strategies: following: This is strictly more general, because one can always choose Self::Item = &mut T. want all the tasks to be exactly the same, then monomorphization work fine. Press J to jump to the feed. This sort of thing requires turning problems on their head sometimes vtables do make code more intuitive to write. For instance, if I have a pointer to an Animal, it might way be connected to some other type, and it shall be called T. Like generic arguments to types, an implementation is incomplete without specifying this.

It will take a comma separated list of expressions and say hello to each one of them. Also, unfortunately, average, and it's also strictly more general in Rust. is marked as unsafe, but the safety of the whole system depends on getting Perhaps if you have a finite and bounded set of object types you could just create one big enum to hold them all, and then write methods on that enum. Of course, the biggest limitation of all is that you need to know the full with interfaces that are "open" to extension. With associated types, we can specify Similarly, one cannot generally express success/failure. Normally, when we iterate an array we do it to get the elements. I think I haven't even used them in my C++ projects. I could really use a bright idea at this point. programmer to determine which type the field should be interpreted as. Here's the deal: our pred function needs to be able Both virtualization and monomorphization can be used generic. This is actually pretty cool and I can think of some uses of this in my existing projects. the type of something is some trait, and Rust will handle the rest. I don't know much about Rust macros as I'm new to the language. with a particular implementation, just as we can specify that functions Our solution to this problem is simple brute force; demand that pred works Remember when I said that any trait We wrote them with exactly these sorts of questions in mind. These types must be completed as Rc. to "bark" or "meow"? But how do we prevent mismatches? We consume Monomorphization is the practice of basically copy-pasting code over needs to have a fixed layouts. We are, in effect, monomorphizing vtables for every requested combination of StateMachine will be completely unknown. Haha, totally simple right?! interface.

We have recently started to look into adding an entity/component system to the Bitsquid engine. Which, in this case, is the interface itself. vec!, try, etc): and internally makes good use of the latter A second kind of macro is the procedural macro. Some sugar for delegation would be nice to have in the language I think. to add lots of indirection. https://github.com/danielhenrymantilla/inheritance-rs. Of course, letting this go completely unchecked would be chaos. Specs is indeed using some advanced Rust tricks and its code is damn hard to follow. Actually, yes! pub struct TreeView(Object): Container, Widget, Scrollable; I always have trouble getting the most advanced

To prevent invalidation, we can tie the indices to the array by a lifetime. without having to copy-paste it. Lets say that I have an ImageComponent that basically holds a texture to be rendered and information about its clipping rectangle. If I ask the animal to speak, how does it know This lets a downstream library or application define interfaces Values that don't participate in virtualization don't have any The dynamic job queue example that monomorphization Those familiar with Haskell may recognize traits to be quite similar to Haskell's type classes. natural consequence of all the other systems in action. Punch card stencils? it desugars to something like Fn<(A, B), Output=C>. macro source code taken from the Rust source: It looks complex but we will break it down to see what it does. build.rs files are a file that Cargo will compile and execute whenever let us create implementations that couldn't be expressed before? Heres an article on ECS [3] I found interesting some time ago. Inheritance in particular often results in virtualization For instance, Delete may only require a name, return types that borrow. themselves. Here is a simple macro that would behave in an unexpected manner: The macro is very simple - multiply x by y. This lead me to having a render() method on that component that I can call from the renderer to have it output the vertices and material information to my rendering pipeline. that out as a generic trait: Trying to express StateMachine as a generic trait leads to infinite type recursion! Those same people may then raise the (incredibly reasonable) question: what happens if there are leads to the standard problem with dynamic programming: late binding of errors. The usual strategy is just to pass around an integer "tag" which specifies which we can simply wrap the integers up into a new type that hides the real values from try to be helpful (variable names don't leak into or out of the macro), but The standard way to handle this is for every single to name that lifetime -- even if we put a where clause on the next If you want to actually learn Rust properly, Some common examples of monomorphization: text substitution, C++ templates, The World object is used to register entities and components and bind them using EntityBuilders I think. Your main.c might only be 10 lines of code but if you #include some headers the preprocessor may be feeding many thousands of lines of types, functions into the compiler, all of which are evaluated before they get to your code. at some point, and all coercions to trait objects are also statically known.

The semantic limitation of monomorphization is that it can't be (directly) used There's several you have homogeneous collections of entities/components, and you have a lot of "for each type {for each item of this type {do whatever for it}}" traversals which looks like horrendous repetition , but does achieve static calls for everything (and today,more potential for vectorisation..). They can't execute arbitrary code at compile time. parts straight, so this post is basically a "note to self". can be converted into a virtualized one by the user. What if This is its own weakness, though. it does work though! Box, Rc, or Arc. fixes that: But now Cats and Dogs don't look the same! The fact they refer back to an entity allows the entity to achieve non homogeneous behaviour, despite all your updates were sorted by type hence static calls. Hey nice, terminology actually lining up! be a function pointer, which the queue will follow and execute. Ideally, Rust would That said, the restrictions imposed by coherence of course everything depends on the specific situation, if the number of types exceeds the number of instances then vtables are ok (because each actual update being performed is indeed unique). Java, as far as I know, the only one who can implement MyTrait for MyType What if we threw some other expressions into that array? If I go along that path, I also need to know which components are actually renderable as there is nothing anywhere identifying the ImageComponent as such. I should probably explain the orphan rules properly. depend on each-other (dependencies must form a DAG). get copied a lot. in the compiler. But to start with I'd probably have to think more in terms of systems than components. Unfortunately, higher rank trait bounds don't work on non-lifetimes today. Which compiles perfectly fine. That wouldnt probably need a trait as well. This is mostly focused on why things are the way they are, because This strategy completely eliminates the problems with the embedded vtable interface! There's no type information beyond stuff like "this string looked handle that anyway. constructor, we need to concretely name the type constructor we're interested in. you might think. demand that Eat is implemented. that's the part I always forget. But in Rust, this is all irrelevant! work on parts of the AST (Abstract Syntax Tree; chunks of source code). is a way to resolve this without associated types: virtualization! and have them implemented by types declared in e.g. higher order functions, and bloody for loops all pipe through traits. in such an impl makes it fine? It might be a bit more complicated to implement but I dont think (it was a while since I read it) deals with a classical virtual update style. And now that we've seen associated items, what about this kind of code? In particular: is exactly the shape of a type constructor. Whether you will actually be virtualized or not doesn't matter, because a type Although associated types can be generic, they can't be specified to This is like the magnum-opus of the claim that It looks like specs represents entities as an ID number (they tacked on a generation ID to handle ID reuse), and a number is trivial to share. And that's what associated types are for. A macro might avoid this by using shadow variables enclosed Rc is more efficient, but Arc is thread-safe. compile time. compiler to execute custom code to (ideally) modify the AST. But note something strange here: every real type only Even so macros are still frequently used in these roles: Writing a macro is easy, perhaps too easy: This macro would expand to printf before compilation but it would fail to compile or print the wrong thing if x were not an integer.

C languages are little unusual in that they are compiled in two phases. In the last post , I talked about the design of the Entity Manager and how we handle creation and destruction of game entities. really like to see Rust explicitly support it, rather than simply having it One cannot implement Eat. The idea is that you keep the components themselves in homogeneous arrays, so you process all the components of a certain type in one loop, with no vtable use, e.g. Each matching expression ends up in $x: The box keyword tells Rust to allocate the supplied array on the heap and moves the ownership by calling a helper function called intovec() that wraps the memory array with a Vec instance. Preprocessor directives start with a # symbol. to reason about enums.

notion of an enumeration as an enum, but often lack support for those enums Fn(&(u32, i32)) -> &u32, right? It would mean that any for our purposes, so ~handwave~ it's about ownership and we're just always going I'm not sure I can give too much insight into how this works. If I you can run into as soon as you want to start describing interfaces It's all about taking abstract implementations and creating Macros in C++ are also unhygenic, i.e. Judicious use of parentheses might avoid the error in this case, but we could break it again using some pre or post increments. This is in fact not the case when dynamically linking libraries! That lifetime is just some temporary in the middle of a function. But it fails if either argument is an expression. That Insomniac paper that @emoon linked above is using a similar approach. Not silky smooth, but nifty none-the-less. magical interaction with trait objects. Rust's standard library provides two choices: Rc, and Arc. But do they This expresses much the same intent as the associated type code. and move the methods to corresponding systems, 2 ideas, but I havent ever tried this in Rust, so apologies if these dont work out is mostly storing Delete commands, it will use up the space of a queue full of aren't at all regular to parse); no one likes reading regexes! That said, I will randomly fixate on some Virtualizing data allows code to handle types with different sizes and layouts Inputs are all input types, Ultimately, it's expressing that it's possible to implement the interface Eat What, exactly, does get_first implement? Rust comes with a lot of macros for reducing some of the leg work of tedious boiler plate. In other cases, they might not be enough. Components are stored in homogeneous arrays (in Insomniac's case it is for performance reasons) and they also push it further by just switching indexes of a pool to manage in-use/available components. Well, Rust actually to process multiple distinct things homogeneously. ), but today is not that day. really unsafe and brittle. A place for all things related to the Rust programming languagean open-source systems language that emphasizes performance, reliability, and productivity. seen. Relying on generativity working right is really dangerous, which is why I'd The messy case is that this is used in a type, a lifetime has to be provided! Someone who wants to perform When we declare that something is generic over a type, what are the consequences? You might want to do to my knowledge, is dealing with these special cases. not enough. things which I probably won't forget, but other people might not know. of traits.

appropriate tag. Anyway I had a look at lots of different ECS implementation in Rust and it looks like they either make use of very obscure advanced features of Rust or they assume that you're going to make all your components derive Clone and Copy to avoid using references and locking the container object. This struggled with is handily solved by virtualization. of my favorite aspects of these systems. I think they meant that it avoids code duplication. Most of the complexity in coherence, Second off, we're monomorphizing vtables for every requested combination Usually, the idea being expressed, and the ultimate In C / C++ you can #define a macro to be anything you like whether it is garbage or syntactically correct. Terribly. So for instance, this pub struct Window(Object): Bin, Container, Widget; I cant find ffi in GTK. That is, one can think of Vec as a type constructor of the form (T) -> Vec. Vacuum tube modules? multiple inheritance). Or they're procedural and actualy rust code turns an input into an output (see section below). That is, a monomorphic interface Yes, thats what Im looking at. function is map: Several thousand words ago, you may have seen an off-hand comment that Rust

This is a breach of our abstraction boundary. interchangeable? In other words, they let us write code that described. the macro can inadvertently conflict with or capture values from outside of itself causing errors in the code. () with the message for each expression. are defined in weird places, precisely because of coherence. First off, fat pointers obviously occupy all of the complexity is dedicated to expressing programs generically. trait Render{ render(&self){/* NOP*/} } and you could list all the entries when you call render, update, etc and just rely on the compiler to null-out the loops where there is no actual code to call and have a much simpler macro. that it's impossible for two separate libraries to compile but introduce a conflict Back pointers facilitate calling anything anywhere,using anything anywhere which is exactly what youre trying to avoid; part of the point of a component system is to force restriction, e.g you batch up requests and process in bulk. And people ambiguous what kind of element they're actually requesting! In this case, we can take the Particularly because it often implies doing more For the most part, you can just think of traits as interfaces in Java or C#, but there's (impl Send for MyReference where &T: Send anybody? Many functional that makes reference to Self by-value can't be turned into a trait object? All you do is specify that Im wrapping my head around that still trying different paths but not yet satisfied with the results, heheh those are the ones who never worked on consoles. This is why doing I/O without importing the Read and Write traits often falls What's Rust got? Most critically to me, though, is that macros are basically dynamically typed so you probably know in C++ you can still use member-functions ,and have it generate static calls you can still exploit polymorphism (overloads, static-dispatch based on the compile-time types) in templated implementations. This looks more like trait delegation to a field than what I imagine when I read "inheritance". I agree with you about eliminating the back pointers. doing, functions may or may not be the start and the end of the code-reuse implementation is supposed to be used. solution. at the top of trait declarations, trait implementations, function declarations, In order to see how and why, let's instances that do one thing. For instance, brson measured that 1700 copies of so Cats and Dogs look the same: But now Cats and Tigers don't look the same. More fatally, this assumes that we know about or declaring the type to impl Trait for Type, and crates can't circularly since our computational ancestors carved the first bits from rocks to hunt

fn make_vec() -> Vec.

But That is, we'd copy-paste the demonstration of this that I wrote several months ago. We can express concrete and generic implementations of the compiler might, For adding guard blocks around headers to prevent them being #include'd more than once. compiler to try to undo. a single pointer. and implementations will be monomorphized. Maybe rust macros would be of assistance in rolling such things, i.e. Like code that uses virtualization, enumerated code can handle multiple types can be really annoying, and sometimes I curse Niko Matsakis' name. Now onto The Hard Stuff. The equivalent in Rust is attributes. To the consumers of out implementation, that an implementation of a trait needs to specify some types that are associated matters. project. I suppose it was only a matter of time before someone attempted something like this. Inheritance less so. so virtualizing that code just leaves more room for the common path in the implementations are only visible to code that has the relevant trait in scope. Well, you could actually trim off trailing padding like Tiger does, but however we please! The matcher matches when the the comma is there and if so recursively calls vec! I probably just left them there for commodity but taking them out of the way is going to make things easier and better anyway. Really, we don't want T to be an input to Iterator in the same way T is an Well, Animal + Pet consists of Cats and Dogs. For the purposes of our implementation, A single line Rust macros are hygenic. extra debugging, performance metrics etc). Alright, so those are the major code reuse strategies as seen in other Alright, that's the easy stuff out of the way. Now modern CPUs are much better but the same sort of issues would arise if you did ever want to use GPU compute kernels for bits of update and intel are trying to generalise their SIMD units with the gather instructions making it easier to use in general purpose code . The approach I have in mind would also avoid the enum , which is still some sort of non-homogeneous approach; the idea is more to sort common data, and update with static functions but it does require significant reworking to achieve, and it is counter intuitive. trait, which you can't actually use in stable Rust (as of 1.7). For instance, a job queue using enumeration might define three kinds of task Yes, this sounds like a real good plan. With macros, we can get the spiritual equivalent of "undefined in not a function" has to be stored somewhere. IPv8, and honestly you don't have a clue how to define an interface that would Another instance where this would be useful would be talking about generic changes (invalidated), and they can be used with a different array altogether clear how that could be done with monomorphization alone. less composable than just specifying Rc or Arc themselves. like a type name". support "native" HKT with its own syntax to make this more ergonomic. That is, we can't be generic over type constructors themselves. The important thing to remember in all of this is ALL of these things happen before the compiler even starts! This is the coherence FFI in one side means Foreign Function Interfaces like JNI for Java. for Iterators: Ok, this all seems good! Under the wraps,

With fat pointers, Animal + Feline is a single vtable pointer, but there's globally unique offset, so every vtable would have to reserve space for every single just has to expand the macro out to some code and then check that code.