Question about how to think about Enum data type and just struct: Let’s assume that there is a piece of code in language which has traditional OOP. There is a need to have an array of some objects which have to be processed in a bulk. So let’s assume that all items in array have to be the same type. So there could be base class which is abstract (or might be a trait) and other child classes which extends base abstract class and contains specific for child classes’ fields. Taking an example in Scala it would look like this:
trait Fruit
case object Apple extends Fruit
case object Orange extends Fruit
case object Plum extends Fruit
case class GlassOfJuice(fruits: Set[Fruit])
// ...
val fruits = Seq(Apple, Orange, Plum)
def squeezer(fruits: Seq[Fruit]): GlassOfJuice =
fruits.foldLeft(GlassOfJuice(Set.empty))((glass, fruit) => glass.copy(fruits = glass.fruits + fruit))How similar code might look like in rust? Should Fruit and all child be a enum data type? Or in case of usage struct data type for it. How squeezerfunction would look like since structs can’t extends other structs.
Why should
Means, the initialization of the
squeezer return GlassOfJuice with a set of fruits, while the class only contains a set of fruits?Means, the initialization of the
GlassOfJuice could take the fruits and set it as itselfs only property.
well, propbably a bad example :-) I just wanted to simplify things for the example. I’m actually interesting in how the same code could be expressed in rust if not take into account that
GlassOfJuice could be simle initiated like this: GlassOfJiuce(fruits.toSet)
Maybe like this.
aha, I see. So enum is the way to go in such example
I'm sure, this will depend on your problem.
@btbvoy There is no one way to go about this. Traits seem like a reasonable solution as well in this example.
Yeah, for sure.
This question very much reminds me of the RustConf talk without_boats gave: https://www.youtube.com/watch?v=erJdCIti_O8&list=PLE7tQUdRKcybLShxegjn0xyTTDJeYwEkI&index=8
I should watch it. Later. Otherwise, I will stuck on conf videos the whole day. :shipit:
originally I’m trying to implement an application using CQRS Event Sourcing. In my original use case I have a list of Commands (which in my example - represented as
Fruits) and a bulk of commands are translated into a sequence of events. So I started implementation just with structs but later on I understand if I want to have separated handler of commands and I want to process them in bulk - it need to have base type for all of them or imlement trait for them (but the idea with trait doesn’t look nice since it’s only tagging / putting a label on command in order to sutisfy compiler’s requirements and there is no any method on a command which is required for now). And I re-checked docs again and found an enum data type and was in doubts which way to go :-) and yes, I agree in software there is “it depends” :-)
@iamsebastian thanks for giving an example :-) I’ll give it a try
@jplatte thanks for the link to the talk, following an @iamsebastian I’ll take a look later a bit
Okay, don't expect a clear answer to your question from the talk though :D
The main message I got from it is that it's still an open question which solution would be idiomatic for rust code facing this kind of question. But of course it's an interesting talk, and you should still watch it :)
You're welcome, @btbvoy
anybody use atom? i'm facing a problem with rust-linter package
Yeah, but just on the small MacBook for travel and quick read around.
Vim on the real work station.
Vim on the real work station.
i uninstalled previous rust version then installed the latest version using rustup, now rust-linter gives error :worried:
Well sorry, but I don't think I can help you with this. No MacBook here atm. :partly_sunny:
@jplatte :-) just watched this talk. You are right indeed - there is no clear answer for such kind of questions
anyway that we nice talk :-)
oops, typo: was nice talk*
I am, like, absolutely stuck on how to handle a lifetime borrow issue. Is it acceptable for me to post a short snippet that is causing the issue in here and ask for help?
Hello everyone. I got a question. When you use an extern crate. should you signal it only in main.rs? or in every files that use it?
I only put
extern crate in the main.rs file.
You do need to
use in every file/module that needs it. extern exposes the top-level symbol though.
So like
extern crate chrono; doesn't require use chrono; in the same mod/file, but you will need use chrono; in other files. (You may also want to do something like use chrono::*; to bring symbols into scope unqualified.
@CryptArchy If it's only a few lines, paste it here as a code block. Otherwise I'd say linking to it would be better.
If you scroll up a bit you'll see it's not uncommon to paste code here btw ;)
Sure, the problematic area is pretty small, code block should be fine.
fn start_kafka(tx: Sender<Message>) {
let mut consumer = Consumer::from_hosts(vec!["example.com".to_owned()]); //... some other config omitted
loop {
for ms in consumer.poll().unwrap().iter() {
for m in ms.messages() {
tx.send(m);
}
consumer.consume_messageset(ms);
}
consumer.commit_consumed().unwrap();
}
}
So, the reported problem is that the anonymous value of
consumer.poll().unwrap() doesn't live long enough. But the actual problem is that I'm passing m (the Message) into tx a Sender<Message> type from the std::mpsc lib
I don't know how to fix this though. Conceptually, I think I get it. The channel is holding the value, so it can't be destroyed (which it will be at the end of the loop scope).
I'll try to reproduce it in my playground dir... Sender and Consumer are from std::sync::mpsc, I assume? What's message?
The Message type itself looks like this: (from https://spicavigo.github.io/kafka-rust/kafka/consumer/struct.Message.html)
pub struct Message<'a> {
pub offset: i64,
pub key: &'a [u8],
pub value: &'a [u8],
}It's not marked as Clone so I can't copy the message directly. I've tried copying the values off of it but my own external clone method inherits the 'a lifetime from it, so the copy is still bound to the same lifespan.
Ah ^^
Oh wait, Consumer isn't in std::sync::mpsc
But I think I should be able to drop in kafka as a dependency
No, Consumer is from the rust-kafka crate as well. That
<'a> lifetime from the Message propogates all the way up to the poll method
I'm using
kafka = "0.5" in my dependencies
Hmmm, I used
cargo add (from cargo-edit) and it added 0.5.0, so that seems to be the latest.
The minimal code for making a consumer that would likely build (though not run I'm guessing) would look like
let mut consumer = Consumer::from_hosts(vec!["example.com:9092".to_owned()])
.with_topic_partitions("test".to_owned(), &[0])
.with_fallback_offset(FetchOffset::Earliest)
.with_group("mygroup".to_owned())
.with_offset_storage(GroupOffsetStorage::Kafka)
.create()
.unwrap();
I'm getting different errors though..
Well, maybe not "minimal" but close enough.
error: no method named `poll` found for type `kafka::consumer::Builder` in the current scope
--> src/main.rs:10:28
|
10 | for ms in consumer.poll().unwrap().iter() {
| ^^^^
error: no method named `consume_messageset` found for type `kafka::consumer::Builder` in the current scope
--> src/main.rs:14:22
|
14 | consumer.consume_messageset(ms);
| ^^^^^^^^^^^^^^^^^^
error: no method named `commit_consumed` found for type `kafka::consumer::Builder` in the current scope
--> src/main.rs:16:18
|
16 | consumer.commit_consumed().unwrap();
| ^^^^^^^^^^^^^^^
That is what I get for your code plus
extern crate kafka;
use std::sync::mpsc::Sender;
use kafka::consumer::{Consumer, Message};
Replace the consumer binding with the code I just posted.
Okay
I had elided the critical "create().unwrap()" bit that gives you an actual consumer and not a builder
I believe my usings are like:
use kafka::consumer::{Consumer, FetchOffset, GroupOffsetStorage};
use kafka::client::fetch::Message;
use std::sync::mpsc::{channel, Sender, Receiver, TryRecvError};
still getting a different error interestingly:
error[E0308]: mismatched types
--> src/main.rs:19:25
|
19 | tx.send(m);
| ^ expected struct `kafka::consumer::Message`, found reference
|
= note: expected type `kafka::consumer::Message<'_>`
= note: found type `&kafka::consumer::Message<'_>`
I'll check if this is a compiler branch inconsistency...
Ah, no I think it's just leftover from experiments I was doing. Sorry about that, change the signature to be
fn start_kafka(tx: Sender<Message>) {
Okay, no, would have been weird too
that's how it looks..
I'll change it to send(*m) and see if that works
It'll give you a "moving out of borrowed context" error
I'm sorry, the opposite of what I just said, change the signature to be:
fn start_kafka(tx: Sender<&Message>) {
Hah, okay
I was trying to use a clone method that would take the &Message and copy the values into a new Message so that the Sender would own it (didn't work)
ahh, yeah okay, now I see the error you were talking about
So it complains about the
poll line but if you comment out the tx.send(m) it will compile so the source of the problem is trying to hold onto that m value which is then just propagating back up.
Hm, I think I lack the knowledge about the context and what exactly this kafka library does to help. Maybe I can have a closer look tomorrow, if nobody else figures this out until then.
You should probably ask on the IRC channel too if you haven't done that yet.
There are more active people there