How to set a variable to implementations of generic typed Trait in rust?

Question

I am new to rust and I am trying to write an app that basically uses one of many possible services to fetch some data, transform it and save to my database.

I am trying to do something like a generic interface from Java, to instantiate the correct service based on command line input and use it throughout the program.

I have coded something like this:

use anyhow::Result;

pub trait Service<T> {
    fn get_values(&self, id: u64) -> T;
    fn convert_data(&self, input: T, output: &mut Vec<MyDataStruct>) -> Result<String>;
}

and two different implementations:

struct ServiceA {
    clientA: DataRetrieverForA,
}

struct ServiceB {
    clientB: DataRetrieverForB,
}

impl Service<Result<ADataStruct>> for ServiceA {
    fn get_values(&self, id: u64) -> Result<ADataStruct>;
    fn convert_data(&self, input: Result<ADataStruct>, output: &mut Vec<MyDataStruct>) -> Result<String>;
}

impl Service<BDataStruct> for ServiceB {
    fn get_values(&self, id: u64) -> BDataStruct;
    fn convert_data(&self, input: BDataStruct, output: &mut Vec<MyDataStruct>) -> Result<String>;
}

the self.get_values(id) uses the self.clientX to retrieve data and self.convert_data(input, &mut output) transforms data from ADataStruct and BDataStruct to MyDataStruct before saving it to my database.

The app will run using either ServiceA or ServiceB depending on command line input:

fn main() {
    // ...

    let service: Box<&(dyn Service<_>)> = match cli_service {
        Service::A => { Box::new(&ServiceA::new(...) }
        Service::B => { Box::new(&ServiceB::new(...) }
    };

    //...
}

I have tried many changes, mostly based on https://doc.rust-lang.org/book/ch10-02-traits.html and https://doc.rust-lang.org/book/ch17-02-trait-objects.html but I can't find an example that handles functions that use a generic type from trait definition. When I removed the generic parameter and fixed it to some common struct for testing, the code compiled and ran with no errors. So my guess is my mistake is with generic/trait usage.

The error I get with this code:


error[E0277]: the trait bound `ServiceB: Service<ADataStruct>` is not satisfied
  --> ori-runner\src\main.rs:40:37
   |
40 | ... { Box::new(&ServiceB::new(params)...
   |       -------- ^^^^^^^^^^^^^^^^^^^^^^ the trait `Service<ADataStruct>` is not implemented for `ServiceB`
   |       |
   |       required by a bound introduced by this call
   |
   = help: the following implementations were found:
             <ServiceB as Service<BDataStructure>>
   = note: required for the cast to the object type `dyn Service<ADataStruct>>`

What am I doing wrong? It is obvious the first match type is defining the '_' of the dyn Service variable, but I am out of ideas and google searches...

Thanks!

the problem is that your pointer should use the same type signature, Box<dyn Service<SOME_TYPE>>, SOME_TYPE is different for your implementations, so it is never gonna compile. — Netwave
– Netwave, Commented Mar 26, 2022 at 21:00
Yes, that is the problem. But if every service would return something different, which approach should I use then? I thought the _ was like a ? in java. If I can't use Generic typed traits to define method signatures, wouldn't they be useless? I feel I am missing something here... — oak
– oak, Commented Mar 26, 2022 at 21:39
if each service doesnt use the same data you cannot do it in this way. — Netwave
– Netwave, Commented Mar 26, 2022 at 21:45
Understood. Would you have a suggestion to give me? I would appreciate it if you could point me to some direction on how to implement this. — oak
– oak, Commented Mar 26, 2022 at 21:52
One possible solution is to put the rest of your main in a helper function/struct that is generic over a service (<T, S: Service<T>>) and then call the helper with the appropriate Service type based on the value of cli_service. — EvilTak
– EvilTak, Commented Mar 26, 2022 at 21:55

Netwave · Accepted Answer · 2022-03-26 21:55:08Z

2

Since the types are different, one option would be to wrap them in an enum and have some method/s for computing whatever needed depending on the decision. The enum wrapper would abstract the services operations.

struct DataRetrieverForA {}
struct DataRetrieverForB {}

struct ADataStruct {}
struct BDataStruct {}
struct MyDataStruct {}

struct ServiceA {
    clientA: DataRetrieverForA,
}

struct ServiceB {
    clientB: DataRetrieverForB,
}

impl ServiceA {
    fn get_values(&self, id: u64) -> Result<ADataStruct, ()> {
        Ok(ADataStruct {})
    }
    fn convert_data(
        &self,
        input: Result<ADataStruct, ()>,
        output: &mut Vec<MyDataStruct>,
    ) -> Result<String, ()> {
        Ok("foo".into())
    }
}

impl ServiceB {
    fn get_values(&self, id: u64) -> BDataStruct {
        BDataStruct {}
    }
    fn convert_data(
        &self,
        input: BDataStruct,
        output: &mut Vec<MyDataStruct>,
    ) -> Result<String, ()> {
        Ok("bar".into())
    }
}

enum Services {
    A(ServiceA),
    B(ServiceB),
}

impl Services {
    fn a() -> Self {
        Self::A(ServiceA {
            clientA: DataRetrieverForA {},
        })
    }

    fn b() -> Self {
        Self::B(ServiceB {
            clientB: DataRetrieverForB {},
        })
    }

    fn compute(self) {
        todo!()
    }
}

fn main() {
    let arg = "a";

    let service = match arg {
        "a" => Services::a(),
        _ => Services::b(),
    };
    service.compute();
}

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answered Mar 26, 2022 at 21:55

Netwave

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2 Comments

oak Over a year ago

I am trying this approach, thanks for your example. How would I get access to get_values and convert_data service methods from the enum's compute function?

Netwave Over a year ago

you would have to match the enum: match self {...}

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