Struggling with generics traits

help
1

Hey there,
I spent the last couple of weeks reading the Book and after that, I started making some simple exercises, right now I'm stuck with a problem that involves Traits.
For this exercise I want to create a Farm generic trait and the data type bound must be another trait (with fantasy I called it Animal).
I defined the traits as follows:

use anyhow::Result;

pub trait Animal {
    fn animal_type() -> String;
}

pub trait Farm<T> where T: Animal {
    fn add_animal(&self, animal: T) -> Result<()>;
    fn remove_animal(&self, animal: T) -> Result<()>;
    fn merge_farm(self, farm: dyn Farm<T>) -> dyn Farm<T>;
}

The first two functions are easy to implement, I will just need to mutate the internal HashMap to add or remove animals.
But I don't know how to implement the merge_farm function. Well I know the logic, add animals from both farms into a new one but how can I create the farm struct and return it?
And second question: why the compiler is telling me to use the keyword dyn in the second parameter and in the type to return?

2

This type seems really confused to me. You see, the dyn keyword is only used to specify you want to use a trait object i.e. dynamic dispatch. But that only works with traits.
Assuming Farm<T> is indeed a type, the suggestion makes no sense. Even if the type was a dynamically-sized type (DST), I wouldn't expect a suggestion to add the dyn keyword.

Can you post the definition of the type?

3

It seems to be a trait, based on the code OP posted.

@fbrv, as for your question, you probably want

fn merge_farm<F, G>(self, farm: F) -> G
    where
        F: Farm<T>,
        G: Farm<T>;

or some variant thereof, e.g.:

fn merge_farm<F, G>(self, farm: F) -> Self
    where
        F: Farm<T>;

both of which do use generics. Using dynamic dispatch (dyn Trait) is the exception, and not the rule, in Rust.

1 Like
4

Wouldn't

fn merge_farm(self, farm: Self) -> Self;

be enough here ?

5

I don't know; the first case is strictly more general, as it allows merging non-Self-typed values as well.

1 Like
6

dyn Trait can't exist on its own. It's never possible to pass, store or return such type.

You can only have it hidden behind a pointer-based type, like Box<dyn Trait> or &dyn Trait.

7

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