Dependend trait def problem

help
1

Suppose I have this

pub trait A {
    type valueA;
    fn foo(&Self) -> bool;
}

pub trait B<T:A> {
    type valueB;    
    fn bar(&Self::valueA) -> valueB;
}

And I want to do something like this:

pub trait traid_collection: A + B <A> + ... more traits {
    type version;
}

impl <T> traid_collection for T
    where T: A + B<A> + more traits
{ 
        type version = u64;
}

This does not compile. The reason to use trait_collection is to make change in the amount and dependencies of the traits in the system more manageable. It there a way to get this to compile? The idea arose from a discussion here: Messy generic type dependencies

2 
pub trait A {
    type ValueA;
    fn foo(&self) -> bool;
}

pub trait B<T:A> {
    type ValueB;    
    fn bar(_: &T::ValueA) -> Self::ValueB;
}

pub trait Traits<T: A>: A + B<T> + Send + Sync {
    type Version;
}

impl <T, U: A> Traits<U> for T
    where T: A + B<U> + Send + Sync
{ 
        type Version = u64;
}
3

Unfortunately this defies the entire purpose of trait-collection. We don't want many type parameters but capture everything inside a single one.

The reason is, that everytime the code base advances and a new trait appears that might depend complicated on other traits and generics, one must rewrite the signuters of all types that depend on this. This is not fixable locally (inside a single file) but requires changes on many files.

4

Different types can implement trait B<T> for different T's. You can't capture that in a single umbrella trait.

1 Like
5

Then how do programmers handle this without tweaking the entire dependent codebase everytime a new trait appears?

6

It does not depend on the type only on the trait bound of that type, so the type can stay abstract

7

No it can't, using an associated type is not just using the trait bound. It can play into layouts, so you can't just erase the type. That's not how dynamic dispatch works.

@red75prime's solution does make things simpler because you only need to refer to the umbrella trait instead of all it's parts.

8

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