What is the most idiomatic way to merge two error types?

Question

I have a type Foo whose methods may "raise" errors of an associated type Foo::Err.

pub trait Foo {
    type Err;
    
             


        

Answer 1

You should use a trait object Error, and you return the first error that you encounter:

pub trait Bar {
    fn bar(&mut self, foo: F) -> Result>;
}

or implement your trait like this:

impl Bar for MyType {
    type Err = Box;

    fn bar(&mut self, foo: F) -> Result;
}

If you really want to have your two errors (but this is strange because one error suffices to make the process not ok), you can use a crate like failure to create an "error trace".

As a general advice, you should not forget to use the traits from std to add more semantic to your code.