chainerror/booksrc/tutorial2.md

Simple Chained String Errors

With relatively small changes and the help of the cherr! macro of the chainerror crate the String errors are now chained together.

Press the play button in the upper right corner and see the nice debug output.

use crate::chainerror::*;
{{#include ../examples/tutorial2.rs:2:}}
# #[allow(dead_code)]
# mod chainerror {
{{#includecomment ../src/lib.rs}}
# }

What did we do here?

{{#include ../examples/tutorial2.rs:13:15}}

The macro cherr!(olderror, newerror) stores olderror as the source/cause of newerror along with the filename (file!()) and line number (line!()) and returns newerror.

Err()? then returns the inner error applying .into(), so that we again have a Err(Box<Error>) as a result.

The Debug implementation of ChainError<T> (which is returned by cherr!()) prints the Debug of T prefixed with the stored filename and line number.

ChainError<T> in our case is ChainError<String>.