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Introduce some {:#} and {:#?} alt formatting representations (#42)
When you print an error object using "{}" or to_string(), only the outermost underlying error or context is printed, not any of the lower level causes. This is exactly as if you had called the Display impl of the error from which you constructed your anyhow::Error.
Failed to read instrs from ./path/to/instrs.json
To print causes as well using anyhow's default formatting of causes, use the alternate selector "{:#}".
Failed to read instrs from ./path/to/instrs.json: No such file or directory (os error 2)
The Debug format "{:?}" includes your backtrace if one was captured. Note that this is the representation you get by default if you return an error from
fn main
instead of printing it explicitly yourself.Error: Failed to read instrs from ./path/to/instrs.json Caused by: No such file or directory (os error 2) Stack backtrace: 0: <E as anyhow::context::ext::StdError>::ext_context at /git/anyhow/src/backtrace.rs:26 1: core::result::Result<T,E>::map_err at /git/rustc/src/libcore/result.rs:596 2: anyhow::context::<impl anyhow::Context<T,E> for core::result::Result<T,E>>::with_context at /git/anyhow/src/context.rs:58 3: testing::main at src/main.rs:5 4: std::rt::lang_start at /git/rustc/src/libstd/rt.rs:61 5: main 6: __libc_start_main 7: _start
To see a conventional struct-style Debug representation, use "{:#?}".
Error { context: "Failed to read instrs from ./path/to/instrs.json", source: Os { code: 2, kind: NotFound, message: "No such file or directory", }, }
If none of the built-in representations are appropriate and you would prefer to render the error and its cause chain yourself, it can be done something like this:
use anyhow::{Context, Result}; fn main() { if let Err(err) = try_main() { eprintln!("ERROR: {}", err); err.chain().skip(1).for_each(|cause| eprintln!("because: {}", cause)); std::process::exit(1); } } fn try_main() -> Result<()> { ... }