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< ol class = "chapter" > < li class = "expanded affix " > < a href = "index.html" > chainerror< / a > < / li > < li class = "expanded " > < a href = "tutorial1.html" > < strong aria-hidden = "true" > 1.< / strong > Simple String Errors< / a > < / li > < li class = "expanded " > < a href = "tutorial2.html" > < strong aria-hidden = "true" > 2.< / strong > Simple Chained String Errors< / a > < / li > < li class = "expanded " > < a href = "tutorial3.html" > < strong aria-hidden = "true" > 3.< / strong > Mapping Errors< / a > < / li > < li class = "expanded " > < a href = "tutorial4.html" > < strong aria-hidden = "true" > 4.< / strong > Saving coding chars< / a > < / li > < li class = "expanded " > < a href = "tutorial5.html" > < strong aria-hidden = "true" > 5.< / strong > The source() of Errors< / a > < / li > < li class = "expanded " > < a href = "tutorial6.html" > < strong aria-hidden = "true" > 6.< / strong > Downcast the Errors< / a > < / li > < li class = "expanded " > < a href = "tutorial7.html" > < strong aria-hidden = "true" > 7.< / strong > The root cause of all Errors< / a > < / li > < li class = "expanded " > < a href = "tutorial8.html" > < strong aria-hidden = "true" > 8.< / strong > Finding an Error cause< / a > < / li > < li class = "expanded " > < a href = "tutorial9.html" > < strong aria-hidden = "true" > 9.< / strong > Selective Error Handling< / a > < / li > < li class = "expanded " > < a href = "tutorial10.html" > < strong aria-hidden = "true" > 10.< / strong > ErrorKind to the rescue< / a > < / li > < li class = "expanded " > < a href = "tutorial11.html" > < strong aria-hidden = "true" > 11.< / strong > Debug for the ErrorKind< / a > < / li > < li class = "expanded " > < a href = "tutorial12.html" > < strong aria-hidden = "true" > 12.< / strong > Deref for the ErrorKind< / a > < / li > < li class = "expanded " > < a href = "tutorial13.html" > < strong aria-hidden = "true" > 13.< / strong > Writing a library< / a > < / li > < li class = "expanded " > < a href = "tutorial14.html" > < strong aria-hidden = "true" > 14.< / strong > Going back to std< / a > < / li > < li class = "expanded affix " > < a href = "end.html" > The End< / a > < / li > < / ol >
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< main >
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< h1 > < a class = "header" href = "#chainerror" id = "chainerror" > chainerror< / a > < / h1 >
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< p > < a href = "https://travis-ci.org/haraldh/chainerror" > < img src = "https://travis-ci.org/haraldh/chainerror.svg?branch=master" alt = "Build Status" / > < / a >
< a href = "https://crates.io/crates/chainerror" > < img src = "https://img.shields.io/crates/v/chainerror.svg" alt = "Crate" / > < / a >
< a href = "https://docs.rs/chainerror/" > < img src = "https://img.shields.io/badge/api-rustdoc-blue.svg" alt = "Rust Documentation" / > < / a > < / p >
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< p > < code > chainerror< / code > provides an error backtrace like < code > failure< / code > without doing a real backtrace, so even after you < code > strip< / code > your
binaries, you still have the error backtrace.< / p >
< p > < code > chainerror< / code > has no dependencies!< / p >
< p > < code > chainerror< / code > uses < code > .source()< / code > of < code > std::error::Error< / code > along with < code > line()!< / code > and < code > file()!< / code > to provide a nice debug error backtrace.
It encapsulates all types, which have < code > Display + Debug< / code > and can store the error cause internally.< / p >
< p > Along with the < code > ChainError< T> < / code > struct, < code > chainerror< / code > comes with some useful helper macros to save a lot of typing.< / p >
< p > Debug information is worth it!< / p >
< p > Now continue reading the
< a href = "https://haraldh.github.io/chainerror/tutorial1.html" > Tutorial< / a > < / p >
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< h2 > < a class = "header" href = "#example" id = "example" > Example:< / a > < / h2 >
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< p > Output:< / p >
< pre > < code > $ cargo run -q --example example
Main Error Report: func1 error calling func2
Error reported by Func2Error: func2 error: calling func3
The root cause was: std::io::Error: Kind(
NotFound
)
Debug Error:
examples/example.rs:45: func1 error calling func2
Caused by:
examples/example.rs:20: Func2Error(func2 error: calling func3)
Caused by:
examples/example.rs:13: Error reading 'foo.txt'
Caused by:
Kind(NotFound)
< / code > < / pre >
< pre > < code class = "language-rust ignore" > use chainerror::*;
use std::error::Error;
use std::io;
use std::result::Result;
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fn do_some_io() -> Result< (), Box< Error + Send + Sync> > {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
Ok(())
}
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fn func3() -> Result< (), Box< Error + Send + Sync> > {
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let filename = " foo.txt" ;
do_some_io().map_err(mstrerr!(" Error reading '{}'" , filename))?;
Ok(())
}
derive_str_cherr!(Func2Error);
fn func2() -> ChainResult< (), Func2Error> {
func3().map_err(mstrerr!(Func2Error, " func2 error: calling func3" ))?;
Ok(())
}
enum Func1Error {
Func2,
IO(String),
}
impl ::std::fmt::Display for Func1Error {
fn fmt(& self, f: & mut ::std::fmt::Formatter) -> ::std::fmt::Result {
match self {
Func1Error::Func2 => write!(f, " func1 error calling func2" ),
Func1Error::IO(filename) => write!(f, " Error reading '{}'" , filename),
}
}
}
impl ::std::fmt::Debug for Func1Error {
fn fmt(& self, f: & mut ::std::fmt::Formatter) -> ::std::fmt::Result {
write!(f, " {}" , self)
}
}
fn func1() -> ChainResult< (), Func1Error> {
func2().map_err(|e| cherr!(e, Func1Error::Func2))?;
let filename = String::from(" bar.txt" );
do_some_io().map_err(|e| cherr!(e, Func1Error::IO(filename)))?;
Ok(())
}
fn main() {
if let Err(e) = func1() {
match e.kind() {
Func1Error::Func2 => eprintln!(" Main Error Report: func1 error calling func2" ),
Func1Error::IO(filename) => {
eprintln!(" Main Error Report: func1 error reading '{}'" , filename)
}
}
if let Some(e) = e.find_chain_cause::< Func2Error> () {
eprintln!(" \nError reported by Func2Error: {}" , e)
}
if let Some(e) = e.root_cause() {
let ioerror = e.downcast_ref::< io::Error> ().unwrap();
eprintln!(" \nThe root cause was: std::io::Error: {:#?}" , ioerror);
}
eprintln!(" \nDebug Error:\n{:?}" , e);
}
}
< / code > < / pre >
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< h2 > < a class = "header" href = "#features" id = "features" > Features< / a > < / h2 >
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< p > < code > no-fileline< / code >
: completely turn off storing filename and line< / p >
< p > < code > display-cause< / code >
: turn on printing a backtrace of the errors in < code > Display< / code > < / p >
< p > < code > no-debug-cause< / code >
: turn off printing a backtrace of the errors in < code > Debug< / code > < / p >
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< h1 > < a class = "header" href = "#simple-string-errors" id = "simple-string-errors" > Simple String Errors< / a > < / h1 >
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< p > An easy way of doing error handling in rust is by returning < code > String< / code > as a < code > Box< std::error::Error> < / code > .< / p >
< p > If the rust < code > main< / code > function returns an < code > Err()< / code > , this < code > Err()< / code > will be displayed with < code > std::fmt::Debug< / code > .< / p >
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< p > As you can see by running the example (by pressing the " Play" button in upper right of the code block),
this only
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prints out the last < code > Error< / code > .< / p >
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< pre > < code > Error: StringError(" func1 error" )
< / code > < / pre >
< p > The next chapters of this tutorial show how < code > chainerror< / code > adds more information
and improves inspecting the sources of an error.< / p >
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< p > You can also run the tutorial examples in the checked out
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< a href = "https://github.com/haraldh/chainerror" > chainerror git repo< / a > .< / p >
< pre > < code > $ cargo run -q --example tutorial1
< / code > < / pre >
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< pre > < pre class = "playpen" > < code class = "language-rust" > use std::error::Error;
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use std::io;
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use std::result::Result;
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fn do_some_io() -> Result< (), Box< Error + Send + Sync> > {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
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Ok(())
}
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fn func2() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(_) = do_some_io() {
Err(" func2 error" )?;
}
Ok(())
}
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fn func1() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(_) = func2() {
Err(" func1 error" )?;
}
Ok(())
}
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fn main() -> Result< (), Box< Error + Send + Sync> > {
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func1()
}
< / code > < / pre > < / pre >
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< h1 > < a class = "header" href = "#simple-chained-string-errors" id = "simple-chained-string-errors" > Simple Chained String Errors< / a > < / h1 >
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< p > With relatively small changes and the help of the < code > cherr!< / code > macro of the < code > chainerror< / code > crate
the < code > String< / code > errors are now chained together.< / p >
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< p > Press the play button in the upper right corner and see the nice debug output.< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > use chainerror::*;
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use std::error::Error;
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use std::io;
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use std::result::Result;
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fn do_some_io() -> Result< (), Box< Error + Send + Sync> > {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
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Ok(())
}
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fn func2() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(e) = do_some_io() {
Err(cherr!(e, " func2 error" ))?;
}
Ok(())
}
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fn func1() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(e) = func2() {
Err(cherr!(e, " func1 error" ))?;
}
Ok(())
}
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fn main() -> Result< (), Box< Error + Send + Sync> > {
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func1()
}
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< span class = "boring" > #[allow(dead_code)]
< / span > < span class = "boring" > mod chainerror {
< / span > {{#includecomment ../src/lib.rs}}
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
< h3 > < a class = "header" href = "#what-did-we-do-here" id = "what-did-we-do-here" > What did we do here?< / a > < / h3 >
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< pre > < code class = "language-rust ignore" > if let Err(e) = do_some_io() {
Err(cherr!(e, " func2 error" ))?;
}
< / code > < / pre >
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< p > The macro < code > cherr!(olderror, newerror)< / code > stores < code > olderror< / code > as the source/cause of < code > newerror< / code >
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along with the filename (< code > file!()< / code > ) and line number (< code > line!()< / code > )
and returns < code > newerror< / code > .< / p >
< p > < code > Err()?< / code > then returns the inner error applying < code > .into()< / code > , so that we
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again have a < code > Err(Box< Error + Send + Sync> )< / code > as a result.< / p >
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< p > The < code > Debug< / code > implementation of < code > ChainError< T> < / code > (which is returned by < code > cherr!()< / code > )
prints the < code > Debug< / code > of < code > T< / code > prefixed with the stored filename and line number.< / p >
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< p > < code > ChainError< T> < / code > in our case is < code > ChainError< String> < / code > .< / p >
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< h1 > < a class = "header" href = "#mapping-errors" id = "mapping-errors" > Mapping Errors< / a > < / h1 >
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< p > Now let's get more rust idiomatic by using < code > .map_err()< / code > .< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > use chainerror::*;
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use std::error::Error;
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use std::io;
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use std::result::Result;
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fn do_some_io() -> Result< (), Box< Error + Send + Sync> > {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
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Ok(())
}
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fn func2() -> Result< (), Box< Error + Send + Sync> > {
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do_some_io().map_err(|e| cherr!(e, " func2 error" ))?;
Ok(())
}
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fn func1() -> Result< (), Box< Error + Send + Sync> > {
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func2().map_err(|e| cherr!(e, " func1 error" ))?;
Ok(())
}
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fn main() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(e) = func1() {
eprintln!(" {:?}" , e);
}
Ok(())
}
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< span class = "boring" > #[allow(dead_code)]
< / span > < span class = "boring" > mod chainerror {
< / span > {{#includecomment ../src/lib.rs}}
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
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< p > If you compare the output to the previous example, you will see,
that:< / p >
< pre > < code > Error: src/main.rs:19: " func1 error"
< / code > < / pre >
< p > changed to just:< / p >
< pre > < code > src/main.rs:16: " func1 error"
< / code > < / pre >
< p > This is, because we caught the error of < code > func1()< / code > in < code > main()< / code > and print it out ourselves.< / p >
< p > We can now control, whether to output in < code > Debug< / code > or < code > Display< / code > mode.
Maybe depending on < code > --debug< / code > as a CLI argument.< / p >
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< h1 > < a class = "header" href = "#saving-coding-chars" id = "saving-coding-chars" > Saving coding chars< / a > < / h1 >
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< p > Because decorating an error with more information should not
let you jump through hoops, < code > chainerror< / code > has a quick macro for that.< / p >
< p > < code > mstrerror!()< / code > fits right into < code > .map_err()< / code > letting you quickly add
more debug strings.< / p >
< p > < code > mstrerror!()< / code > even understands < code > format!()< / code > syntax like < code > println!()< / code > .< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > use chainerror::*;
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use std::error::Error;
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use std::io;
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use std::result::Result;
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fn do_some_io() -> Result< (), Box< Error + Send + Sync> > {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
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Ok(())
}
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fn func2() -> Result< (), Box< Error + Send + Sync> > {
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let filename = " foo.txt" ;
do_some_io().map_err(mstrerr!(" Error reading '{}'" , filename))?;
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Ok(())
}
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fn func1() -> Result< (), Box< Error + Send + Sync> > {
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func2().map_err(mstrerr!(" func1 error" ))?;
Ok(())
}
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fn main() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(e) = func1() {
eprintln!(" {:?}" , e);
}
Ok(())
}
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< span class = "boring" > #[allow(dead_code)]
< / span > < span class = "boring" > mod chainerror {
< / span > {{#includecomment ../src/lib.rs}}
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
< h1 > < a class = "header" href = "#the-source-of-errors" id = "the-source-of-errors" > The source() of Errors< / a > < / h1 >
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< p > Sometimes you want to inspect the < code > source()< / code > of an < code > Error< / code > .
< code > chainerror< / code > implements < code > std::error::Error::source()< / code > , so you can get the cause of an error.< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > use chainerror::*;
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use std::error::Error;
use std::io;
use std::result::Result;
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fn do_some_io() -> Result< (), Box< Error + Send + Sync> > {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
Ok(())
}
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fn func2() -> Result< (), Box< Error + Send + Sync> > {
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let filename = " foo.txt" ;
do_some_io().map_err(mstrerr!(" Error reading '{}'" , filename))?;
Ok(())
}
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fn func1() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(e) = func2() {
if let Some(s) = e.source() {
eprintln!(" func2 failed because of '{}'" , s);
Err(e).map_err(mstrerr!(" func1 error" ))?;
}
}
Ok(())
}
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fn main() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(e) = func1() {
eprintln!(" {}" , e);
}
Ok(())
}
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< span class = "boring" > #[allow(dead_code)]
< / span > < span class = "boring" > mod chainerror {
< / span > {{#includecomment ../src/lib.rs}}
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
< p > Note, that because we changed the output of the error in < code > main()< / code > from
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< code > Debug< / code > to < code > Display< / code > , we don't see the error backtrace with filename and line number.< / p >
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< p > To enable the < code > Display< / code > backtrace, you have to enable the feature < code > display-cause< / code > for < code > chainerror< / code > .< / p >
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< h1 > < a class = "header" href = "#downcast-the-errors" id = "downcast-the-errors" > Downcast the Errors< / a > < / h1 >
< p > < code > std::error::Error< / code > comes with some helper methods to get to the original object of the
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< code > & (dyn Error + 'static)< / code > returned by < code > .source()< / code > .< / p >
< pre > < code class = "language-rust ignore" > pub fn downcast_ref< T: Error + 'static> (& self) -> Option< & T>
pub fn downcast_mut< T: Error + 'static> (& mut self) -> Option< & mut T>
< / code > < / pre >
< p > This is how it looks like, when using those:< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > use chainerror::*;
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use std::error::Error;
use std::io;
use std::result::Result;
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fn do_some_io() -> Result< (), Box< Error + Send + Sync> > {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
Ok(())
}
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fn func2() -> Result< (), Box< Error + Send + Sync> > {
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let filename = " foo.txt" ;
do_some_io().map_err(mstrerr!(" Error reading '{}'" , filename))?;
Ok(())
}
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fn func1() -> Result< (), Box< Error + Send + Sync> > {
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func2().map_err(mstrerr!(" func1 error" ))?;
Ok(())
}
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fn main() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(e) = func1() {
eprintln!(" Error: {}" , e);
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let mut s : & (dyn Error) = e.as_ref();
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while let Some(c) = s.source() {
if let Some(ioerror) = c.downcast_ref::< io::Error> () {
eprintln!(" caused by: std::io::Error: {}" , ioerror);
match ioerror.kind() {
io::ErrorKind::NotFound => eprintln!(" of kind: std::io::ErrorKind::NotFound" ),
_ => {}
}
} else {
eprintln!(" caused by: {}" , c);
}
s = c;
}
}
Ok(())
}
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< span class = "boring" > #[allow(dead_code)]
< / span > < span class = "boring" > mod chainerror {
< / span > {{#includecomment ../src/lib.rs}}
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
< h1 > < a class = "header" href = "#the-root-cause-of-all-errors" id = "the-root-cause-of-all-errors" > The root cause of all Errors< / a > < / h1 >
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< p > < code > chainerror< / code > also has some helper methods:< / p >
< pre > < code class = "language-rust ignore" > fn is_chain< T: 'static + Display + Debug> (& self) -> bool
fn downcast_chain_ref< T: 'static + Display + Debug> (& self) -> Option< & ChainError< T> >
fn downcast_chain_mut< T: 'static + Display + Debug> (& mut self) -> Option< & mut ChainError< T> >
fn root_cause(& self) -> Option< & (dyn Error + 'static)>
fn find_cause< U: Error + 'static> (& self) -> Option< & U>
fn find_chain_cause< U: Error + 'static> (& self) -> Option< & ChainError< U> >
fn kind< 'a> (& 'a self) -> & 'a T
< / code > < / pre >
< p > Using < code > downcast_chain_ref::< String> ()< / code > gives a < code > ChainError< String> < / code > , which can be used
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to call < code > .find_cause::< io::Error> ()< / code > . < / p >
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< pre > < code class = "language-rust ignore" > if let Some(s) = e.downcast_chain_ref::< String> () {
if let Some(ioerror) = s.find_cause::< io::Error> () {
< / code > < / pre >
< p > or to use < code > .root_cause()< / code > , which of course can be of any type implementing < code > std::error::Error< / code > .< / p >
< pre > < code class = "language-rust ignore" > if let Some(e) = s.root_cause() {
< / code > < / pre >
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< pre > < pre class = "playpen" > < code class = "language-rust" > use chainerror::*;
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use std::error::Error;
use std::io;
use std::result::Result;
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fn do_some_io() -> Result< (), Box< Error + Send + Sync> > {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
Ok(())
}
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fn func2() -> Result< (), Box< Error + Send + Sync> > {
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let filename = " foo.txt" ;
do_some_io().map_err(mstrerr!(" Error reading '{}'" , filename))?;
Ok(())
}
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fn func1() -> Result< (), Box< Error + Send + Sync> > {
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func2().map_err(mstrerr!(" func1 error" ))?;
Ok(())
}
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fn main() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(e) = func1() {
eprintln!(" Error: {}" , e);
if let Some(s) = e.downcast_chain_ref::< String> () {
if let Some(ioerror) = s.find_cause::< io::Error> () {
eprintln!(" caused by: std::io::Error: {}" , ioerror);
match ioerror.kind() {
io::ErrorKind::NotFound => eprintln!(" of kind: std::io::ErrorKind::NotFound" ),
_ => {}
}
}
if let Some(e) = s.root_cause() {
let ioerror = e.downcast_ref::< io::Error> ().unwrap();
eprintln!(" The root cause was: std::io::Error: {:#?}" , ioerror);
}
}
}
Ok(())
}
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< span class = "boring" > #[allow(dead_code)]
< / span > < span class = "boring" > mod chainerror {
< / span > {{#includecomment ../src/lib.rs}}
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
< h1 > < a class = "header" href = "#finding-an-error-cause" id = "finding-an-error-cause" > Finding an Error cause< / a > < / h1 >
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< p > To distinguish the errors occuring in various places, we can define named string errors with the
" new type" pattern.< / p >
< pre > < code class = "language-rust ignore" > derive_str_cherr!(Func2Error);
derive_str_cherr!(Func1Error);
< / code > < / pre >
< p > Instead of < code > ChainError< String> < / code > we now have < code > struct Func1Error(String)< / code > and < code > ChainError< Func1Error> < / code > .< / p >
< p > In the < code > main< / code > function you can see, how we can match the different errors.< / p >
< p > Also see:< / p >
< pre > < code class = "language-rust ignore" > if let Some(f2err) = f1err.find_chain_cause::< Func2Error> () {
< / code > < / pre >
< p > as a shortcut to< / p >
< pre > < code class = "language-rust ignore" > if let Some(f2err) = f1err.find_cause::< ChainError< Func2Error> > () {
< / code > < / pre >
< p > hiding the < code > ChainError< T> < / code > implementation detail.< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > use chainerror::*;
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use std::error::Error;
use std::io;
use std::result::Result;
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fn do_some_io() -> Result< (), Box< Error + Send + Sync> > {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
Ok(())
}
derive_str_cherr!(Func2Error);
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fn func2() -> Result< (), Box< Error + Send + Sync> > {
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let filename = " foo.txt" ;
do_some_io().map_err(mstrerr!(Func2Error, " Error reading '{}'" , filename))?;
Ok(())
}
derive_str_cherr!(Func1Error);
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fn func1() -> Result< (), Box< Error + Send + Sync> > {
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func2().map_err(mstrerr!(Func1Error, " func1 error" ))?;
Ok(())
}
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fn main() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(e) = func1() {
if let Some(f1err) = e.downcast_chain_ref::< Func1Error> () {
eprintln!(" Func1Error: {}" , f1err);
if let Some(f2err) = f1err.find_cause::< ChainError< Func2Error> > () {
eprintln!(" Func2Error: {}" , f2err);
}
if let Some(f2err) = f1err.find_chain_cause::< Func2Error> () {
eprintln!(" Debug Func2Error:\n{:?}" , f2err);
}
}
}
Ok(())
}
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< span class = "boring" > #[allow(dead_code)]
< / span > < span class = "boring" > mod chainerror {
< / span > {{#includecomment ../src/lib.rs}}
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
< h1 > < a class = "header" href = "#selective-error-handling" id = "selective-error-handling" > Selective Error Handling< / a > < / h1 >
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< p > What about functions returning different Error types?< / p >
< p > In this example < code > func1()< / code > can return either < code > Func1ErrorFunc2< / code > or < code > Func1ErrorIO< / code > .< / p >
< p > We might want to < code > match< / code > on < code > func1()< / code > with something like:< / p >
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< pre > < code class = "language-rust ignore" > fn main() -> Result< (), Box< Error + Send + Sync> > {
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match func1() {
Err(e) if let Some(s) = e.downcast_chain_ref::< Func1ErrorIO> () =>
eprintln!(" Func1ErrorIO:\n{:?}" , s),
Err(e) if let Some(s) = e.downcast_chain_ref::< Func1ErrorFunc2> () =>
eprintln!(" Func1ErrorFunc2:\n{:?}" , s),
Ok(_) => {},
}
Ok(())
}
< / code > < / pre >
< p > but this is not valid rust code, so we end up doing it the hard way.
In the next chapter, we will see, how to solve this more elegantly.< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > use chainerror::*;
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use std::error::Error;
use std::io;
use std::result::Result;
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fn do_some_io() -> Result< (), Box< Error + Send + Sync> > {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
Ok(())
}
derive_str_cherr!(Func2Error);
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fn func2() -> Result< (), Box< Error + Send + Sync> > {
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let filename = " foo.txt" ;
do_some_io().map_err(mstrerr!(Func2Error, " Error reading '{}'" , filename))?;
Ok(())
}
derive_str_cherr!(Func1ErrorFunc2);
derive_str_cherr!(Func1ErrorIO);
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fn func1() -> Result< (), Box< Error + Send + Sync> > {
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func2().map_err(mstrerr!(Func1ErrorFunc2, " func1 error calling func2" ))?;
let filename = " bar.txt" ;
do_some_io().map_err(mstrerr!(Func1ErrorIO, " Error reading '{}'" , filename))?;
Ok(())
}
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fn main() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(e) = func1() {
if let Some(s) = e.downcast_ref::< ChainError< Func1ErrorIO> > () {
eprintln!(" Func1ErrorIO:\n{:?}" , s);
}
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if let Some(s) = e.downcast_chain_ref::< Func1ErrorFunc2> () {
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eprintln!(" Func1ErrorFunc2:\n{:?}" , s);
}
}
Ok(())
}
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< span class = "boring" > #[allow(dead_code)]
< / span > < span class = "boring" > mod chainerror {
< / span > {{#includecomment ../src/lib.rs}}
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
< h1 > < a class = "header" href = "#errorkind-to-the-rescue" id = "errorkind-to-the-rescue" > ErrorKind to the rescue< / a > < / h1 >
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< p > To cope with different kind of errors, we introduce the kind of an error < code > Func1ErrorKind< / code > with an enum.< / p >
< p > Because we derive < code > Debug< / code > and implement < code > Display< / code > our < code > Func1ErrorKind< / code > enum, this enum can be used as
a < code > std::error::Error< / code > .< / p >
< p > Not using < code > String< / code > errors anymore, the < code > cherr!()< / code > macro seen in the beginning of
the tutorial has to be used again.< / p >
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< p > Only returning < code > Func1ErrorKind< / code > in < code > func1()< / code > now let us get rid of < code > Result< (), Box< Error + Send + Sync> > < / code > and we can
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use < code > ChainResult< (), Func1ErrorKind> < / code > .< / p >
< p > In < code > main< / code > we can now directly use the methods of < code > ChainError< T> < / code > without downcasting the error first.< / p >
2020-03-03 15:08:09 +01:00
< p > Also a nice < code > match< / code > on < code > ChainError< T> .kind()< / code > is now possible, which returns < code > & T< / code > , meaning
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< code > & Func1ErrorKind< / code > here.< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > use chainerror::*;
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use std::error::Error;
use std::io;
use std::result::Result;
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fn do_some_io() -> Result< (), Box< Error + Send + Sync> > {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
Ok(())
}
derive_str_cherr!(Func2Error);
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fn func2() -> Result< (), Box< Error + Send + Sync> > {
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let filename = " foo.txt" ;
do_some_io().map_err(mstrerr!(Func2Error, " Error reading '{}'" , filename))?;
Ok(())
}
#[derive(Debug)]
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enum Func1ErrorKind {
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Func2,
IO(String),
}
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impl ::std::fmt::Display for Func1ErrorKind {
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fn fmt(& self, f: & mut ::std::fmt::Formatter) -> ::std::fmt::Result {
match self {
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Func1ErrorKind::Func2 => write!(f, " func1 error calling func2" ),
Func1ErrorKind::IO(filename) => write!(f, " Error reading '{}'" , filename),
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}
}
}
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impl ::std::error::Error for Func1ErrorKind {}
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fn func1() -> ChainResult< (), Func1ErrorKind> {
func2().map_err(|e| cherr!(e, Func1ErrorKind::Func2))?;
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let filename = String::from(" bar.txt" );
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do_some_io().map_err(|e| cherr!(e, Func1ErrorKind::IO(filename)))?;
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Ok(())
}
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fn main() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(e) = func1() {
match e.kind() {
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Func1ErrorKind::Func2 => eprintln!(" Main Error Report: func1 error calling func2" ),
Func1ErrorKind::IO(filename) => {
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eprintln!(" Main Error Report: func1 error reading '{}'" , filename)
}
}
if let Some(e) = e.find_chain_cause::< Func2Error> () {
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eprintln!(" \nError reported by Func2Error: {}" , e)
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}
eprintln!(" \nDebug Error:\n{:?}" , e);
}
Ok(())
}
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< span class = "boring" > #[allow(dead_code)]
< / span > < span class = "boring" > mod chainerror {
< / span > {{#includecomment ../src/lib.rs}}
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
< h1 > < a class = "header" href = "#debug-for-the-errorkind" id = "debug-for-the-errorkind" > Debug for the ErrorKind< / a > < / h1 >
2019-02-07 09:02:01 +01:00
< p > One small improvement at the end of the tutorial is to fix the debug output of
< code > Func1ErrorKind< / code > . As you probably noticed, the output doesn't say much of the enum.< / p >
< pre > < code > Debug Error:
src/main.rs:35: Func2
[…]
< / code > < / pre >
< p > As a lazy shortcut, we implement < code > Debug< / code > by calling < code > Display< / code > and end up with< / p >
< pre > < code > Debug Error:
src/main.rs:40: func1 error calling func2
[…}
< / code > < / pre >
< p > which gives us a lot more detail.< / p >
< p > To create your own Errors, you might find < a href = "https://crates.io" > crates< / a > which create enum < code > Display+Debug< / code > via derive macros.< / p >
< p > Also noteworthy is < a href = "https://crates.io/crates/custom_error" > custom_error< / a > to define your custom errors,
which can then be used with < code > chainerror< / code > .< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > use chainerror::*;
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use std::error::Error;
use std::io;
use std::result::Result;
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fn do_some_io() -> Result< (), Box< Error + Send + Sync> > {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
Ok(())
}
derive_str_cherr!(Func2Error);
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fn func2() -> Result< (), Box< Error + Send + Sync> > {
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let filename = " foo.txt" ;
do_some_io().map_err(mstrerr!(Func2Error, " Error reading '{}'" , filename))?;
Ok(())
}
enum Func1ErrorKind {
Func2,
IO(String),
}
impl ::std::fmt::Display for Func1ErrorKind {
fn fmt(& self, f: & mut ::std::fmt::Formatter) -> ::std::fmt::Result {
match self {
Func1ErrorKind::Func2 => write!(f, " func1 error calling func2" ),
Func1ErrorKind::IO(filename) => write!(f, " Error reading '{}'" , filename),
}
}
}
impl ::std::fmt::Debug for Func1ErrorKind {
fn fmt(& self, f: & mut ::std::fmt::Formatter) -> ::std::fmt::Result {
write!(f, " {}" , self)
}
}
impl ::std::error::Error for Func1ErrorKind {}
fn func1() -> ChainResult< (), Func1ErrorKind> {
func2().map_err(|e| cherr!(e, Func1ErrorKind::Func2))?;
let filename = String::from(" bar.txt" );
do_some_io().map_err(|e| cherr!(e, Func1ErrorKind::IO(filename)))?;
Ok(())
}
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fn main() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(e) = func1() {
match e.kind() {
Func1ErrorKind::Func2 => eprintln!(" Main Error Report: func1 error calling func2" ),
Func1ErrorKind::IO(filename) => {
eprintln!(" Main Error Report: func1 error reading '{}'" , filename)
}
}
if let Some(e) = e.find_chain_cause::< Func2Error> () {
eprintln!(" \nError reported by Func2Error: {}" , e)
}
eprintln!(" \nDebug Error:\n{:?}" , e);
}
Ok(())
}
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< span class = "boring" > #[allow(dead_code)]
< / span > < span class = "boring" > mod chainerror {
< / span > {{#includecomment ../src/lib.rs}}
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
< h1 > < a class = "header" href = "#deref-for-the-errorkind" id = "deref-for-the-errorkind" > Deref for the ErrorKind< / a > < / h1 >
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< p > Because ChainError< T > implements Deref to & T, we can also match on < code > *e< / code > instead of < code > e.kind()< / code >
or call a function with < code > & e< / code > < / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > use chainerror::*;
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use std::error::Error;
use std::io;
use std::result::Result;
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fn do_some_io() -> Result< (), Box< Error + Send + Sync> > {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
Ok(())
}
derive_str_cherr!(Func2Error);
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fn func2() -> Result< (), Box< Error + Send + Sync> > {
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let filename = " foo.txt" ;
do_some_io().map_err(mstrerr!(Func2Error, " Error reading '{}'" , filename))?;
Ok(())
}
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enum Func1ErrorKind {
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Func2,
IO(String),
}
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impl ::std::fmt::Display for Func1ErrorKind {
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fn fmt(& self, f: & mut ::std::fmt::Formatter) -> ::std::fmt::Result {
match self {
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Func1ErrorKind::Func2 => write!(f, " func1 error calling func2" ),
Func1ErrorKind::IO(filename) => write!(f, " Error reading '{}'" , filename),
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}
}
}
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impl ::std::fmt::Debug for Func1ErrorKind {
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fn fmt(& self, f: & mut ::std::fmt::Formatter) -> ::std::fmt::Result {
write!(f, " {}" , self)
}
}
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impl ::std::error::Error for Func1ErrorKind {}
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fn func1() -> ChainResult< (), Func1ErrorKind> {
func2().map_err(|e| cherr!(e, Func1ErrorKind::Func2))?;
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let filename = String::from(" bar.txt" );
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do_some_io().map_err(|e| cherr!(e, Func1ErrorKind::IO(filename)))?;
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Ok(())
}
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fn handle_func1errorkind(e: & Func1ErrorKind) {
match e {
Func1ErrorKind::Func2 => eprintln!(" Main Error Report: func1 error calling func2" ),
Func1ErrorKind::IO(ref filename) => {
eprintln!(" Main Error Report: func1 error reading '{}'" , filename)
}
}
}
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fn main() -> Result< (), Box< Error + Send + Sync> > {
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if let Err(e) = func1() {
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match *e {
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Func1ErrorKind::Func2 => eprintln!(" Main Error Report: func1 error calling func2" ),
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Func1ErrorKind::IO(ref filename) => {
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eprintln!(" Main Error Report: func1 error reading '{}'" , filename)
}
}
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handle_func1errorkind(& e);
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if let Some(e) = e.find_chain_cause::< Func2Error> () {
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eprintln!(" \nError reported by Func2Error: {}" , e)
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}
eprintln!(" \nDebug Error:\n{:?}" , e);
}
Ok(())
}
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< span class = "boring" > #[allow(dead_code)]
< / span > < span class = "boring" > mod chainerror {
< / span > {{#includecomment ../src/lib.rs}}
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
< h1 > < a class = "header" href = "#writing-a-library" id = "writing-a-library" > Writing a library< / a > < / h1 >
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< p > I would advise to only expose an < code > mycrate::ErrorKind< / code > and type alias < code > mycrate::Error< / code > to < code > ChainError< mycrate::ErrorKind> < / code >
so you can tell your library users to use the < code > .kind()< / code > method as < code > std::io::Error< / code > does.< / p >
< p > If you later decide to make your own < code > Error< / code > implementation, your library users don't
have to change much or anything.< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > < span class = "boring" > #[allow(dead_code)]
< / span > < span class = "boring" > #[macro_use]
< / span > < span class = "boring" > pub mod chainerror {
< / span > {{#includecomment ../src/lib.rs}}
< span class = "boring" > }
< / span > pub mod mycrate {
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use crate::chainerror::*; // omit the `crate::` part
use std::io;
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fn do_some_io() -> std::result::Result< (), Box< std::error::Error + Send + Sync> > {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
Ok(())
}
derive_str_cherr!(Func2Error);
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fn func2() -> std::result::Result< (), Box< std::error::Error + Send + Sync> > {
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let filename = " foo.txt" ;
do_some_io().map_err(mstrerr!(Func2Error, " Error reading '{}'" , filename))?;
Ok(())
}
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#[derive(Debug, Clone)]
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pub enum ErrorKind {
Func2,
IO(String),
}
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derive_err_kind!(Error, ErrorKind);
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pub type Result< T> = std::result::Result< T, Error> ;
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impl std::fmt::Display for ErrorKind {
fn fmt(& self, f: & mut ::std::fmt::Formatter) -> std::fmt::Result {
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match self {
ErrorKind::Func2 => write!(f, " func1 error calling func2" ),
ErrorKind::IO(filename) => write!(f, " Error reading '{}'" , filename),
}
}
}
pub fn func1() -> Result< ()> {
func2().map_err(|e| cherr!(e, ErrorKind::Func2))?;
let filename = String::from(" bar.txt" );
do_some_io().map_err(|e| cherr!(e, ErrorKind::IO(filename)))?;
Ok(())
}
}
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fn main() -> Result< (), Box< std::error::Error + Send + Sync> > {
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use mycrate::func1;
use mycrate::ErrorKind;
use std::error::Error;
use std::io;
if let Err(e) = func1() {
match e.kind() {
ErrorKind::Func2 => eprintln!(" Main Error Report: func1 error calling func2" ),
ErrorKind::IO(ref filename) => {
eprintln!(" Main Error Report: func1 error reading '{}'" , filename)
}
}
eprintln!();
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let mut s: & Error = & e;
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while let Some(c) = s.source() {
if let Some(ioerror) = c.downcast_ref::< io::Error> () {
eprintln!(" caused by: std::io::Error: {}" , ioerror);
match ioerror.kind() {
io::ErrorKind::NotFound => eprintln!(" of kind: std::io::ErrorKind::NotFound" ),
_ => {}
}
} else {
eprintln!(" caused by: {}" , c);
}
s = c;
}
eprintln!(" \nDebug Error:\n{:?}" , e);
}
Ok(())
}
< / code > < / pre > < / pre >
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< h1 > < a class = "header" href = "#going-back-to-std" id = "going-back-to-std" > Going back to std< / a > < / h1 >
< p > Not using < code > chainerror< / code > and going full < code > std< / code > would look like this:< / p >
< p > Btw, the code size is bigger than using < code > chainerror< / code > :-)< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" > pub mod mycrate {
use std::error::Error as StdError;
use func2mod::{do_some_io, func2};
pub mod func2mod {
use std::error::Error as StdError;
use std::io;
pub enum ErrorKind {
IO(String),
}
impl std::fmt::Display for ErrorKind {
fn fmt(& self, f: & mut ::std::fmt::Formatter) -> std::fmt::Result {
match self {
ErrorKind::IO(s) => std::fmt::Display::fmt(s, f),
}
}
}
impl std::fmt::Debug for ErrorKind {
fn fmt(& self, f: & mut ::std::fmt::Formatter) -> std::fmt::Result {
match self {
ErrorKind::IO(s) => std::fmt::Display::fmt(s, f),
}
}
}
macro_rules! mcherr {
( $k:expr ) => {{
|e| {
Error(
$k,
Some(Box::from(e)),
Some(concat!(file!(), " :" , line!(), " : " )),
)
}
}};
}
pub struct Error(
ErrorKind,
Option< Box< dyn std::error::Error + 'static> > ,
Option< & 'static str> ,
);
impl Error {
pub fn kind(& self) -> & ErrorKind {
& self.0
}
}
impl From< ErrorKind> for Error {
fn from(e: ErrorKind) -> Self {
Error(e, None, None)
}
}
impl std::error::Error for Error {
fn source(& self) -> Option< & (dyn std::error::Error + 'static)> {
self.1.as_ref().map(|e| e.as_ref())
}
}
impl std::fmt::Display for Error {
fn fmt(& self, f: & mut std::fmt::Formatter) -> std::fmt::Result {
std::fmt::Display::fmt(& self.0, f)
}
}
impl std::fmt::Debug for Error {
fn fmt(& self, f: & mut std::fmt::Formatter) -> std::fmt::Result {
if let Some(ref o) = self.2 {
std::fmt::Display::fmt(o, f)?;
}
std::fmt::Debug::fmt(& self.0, f)?;
if let Some(e) = self.source() {
std::fmt::Display::fmt(" \nCaused by:\n" , f)?;
std::fmt::Debug::fmt(& e, f)?;
}
Ok(())
}
}
pub fn do_some_io() -> std::result::Result< (), Box< dyn std::error::Error> > {
Err(io::Error::from(io::ErrorKind::NotFound))?;
Ok(())
}
pub fn func2() -> std::result::Result< (), Error> {
let filename = " foo.txt" ;
do_some_io().map_err(mcherr!(ErrorKind::IO(format!(
" Error reading '{}'" ,
filename
))))?;
Ok(())
}
}
#[derive(Debug)]
pub enum ErrorKind {
Func2,
IO(String),
}
impl std::fmt::Display for ErrorKind {
fn fmt(& self, f: & mut ::std::fmt::Formatter) -> std::fmt::Result {
match self {
ErrorKind::Func2 => write!(f, " func1 error calling func2" ),
ErrorKind::IO(filename) => write!(f, " Error reading '{}'" , filename),
}
}
}
macro_rules! mcherr {
( $k:expr ) => {{
|e| {
Error(
$k,
Some(Box::from(e)),
Some(concat!(file!(), " :" , line!(), " : " )),
)
}
}};
}
pub struct Error(
ErrorKind,
Option< Box< dyn std::error::Error + 'static> > ,
Option< & 'static str> ,
);
impl Error {
pub fn kind(& self) -> & ErrorKind {
& self.0
}
}
impl From< ErrorKind> for Error {
fn from(e: ErrorKind) -> Self {
Error(e, None, None)
}
}
impl std::error::Error for Error {
fn source(& self) -> Option< & (dyn std::error::Error + 'static)> {
self.1.as_ref().map(|e| e.as_ref())
}
}
impl std::fmt::Display for Error {
fn fmt(& self, f: & mut std::fmt::Formatter) -> std::fmt::Result {
std::fmt::Display::fmt(& self.0, f)
}
}
impl std::fmt::Debug for Error {
fn fmt(& self, f: & mut std::fmt::Formatter) -> std::fmt::Result {
if let Some(ref o) = self.2 {
std::fmt::Display::fmt(o, f)?;
}
std::fmt::Debug::fmt(& self.0, f)?;
if let Some(e) = self.source() {
std::fmt::Display::fmt(" \nCaused by:\n" , f)?;
std::fmt::Debug::fmt(& e, f)?;
}
Ok(())
}
}
pub type Result< T> = std::result::Result< T, Error> ;
pub fn func1() -> Result< ()> {
func2().map_err(mcherr!(ErrorKind::Func2))?;
let filename = String::from(" bar.txt" );
do_some_io().map_err(mcherr!(ErrorKind::IO(filename)))?;
Ok(())
}
}
fn main() -> Result< (), Box< std::error::Error + Send + Sync> > {
use mycrate::func1;
use mycrate::ErrorKind;
use std::error::Error;
use std::io;
if let Err(e) = func1() {
match e.kind() {
ErrorKind::Func2 => eprintln!(" Main Error Report: func1 error calling func2" ),
ErrorKind::IO(ref filename) => {
eprintln!(" Main Error Report: func1 error reading '{}'" , filename)
}
}
eprintln!();
let mut s: & Error = & e;
while let Some(c) = s.source() {
if let Some(ioerror) = c.downcast_ref::< io::Error> () {
eprintln!(" caused by: std::io::Error: {}" , ioerror);
match ioerror.kind() {
io::ErrorKind::NotFound => eprintln!(" of kind: std::io::ErrorKind::NotFound" ),
_ => {}
}
} else {
eprintln!(" caused by: {}" , c);
}
s = c;
}
eprintln!(" \nDebug Error:\n{:?}" , e);
}
Ok(())
}
< / code > < / pre > < / pre >
< h1 > < a class = "header" href = "#the-end" id = "the-end" > The End< / a > < / h1 >
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< p > That's it for now…< / p >
< p > Happy error handling!< / p >
2018-12-21 14:07:02 +01:00
< p > To report issues, submit pull request or for the source code, examples and the book source, visit
the < a href = "https://github.com/haraldh/chainerror" > Git Repo< / a > .< / p >
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< / main >
< nav class = "nav-wrapper" aria-label = "Page navigation" >
<!-- Mobile navigation buttons -->
< div style = "clear: both" > < / div >
< / nav >
< / div >
< / div >
< nav class = "nav-wide-wrapper" aria-label = "Page navigation" >
< / nav >
< / div >
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< script type = "text/javascript" >
window.playpen_copyable = true;
< / script >
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< script src = "elasticlunr.min.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "mark.min.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "searcher.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "clipboard.min.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "highlight.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "book.js" type = "text/javascript" charset = "utf-8" > < / script >
<!-- Custom JS scripts -->
< script type = "text/javascript" >
window.addEventListener('load', function() {
window.setTimeout(window.print, 100);
});
< / script >
< / body >
< / html >
