I followed the code to open a file from Rust by Example:
use std::{env, fs::File, path::Path};
fn main() {
let args: Vec<_> = env::args().collect();
let pattern = &args[1];
if let Some(a) = env::args().nth(2) {
let path = Path::new(&a);
let mut file = File::open(&path);
let mut s = String::new();
file.read_to_string(&mut s);
println!("{:?}", s);
} else {
//do something
}
}
However, I got a message like this:
error[E0599]: no method named `read_to_string` found for type `std::result::Result<std::fs::File, std::io::Error>` in the current scope
--> src/main.rs:11:14
|
11 | file.read_to_string(&mut s);
| ^^^^^^^^^^^^^^ method not found in `std::result::Result<std::fs::File, std::io::Error>`
What am I doing wrong?
Let's look at your error message:
error[E0599]: no method named `read_to_string` found for type `std::result::Result<std::fs::File, std::io::Error>` in the current scope
--> src/main.rs:11:14
|
11 | file.read_to_string(&mut s);
| ^^^^^^^^^^^^^^ method not found in `std::result::Result<std::fs::File, std::io::Error>`
The error message is pretty much what it says on the tin - the type Result does not have the method read_to_string. That's actually a method on the trait Read.
You have a Result because File::open(&path) can fail. Failure is represented with the Result type. A Result may be either an Ok, which is the success case, or an Err, the failure case.
You need to handle the failure case somehow. The easiest is to just die on failure, using expect:
let mut file = File::open(&path).expect("Unable to open");
You'll also need to bring Read into scope to have access to read_to_string:
use std::io::Read;
I'd highly recommend reading through The Rust Programming Language and working the examples. The chapter Recoverable Errors with Result will be highly relevant. I think these docs are top-notch!
If your method returns Result<String, io::Error>, you can use ? on the functions that return a Result:
fn read_username_from_file() -> Result<String, io::Error> {
let mut f = File::open("hello.txt")?;
let mut s = String::new();
f.read_to_string(&mut s)?;
Ok(s)
}
If you cannot return a Result<String, io::Error> then you have to handle error case using expect as mentioned in the accepted answer or matching on the Result and panicking:
let file = File::open(&opt_raw.config);
let file = match file {
Ok(file) => file,
Err(error) => {
panic!("Problem opening the file: {:?}", error)
}
};
For more information, please refer to Recoverable Errors with Result.
Related
I followed the code to open a file from Rust by Example:
use std::{env, fs::File, path::Path};
fn main() {
let args: Vec<_> = env::args().collect();
let pattern = &args[1];
if let Some(a) = env::args().nth(2) {
let path = Path::new(&a);
let mut file = File::open(&path);
let mut s = String::new();
file.read_to_string(&mut s);
println!("{:?}", s);
} else {
//do something
}
}
However, I got a message like this:
error[E0599]: no method named `read_to_string` found for type `std::result::Result<std::fs::File, std::io::Error>` in the current scope
--> src/main.rs:11:14
|
11 | file.read_to_string(&mut s);
| ^^^^^^^^^^^^^^ method not found in `std::result::Result<std::fs::File, std::io::Error>`
What am I doing wrong?
Let's look at your error message:
error[E0599]: no method named `read_to_string` found for type `std::result::Result<std::fs::File, std::io::Error>` in the current scope
--> src/main.rs:11:14
|
11 | file.read_to_string(&mut s);
| ^^^^^^^^^^^^^^ method not found in `std::result::Result<std::fs::File, std::io::Error>`
The error message is pretty much what it says on the tin - the type Result does not have the method read_to_string. That's actually a method on the trait Read.
You have a Result because File::open(&path) can fail. Failure is represented with the Result type. A Result may be either an Ok, which is the success case, or an Err, the failure case.
You need to handle the failure case somehow. The easiest is to just die on failure, using expect:
let mut file = File::open(&path).expect("Unable to open");
You'll also need to bring Read into scope to have access to read_to_string:
use std::io::Read;
I'd highly recommend reading through The Rust Programming Language and working the examples. The chapter Recoverable Errors with Result will be highly relevant. I think these docs are top-notch!
If your method returns Result<String, io::Error>, you can use ? on the functions that return a Result:
fn read_username_from_file() -> Result<String, io::Error> {
let mut f = File::open("hello.txt")?;
let mut s = String::new();
f.read_to_string(&mut s)?;
Ok(s)
}
If you cannot return a Result<String, io::Error> then you have to handle error case using expect as mentioned in the accepted answer or matching on the Result and panicking:
let file = File::open(&opt_raw.config);
let file = match file {
Ok(file) => file,
Err(error) => {
panic!("Problem opening the file: {:?}", error)
}
};
For more information, please refer to Recoverable Errors with Result.
Why does this code not compile?
use std::{fs, path::Path};
fn main() {
let dir = Path::new("../FileSystem");
if !dir.is_dir() {
println!("Is not a directory");
return;
}
for item in try!(fs::read_dir(dir)) {
let file = match item {
Err(e) => {
println!("Error: {}", e);
return;
}
Ok(f) => f,
};
println!("");
}
println!("Done");
}
This is the error I get
error[E0308]: mismatched types
--> src/main.rs:11:17
|
11 | for item in try!(fs::read_dir(dir)) {
| ^^^^^^^^^^^^^^^^^^^^^^^ expected (), found enum `std::result::Result`
|
= note: expected type `()`
found type `std::result::Result<_, _>`
= note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
I also tried the question mark operator:
for item in fs::read_dir(dir)? {
Which had a different error:
error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
--> src/main.rs:11:17
|
11 | for item in fs::read_dir(dir)? {
| ^^^^^^^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
|
= help: the trait `std::ops::Try` is not implemented for `()`
= note: required by `std::ops::Try::from_error`
Previous versions of Rust had a similar error about std::ops::Carrier
Should I avoid try!() and ?? What is the best way to handle errors? Mostly I do it like this:
match error_prone {
Err(e) => {
println!("Error: {}", e);
return;
},
Ok(f) => f,
};
But if I have to use that in a for loop, it's a complete mess
for i in match error_prone {
// match code
} {
// loop code
}
try! is a macro that returns Errs automatically; ? is syntax that does mostly the same thing, but it works with any type that implements the Try trait.
As of Rust 1.22.0, Option implements Try, so it can be used with ?. Before that, ? could only be used in functions that return a Result. try! continues to only work with Results.
As of Rust 1.26.0, main is allowed to return a value that implements Termination. Before that, it doesn't return any value.
As of Rust 1.26.0
Your original code works if you mark main as returning a Result and then return Ok(()) in all the "success" cases:
use std::{fs, io, path::Path};
fn main() -> Result<(), io::Error> {
let dir = Path::new("../FileSystem");
if !dir.is_dir() {
println!("Is not a directory");
return Ok(());
}
for item in fs::read_dir(dir)? {
let file = match item {
Err(e) => {
println!("Error: {}", e);
return Ok(());
}
Ok(f) => f,
};
println!("");
}
println!("Done");
Ok(())
}
Before that
This is how you might transform your code to use ?:
use std::{error::Error, fs, path::Path};
fn print_dir_contents() -> Result<String, Box<Error>> {
let dir = Path::new("../FileSystem");
if !dir.is_dir() {
return Err(Box::from("Is not a directory!"));
}
for entry in fs::read_dir(dir)? {
let path = entry?.path();
let file_name = path.file_name().unwrap();
println!("{}", file_name.to_string_lossy());
}
Ok("Done".into())
}
fn main() {
match print_dir_contents() {
Ok(s) => println!("{}", s),
Err(e) => println!("Error: {}", e.to_string()),
}
}
There's a lot of error handling here that you might not expect - other languages don't tend to require it! But they exist in other languages - Rust just makes you know it. Here are the errors:
entry?
IO errors can happen during iteration.
path.file_name().unwrap()
Not all paths have file names. We can unwrap this because read_dir won't give us a path without a file name.
file_name.to_string_lossy()
You can also to_str and throw an error, but it's nicer to do this. This error exists because not all file names are valid Unicode.
try! and ? throw errors into the return value, converting them to Box::Error. It's actually more reasonable to return an amalgamated error of all the things that can go wrong. Luckily io::Error is just the right type:
use std::io;
// ...
fn print_dir_contents() -> Result<String, io::Error> {
// ...
if !dir.is_dir() {
return Err(io::Error::new(io::ErrorKind::Other, "Is not a directory!"));
}
// ...
}
Frankly, though, this check is already in fs::read_dir, so you can actually just remove the if !dis.is_dir altogether:
use std::{fs, io, path::Path};
fn print_dir_contents() -> Result<String, io::Error> {
let dir = Path::new("../FileSystem");
for entry in fs::read_dir(dir)? {
let path = entry?.path();
let file_name = path.file_name().unwrap();
println!("{}", file_name.to_string_lossy());
}
Ok("Done".into())
}
fn main() {
match print_dir_contents() {
Ok(s) => println!("{}", s),
Err(e) => println!("Error: {}", e.to_string()),
}
}
The ques_in_main RFC got merged recently. Once it's completed, the syntax in the question will indeed compile just fine and work as intended, provided the try!() calls are replaced with the ? operator.
As of Rust 1.26, Rust supports a return value from main(), and thus supports the use of the error-check operator ? (or equivalently the try!() macro) in main() when main() is defined to return a Result:
extern crate failure;
use failure::Error;
use std::fs::File;
type Result<T> = std::result::Result<T, Error>;
fn main() -> Result<()> {
let mut _file = File::open("foo.txt")?; // does not exist; returns error
println!("the file is open!");
Ok(())
}
The above compiles and returns a file not found error (assuming foo.txt does not exist in the local path).
Rust playground example
Veedrac's answer helped me too, although the OP's question is slightly different. While reading the Rust documentation, I saw this snippet:
use std::fs::File;
use std::io::prelude::*;
let mut file = File::open("foo.txt")?;
let mut contents = String::new();
file.read_to_string(&mut contents)?;
assert_eq!(contents, "Hello, world!");
Though in the Rust Book they point out the centrality of the main function, if you run this inside it you'll get a similar error. If you wrap the code inside a function handling the errors the aforementioned snippet works:
use std::error::Error;
use std::io::prelude::*;
use std::fs::File;
fn print_file_content() -> Result<String, Box<Error>> {
let mut f = File::open("foo.txt")?;
let mut contents = String::new();
f.read_to_string(&mut contents)?;
println!("The content: {:?}", contents);
Ok("Done".into())
}
fn main() {
match print_file_content() {
Ok(s) => println!("{}", s),
Err(e) => println!("Error: {}", e.to_string()),
}
}
P.S. I'm learning Rust so these snippets are not intended as good Rust coding :)
Why does this code not compile?
use std::{fs, path::Path};
fn main() {
let dir = Path::new("../FileSystem");
if !dir.is_dir() {
println!("Is not a directory");
return;
}
for item in try!(fs::read_dir(dir)) {
let file = match item {
Err(e) => {
println!("Error: {}", e);
return;
}
Ok(f) => f,
};
println!("");
}
println!("Done");
}
This is the error I get
error[E0308]: mismatched types
--> src/main.rs:11:17
|
11 | for item in try!(fs::read_dir(dir)) {
| ^^^^^^^^^^^^^^^^^^^^^^^ expected (), found enum `std::result::Result`
|
= note: expected type `()`
found type `std::result::Result<_, _>`
= note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
I also tried the question mark operator:
for item in fs::read_dir(dir)? {
Which had a different error:
error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
--> src/main.rs:11:17
|
11 | for item in fs::read_dir(dir)? {
| ^^^^^^^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
|
= help: the trait `std::ops::Try` is not implemented for `()`
= note: required by `std::ops::Try::from_error`
Previous versions of Rust had a similar error about std::ops::Carrier
Should I avoid try!() and ?? What is the best way to handle errors? Mostly I do it like this:
match error_prone {
Err(e) => {
println!("Error: {}", e);
return;
},
Ok(f) => f,
};
But if I have to use that in a for loop, it's a complete mess
for i in match error_prone {
// match code
} {
// loop code
}
try! is a macro that returns Errs automatically; ? is syntax that does mostly the same thing, but it works with any type that implements the Try trait.
As of Rust 1.22.0, Option implements Try, so it can be used with ?. Before that, ? could only be used in functions that return a Result. try! continues to only work with Results.
As of Rust 1.26.0, main is allowed to return a value that implements Termination. Before that, it doesn't return any value.
As of Rust 1.26.0
Your original code works if you mark main as returning a Result and then return Ok(()) in all the "success" cases:
use std::{fs, io, path::Path};
fn main() -> Result<(), io::Error> {
let dir = Path::new("../FileSystem");
if !dir.is_dir() {
println!("Is not a directory");
return Ok(());
}
for item in fs::read_dir(dir)? {
let file = match item {
Err(e) => {
println!("Error: {}", e);
return Ok(());
}
Ok(f) => f,
};
println!("");
}
println!("Done");
Ok(())
}
Before that
This is how you might transform your code to use ?:
use std::{error::Error, fs, path::Path};
fn print_dir_contents() -> Result<String, Box<Error>> {
let dir = Path::new("../FileSystem");
if !dir.is_dir() {
return Err(Box::from("Is not a directory!"));
}
for entry in fs::read_dir(dir)? {
let path = entry?.path();
let file_name = path.file_name().unwrap();
println!("{}", file_name.to_string_lossy());
}
Ok("Done".into())
}
fn main() {
match print_dir_contents() {
Ok(s) => println!("{}", s),
Err(e) => println!("Error: {}", e.to_string()),
}
}
There's a lot of error handling here that you might not expect - other languages don't tend to require it! But they exist in other languages - Rust just makes you know it. Here are the errors:
entry?
IO errors can happen during iteration.
path.file_name().unwrap()
Not all paths have file names. We can unwrap this because read_dir won't give us a path without a file name.
file_name.to_string_lossy()
You can also to_str and throw an error, but it's nicer to do this. This error exists because not all file names are valid Unicode.
try! and ? throw errors into the return value, converting them to Box::Error. It's actually more reasonable to return an amalgamated error of all the things that can go wrong. Luckily io::Error is just the right type:
use std::io;
// ...
fn print_dir_contents() -> Result<String, io::Error> {
// ...
if !dir.is_dir() {
return Err(io::Error::new(io::ErrorKind::Other, "Is not a directory!"));
}
// ...
}
Frankly, though, this check is already in fs::read_dir, so you can actually just remove the if !dis.is_dir altogether:
use std::{fs, io, path::Path};
fn print_dir_contents() -> Result<String, io::Error> {
let dir = Path::new("../FileSystem");
for entry in fs::read_dir(dir)? {
let path = entry?.path();
let file_name = path.file_name().unwrap();
println!("{}", file_name.to_string_lossy());
}
Ok("Done".into())
}
fn main() {
match print_dir_contents() {
Ok(s) => println!("{}", s),
Err(e) => println!("Error: {}", e.to_string()),
}
}
The ques_in_main RFC got merged recently. Once it's completed, the syntax in the question will indeed compile just fine and work as intended, provided the try!() calls are replaced with the ? operator.
As of Rust 1.26, Rust supports a return value from main(), and thus supports the use of the error-check operator ? (or equivalently the try!() macro) in main() when main() is defined to return a Result:
extern crate failure;
use failure::Error;
use std::fs::File;
type Result<T> = std::result::Result<T, Error>;
fn main() -> Result<()> {
let mut _file = File::open("foo.txt")?; // does not exist; returns error
println!("the file is open!");
Ok(())
}
The above compiles and returns a file not found error (assuming foo.txt does not exist in the local path).
Rust playground example
Veedrac's answer helped me too, although the OP's question is slightly different. While reading the Rust documentation, I saw this snippet:
use std::fs::File;
use std::io::prelude::*;
let mut file = File::open("foo.txt")?;
let mut contents = String::new();
file.read_to_string(&mut contents)?;
assert_eq!(contents, "Hello, world!");
Though in the Rust Book they point out the centrality of the main function, if you run this inside it you'll get a similar error. If you wrap the code inside a function handling the errors the aforementioned snippet works:
use std::error::Error;
use std::io::prelude::*;
use std::fs::File;
fn print_file_content() -> Result<String, Box<Error>> {
let mut f = File::open("foo.txt")?;
let mut contents = String::new();
f.read_to_string(&mut contents)?;
println!("The content: {:?}", contents);
Ok("Done".into())
}
fn main() {
match print_file_content() {
Ok(s) => println!("{}", s),
Err(e) => println!("Error: {}", e.to_string()),
}
}
P.S. I'm learning Rust so these snippets are not intended as good Rust coding :)
Why does this code not compile?
use std::{fs, path::Path};
fn main() {
let dir = Path::new("../FileSystem");
if !dir.is_dir() {
println!("Is not a directory");
return;
}
for item in try!(fs::read_dir(dir)) {
let file = match item {
Err(e) => {
println!("Error: {}", e);
return;
}
Ok(f) => f,
};
println!("");
}
println!("Done");
}
This is the error I get
error[E0308]: mismatched types
--> src/main.rs:11:17
|
11 | for item in try!(fs::read_dir(dir)) {
| ^^^^^^^^^^^^^^^^^^^^^^^ expected (), found enum `std::result::Result`
|
= note: expected type `()`
found type `std::result::Result<_, _>`
= note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
I also tried the question mark operator:
for item in fs::read_dir(dir)? {
Which had a different error:
error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
--> src/main.rs:11:17
|
11 | for item in fs::read_dir(dir)? {
| ^^^^^^^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
|
= help: the trait `std::ops::Try` is not implemented for `()`
= note: required by `std::ops::Try::from_error`
Previous versions of Rust had a similar error about std::ops::Carrier
Should I avoid try!() and ?? What is the best way to handle errors? Mostly I do it like this:
match error_prone {
Err(e) => {
println!("Error: {}", e);
return;
},
Ok(f) => f,
};
But if I have to use that in a for loop, it's a complete mess
for i in match error_prone {
// match code
} {
// loop code
}
try! is a macro that returns Errs automatically; ? is syntax that does mostly the same thing, but it works with any type that implements the Try trait.
As of Rust 1.22.0, Option implements Try, so it can be used with ?. Before that, ? could only be used in functions that return a Result. try! continues to only work with Results.
As of Rust 1.26.0, main is allowed to return a value that implements Termination. Before that, it doesn't return any value.
As of Rust 1.26.0
Your original code works if you mark main as returning a Result and then return Ok(()) in all the "success" cases:
use std::{fs, io, path::Path};
fn main() -> Result<(), io::Error> {
let dir = Path::new("../FileSystem");
if !dir.is_dir() {
println!("Is not a directory");
return Ok(());
}
for item in fs::read_dir(dir)? {
let file = match item {
Err(e) => {
println!("Error: {}", e);
return Ok(());
}
Ok(f) => f,
};
println!("");
}
println!("Done");
Ok(())
}
Before that
This is how you might transform your code to use ?:
use std::{error::Error, fs, path::Path};
fn print_dir_contents() -> Result<String, Box<Error>> {
let dir = Path::new("../FileSystem");
if !dir.is_dir() {
return Err(Box::from("Is not a directory!"));
}
for entry in fs::read_dir(dir)? {
let path = entry?.path();
let file_name = path.file_name().unwrap();
println!("{}", file_name.to_string_lossy());
}
Ok("Done".into())
}
fn main() {
match print_dir_contents() {
Ok(s) => println!("{}", s),
Err(e) => println!("Error: {}", e.to_string()),
}
}
There's a lot of error handling here that you might not expect - other languages don't tend to require it! But they exist in other languages - Rust just makes you know it. Here are the errors:
entry?
IO errors can happen during iteration.
path.file_name().unwrap()
Not all paths have file names. We can unwrap this because read_dir won't give us a path without a file name.
file_name.to_string_lossy()
You can also to_str and throw an error, but it's nicer to do this. This error exists because not all file names are valid Unicode.
try! and ? throw errors into the return value, converting them to Box::Error. It's actually more reasonable to return an amalgamated error of all the things that can go wrong. Luckily io::Error is just the right type:
use std::io;
// ...
fn print_dir_contents() -> Result<String, io::Error> {
// ...
if !dir.is_dir() {
return Err(io::Error::new(io::ErrorKind::Other, "Is not a directory!"));
}
// ...
}
Frankly, though, this check is already in fs::read_dir, so you can actually just remove the if !dis.is_dir altogether:
use std::{fs, io, path::Path};
fn print_dir_contents() -> Result<String, io::Error> {
let dir = Path::new("../FileSystem");
for entry in fs::read_dir(dir)? {
let path = entry?.path();
let file_name = path.file_name().unwrap();
println!("{}", file_name.to_string_lossy());
}
Ok("Done".into())
}
fn main() {
match print_dir_contents() {
Ok(s) => println!("{}", s),
Err(e) => println!("Error: {}", e.to_string()),
}
}
The ques_in_main RFC got merged recently. Once it's completed, the syntax in the question will indeed compile just fine and work as intended, provided the try!() calls are replaced with the ? operator.
As of Rust 1.26, Rust supports a return value from main(), and thus supports the use of the error-check operator ? (or equivalently the try!() macro) in main() when main() is defined to return a Result:
extern crate failure;
use failure::Error;
use std::fs::File;
type Result<T> = std::result::Result<T, Error>;
fn main() -> Result<()> {
let mut _file = File::open("foo.txt")?; // does not exist; returns error
println!("the file is open!");
Ok(())
}
The above compiles and returns a file not found error (assuming foo.txt does not exist in the local path).
Rust playground example
Veedrac's answer helped me too, although the OP's question is slightly different. While reading the Rust documentation, I saw this snippet:
use std::fs::File;
use std::io::prelude::*;
let mut file = File::open("foo.txt")?;
let mut contents = String::new();
file.read_to_string(&mut contents)?;
assert_eq!(contents, "Hello, world!");
Though in the Rust Book they point out the centrality of the main function, if you run this inside it you'll get a similar error. If you wrap the code inside a function handling the errors the aforementioned snippet works:
use std::error::Error;
use std::io::prelude::*;
use std::fs::File;
fn print_file_content() -> Result<String, Box<Error>> {
let mut f = File::open("foo.txt")?;
let mut contents = String::new();
f.read_to_string(&mut contents)?;
println!("The content: {:?}", contents);
Ok("Done".into())
}
fn main() {
match print_file_content() {
Ok(s) => println!("{}", s),
Err(e) => println!("Error: {}", e.to_string()),
}
}
P.S. I'm learning Rust so these snippets are not intended as good Rust coding :)
This question already has answers here:
Why do try!() and ? not compile when used in a function that doesn't return Option or Result?
(4 answers)
Closed 5 years ago.
The Rust file examples don't appear compile with Rust 1.18.0.
For example:
use std::fs::File;
use std::io::prelude::*;
fn main() {
let mut file = File::open("foo.txt")?;
let mut contents = String::new();
file.read_to_string(&mut contents)?;
assert_eq!(contents, "Hello, world!");
}
Error log:
rustc 1.18.0 (03fc9d622 2017-06-06)
error[E0277]: the trait bound `(): std::ops::Carrier` is not satisfied
--> <anon>:4:20
|
4 | let mut file = File::open("foo.txt")?;
| ----------------------
| |
| the trait `std::ops::Carrier` is not implemented for `()`
| in this macro invocation
|
= note: required by `std::ops::Carrier::from_error`
error[E0277]: the trait bound `(): std::ops::Carrier` is not satisfied
--> <anon>:6:5
|
6 | file.read_to_string(&mut contents)?;
| -----------------------------------
| |
| the trait `std::ops::Carrier` is not implemented for `()`
| in this macro invocation
|
= note: required by `std::ops::Carrier::from_error`
error: aborting due to 2 previous errors
? is a syntactic sugar that checks a Result: if the result is Err, it is returned as if. If there is no error (aka Ok), the function continue. When you type this:
fn main() {
use std::fs::File;
let _ = File::open("foo.txt")?;
}
that means:
fn main() {
use std::fs::File;
let _ = match File::open("foo.txt") {
Err(e) => return Err(e),
Ok(val) => val,
};
}
Then you understand that for now, you cannot use ? in the main, because main returns unit () and not Result. If you want this stuff to work, you can put it in a function that returns a Result and check it from main:
fn my_stuff() -> std::io::Result<()> {
use std::fs::File;
use std::io::prelude::*;
let mut file = File::open("foo.txt")?;
let mut contents = String::new();
file.read_to_string(&mut contents)?;
// do whatever you want with `contents`
Ok(())
}
fn main() {
if let Err(_) = my_stuff() {
// manage your error
}
}
PS: There is a proposition to make work ? in the main.
They do compile. They just don't compile in a main function like that. If you look at the examples, they all have a big "Run" button on them. Click that and it opens the full, unabridged example on the playpen.
The one you've used above expands to this code:
fn main() {
use std::fs::File;
use std::io::prelude::*;
fn foo() -> std::io::Result<()> {
let mut file = File::open("foo.txt")?;
let mut contents = String::new();
file.read_to_string(&mut contents)?;
assert_eq!(contents, "Hello, world!");
Ok(())
}
}
That code doesn't compile because you've put code that propagates a Result into a function (main in this case) that doesn't return a Result.