Most errors aren’t serious enough to require the program to stop entirely. Sometimes, when a function fails, it’s for a reason that you can easily interpret and respond to. For example, if you try to open a file and that operation fails because the file doesn’t exist, you might want to create the file instead of terminating the process.
Recall from “Handling Potential Failure with the Result
Type” in Chapter 2 that the Result enum is
defined as having two variants, Ok and Err, as follows:
enum Result<T, E> {
Ok(T),
Err(E),
}The T and E are generic type parameters: we’ll discuss generics in more
detail in Chapter 10. What you need to know right now is that T represents
the type of the value that will be returned in a success case within the Ok
variant, and E represents the type of the error that will be returned in a
failure case within the Err variant. Because Result has these generic type
parameters, we can use the Result type and the functions that the standard
library has defined on it in many different situations where the successful
value and error value we want to return may differ.
Let’s call a function that returns a Result value because the function could
fail. In Listing 9-3 we try to open a file:
Filename: src/main.rs
use std::fs::File;
fn main() {
let f = File::open("hello.txt");
}Listing 9-3: Opening a file
How do we know File::open returns a Result? We could look at the standard
library API documentation, or we could ask the compiler! If we give f a type
annotation that we know is not the return type of the function and then try
to compile the code, the compiler will tell us that the types don’t match. The
error message will then tell us what the type of f is. Let’s try it! We
know that the return type of File::open isn’t of type u32, so let’s change
the let f statement to this:
let f: u32 = File::open("hello.txt");Attempting to compile now gives us the following output:
error[E0308]: mismatched types
--> src/main.rs:4:18
|
4 | let f: u32 = File::open("hello.txt");
| ^^^^^^^^^^^^^^^^^^^^^^^ expected u32, found enum
`std::result::Result`
|
= note: expected type `u32`
found type `std::result::Result<std::fs::File, std::io::Error>`
This tells us the return type of the File::open function is a Result<T, E>.
The generic parameter T has been filled in here with the type of the success
value, std::fs::File, which is a file handle. The type of E used in the
error value is std::io::Error.
This return type means the call to File::open might succeed and return a file
handle that we can read from or write to. The function call also might fail:
for example, the file might not exist, or we might not have permission to
access the file. The File::open function needs to have a way to tell us
whether it succeeded or failed and at the same time give us either the file
handle or error information. This information is exactly what the Result enum
conveys.
In the case where File::open succeeds, the value in the variable f will be
an instance of Ok that contains a file handle. In the case where it fails,
the value in f will be an instance of Err that contains more information
about the kind of error that happened.
We need to add to the code in Listing 9-3 to take different actions depending
on the value File::open returns. Listing 9-4 shows one way to handle the
Result using a basic tool, the match expression that we discussed in
Chapter 6.
Filename: src/main.rs
use std::fs::File;
fn main() {
let f = File::open("hello.txt");
let f = match f {
Ok(file) => file,
Err(error) => {
panic!("There was a problem opening the file: {:?}", error)
},
};
}Listing 9-4: Using a match expression to handle the
Result variants that might be returned
Note that, like the Option enum, the Result enum and its variants have been
imported in the prelude, so we don’t need to specify Result:: before the Ok
and Err variants in the match arms.
Here we tell Rust that when the result is Ok, return the inner file value
out of the Ok variant, and we then assign that file handle value to the
variable f. After the match, we can use the file handle for reading or
writing.
The other arm of the match handles the case where we get an Err value from
File::open. In this example, we’ve chosen to call the panic! macro. If
there’s no file named hello.txt in our current directory and we run this
code, we’ll see the following output from the panic! macro:
thread 'main' panicked at 'There was a problem opening the file: Error { repr:
Os { code: 2, message: "No such file or directory" } }', src/main.rs:9:12
As usual, this output tells us exactly what has gone wrong.
The code in Listing 9-4 will panic! no matter why File::open failed. What
we want to do instead is take different actions for different failure reasons:
if File::open failed because the file doesn’t exist, we want to create the
file and return the handle to the new file. If File::open failed for any
other reason—for example, because we didn’t have permission to open the file—we
still want the code to panic! in the same way as it did in Listing 9-4. Look
at Listing 9-5, which adds another arm to the match:
Filename: src/main.rs
use std::fs::File;
use std::io::ErrorKind;
fn main() {
let f = File::open("hello.txt");
let f = match f {
Ok(file) => file,
Err(ref error) if error.kind() == ErrorKind::NotFound => {
match File::create("hello.txt") {
Ok(fc) => fc,
Err(e) => {
panic!(
"Tried to create file but there was a problem: {:?}",
e
)
},
}
},
Err(error) => {
panic!(
"There was a problem opening the file: {:?}",
error
)
},
};
}Listing 9-5: Handling different kinds of errors in different ways
The type of the value that File::open returns inside the Err variant is
io::Error, which is a struct provided by the standard library. This struct
has a method kind that we can call to get an io::ErrorKind value. The enum
io::ErrorKind is provided by the standard library and has variants
representing the different kinds of errors that might result from an io
operation. The variant we want to use is ErrorKind::NotFound, which indicates
the file we’re trying to open doesn’t exist yet.
The condition if error.kind() == ErrorKind::NotFound is called a match
guard: it’s an extra condition on a match arm that further refines the arm’s
pattern. This condition must be true for that arm’s code to be run; otherwise,
the pattern matching will move on to consider the next arm in the match. The
ref in the pattern is needed so error is not moved into the guard condition
but is merely referenced by it. The reason you use ref to create a reference
in a pattern instead of & will be covered in detail in Chapter 18. In short,
in the context of a pattern, & matches a reference and gives you its value,
but ref matches a value and gives you a reference to it.
The condition we want to check in the match guard is whether the value returned
by error.kind() is the NotFound variant of the ErrorKind enum. If it is,
we try to create the file with File::create. However, because File::create
could also fail, we need to add an inner match statement as well. When the
file can’t be opened, a different error message will be printed. The last arm
of the outer match stays the same so the program panics on any error besides
the missing file error.
Using match works well enough, but it can be a bit verbose and doesn’t always
communicate intent well. The Result<T, E> type has many helper methods
defined on it to do various tasks. One of those methods, called unwrap, is a
shortcut method that is implemented just like the match statement we wrote in
Listing 9-4. If the Result value is the Ok variant, unwrap will return
the value inside the Ok. If the Result is the Err variant, unwrap will
call the panic! macro for us. Here is an example of unwrap in action:
Filename: src/main.rs
use std::fs::File;
fn main() {
let f = File::open("hello.txt").unwrap();
}If we run this code without a hello.txt file, we’ll see an error message from
the panic! call that the unwrap method makes:
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error {
repr: Os { code: 2, message: "No such file or directory" } }',
src/libcore/result.rs:906:4
Another method, expect, which is similar to unwrap, lets us also choose the
panic! error message. Using expect instead of unwrap and providing good
error messages can convey your intent and make tracking down the source of a
panic easier. The syntax of expect looks like this:
Filename: src/main.rs
use std::fs::File;
fn main() {
let f = File::open("hello.txt").expect("Failed to open hello.txt");
}We use expect in the same way as unwrap: to return the file handle or call
the panic! macro. The error message used by expect in its call to panic!
will be the parameter that we pass to expect, rather than the default
panic! message that unwrap uses. Here’s what it looks like:
thread 'main' panicked at 'Failed to open hello.txt: Error { repr: Os { code:
2, message: "No such file or directory" } }', src/libcore/result.rs:906:4
Because this error message starts with the text we specified, Failed to open hello.txt, it will be easier to find where in the code this error message is
coming from. If we use unwrap in multiple places, it can take more time to
figure out exactly which unwrap is causing the panic because all unwrap
calls that panic print the same message.
When you’re writing a function whose implementation calls something that might fail, instead of handling the error within this function, you can return the error to the calling code so that it can decide what to do. This is known as propagating the error and gives more control to the calling code, where there might be more information or logic that dictates how the error should be handled than what you have available in the context of your code.
For example, Listing 9-6 shows a function that reads a username from a file. If the file doesn’t exist or can’t be read, this function will return those errors to the code that called this function:
Filename: src/main.rs
use std::io;
use std::io::Read;
use std::fs::File;
fn read_username_from_file() -> Result<String, io::Error> {
let f = File::open("hello.txt");
let mut f = match f {
Ok(file) => file,
Err(e) => return Err(e),
};
let mut s = String::new();
match f.read_to_string(&mut s) {
Ok(_) => Ok(s),
Err(e) => Err(e),
}
}Listing 9-6: A function that returns errors to the
calling code using match
Let’s look at the return type of the function first: Result<String, io::Error>. This means the function is returning a value of the type
Result<T, E> where the generic parameter T has been filled in with the
concrete type String, and the generic type E has been filled in with the
concrete type io::Error. If this function succeeds without any problems, the
code that calls this function will receive an Ok value that holds a
String—the username that this function read from the file. If this function
encounters any problems, the code that calls this function will receive an
Err value that holds an instance of io::Error that contains more
information about what the problems were. We chose io::Error as the return
type of this function because that happens to be the type of the error value
returned from both of the operations we’re calling in this function’s body that
might fail: the File::open function and the read_to_string method.
The body of the function starts by calling the File::open function. Then we
handle the Result value returned with a match similar to the match in
Listing 9-4, only instead of calling panic! in the Err case, we return
early from this function and pass the error value from File::open back to the
calling code as this function’s error value. If File::open succeeds, we store
the file handle in the variable f and continue.
Then we create a new String in variable s and call the read_to_string
method on the file handle in f to read the contents of the file into s. The
read_to_string method also returns a Result because it might fail, even
though File::open succeeded. So we need another match to handle that
Result: if read_to_string succeeds, then our function has succeeded, and we
return the username from the file that’s now in s wrapped in an Ok. If
read_to_string fails, we return the error value in the same way that we
returned the error value in the match that handled the return value of
File::open. However, we don’t need to explicitly say return, because this
is the last expression in the function.
The code that calls this code will then handle getting either an Ok value
that contains a username or an Err value that contains an io::Error. We
don’t know what the calling code will do with those values. If the calling code
gets an Err value, it could call panic! and crash the program, use a
default username, or look up the username from somewhere other than a file, for
example. We don’t have enough information on what the calling code is actually
trying to do, so we propagate all the success or error information upward for
it to handle appropriately.
This pattern of propagating errors is so common in Rust that Rust provides the
question mark operator ? to make this easier.
Listing 9-7 shows an implementation of read_username_from_file that has the
same functionality as it had in Listing 9-6, but this implementation uses the
question mark operator:
Filename: src/main.rs
use std::io;
use std::io::Read;
use std::fs::File;
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)
}Listing 9-7: A function that returns errors to the
calling code using ?
The ? placed after a Result value is defined to work in almost the same way
as the match expressions we defined to handle the Result values in Listing
9-6. If the value of the Result is an Ok, the value inside the Ok will
get returned from this expression, and the program will continue. If the value
is an Err, the value inside the Err will be returned from the whole
function as if we had used the return keyword so the error value gets
propagated to the calling code.
There is a difference between what the match expression from Listing 9-6 and
? do: error values used with ? go through the from function, defined in
the From trait in the standard library, which is used to convert errors from
one type into another. When ? calls the from function, the error type
received is converted into the error type defined in the return type of the
current function. This is useful when a function returns one error type to
represent all the ways a function might fail, even if parts might fail for many
different reasons. As long as each error type implements the from function to
define how to convert itself to the returned error type, ? takes care of the
conversion automatically.
In the context of Listing 9-7, the ? at the end of the File::open call will
return the value inside an Ok to the variable f. If an error occurs, ?
will return early out of the whole function and give any Err value to the
calling code. The same thing applies to the ? at the end of the
read_to_string call.
The ? operator eliminates a lot of boilerplate and makes this function’s
implementation simpler. We could even shorten this code further by chaining
method calls immediately after the ?, as shown in Listing 9-8:
Filename: src/main.rs
use std::io;
use std::io::Read;
use std::fs::File;
fn read_username_from_file() -> Result<String, io::Error> {
let mut s = String::new();
File::open("hello.txt")?.read_to_string(&mut s)?;
Ok(s)
}Listing 9-8: Chaining method calls after ?
We’ve moved the creation of the new String in s to the beginning of the
function; that part hasn’t changed. Instead of creating a variable f, we’ve
chained the call to read_to_string directly onto the result of
File::open("hello.txt")?. We still have a ? at the end of the
read_to_string call, and we still return an Ok value containing the
username in s when both File::open and read_to_string succeed rather than
returning errors. The functionality is again the same as in Listing 9-6 and
Listing 9-7; this is just a different, more ergonomic way to write it.
The ? operator can only be used in functions that have a return type of
Result, because it is defined to work in the same way as the match
expression we defined in Listing 9-6. The part of the match that requires a
return type of Result is return Err(e), so the return type of the function
must be a Result to be compatible with this return.
Let’s look at what happens if we use ? in the main function, which you’ll
recall has a return type of ():
use std::fs::File;
fn main() {
let f = File::open("hello.txt")?;
}When we compile this code, we get the following error message:
error[E0277]: the trait bound `(): std::ops::Try` is not satisfied
--> src/main.rs:4:13
|
4 | let f = File::open("hello.txt")?;
| ------------------------
| |
| the `?` operator can only be used in a function that returns
`Result` (or another type that implements `std::ops::Try`)
| in this macro invocation
|
= help: the trait `std::ops::Try` is not implemented for `()`
= note: required by `std::ops::Try::from_error`
This error points out that we’re only allowed to use ? in a function that
returns Result. In functions that don’t return Result, when you call other
functions that return Result, you’ll need to use a match or one of the
Result methods to handle the Result instead of using ? to potentially
propagate the error to the calling code.
Now that we’ve discussed the details of calling panic! or returning Result,
let’s return to the topic of how to decide which is appropriate to use in which
cases.