Recoverable Errors with Result
Most errors aren’t serious enough to require the program to stop entirely. Sometimes, when a function fails, it’s for a reason that we can easily interpret and respond to. For example, if we try to open a file and that operation fails because the file doesn’t exist, we might want to create the file instead of terminating the process.
Recall from Chapter 2 the section on “Handling Potential Failure with the
Result Type” 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 go into 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: opening a file, shown in Listing 9-2.
Filename: src/main.rs
use std::fs::File;
fn main() {
let f = File::open("hello.txt");
}
Listing 9-2: 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 of some type that we know the return type of the function is not,
then we 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:
let f: u32 = File::open("hello.txt");
Attempting to compile now gives us:
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`
= note: 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 to us
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 we will have 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 from Listing 9-2 to take different actions depending
on the value File::open returned. Listing 9-3 shows one way to handle the
Result with a basic tool: the match expression that we learned about 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 9-3: Using a match expression to handle the
Result variants we might have
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 then 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:8
Matching on Different Errors
The code in Listing 9-3 will panic! no matter the reason that File::open
failed. What we’d really like 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 to panic! in the same way as we
did in Listing 9-3. Let’s look at Listing 9-4, 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 9-4: 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.
io::ErrorKind is an enum provided by the standard library that has variants
representing the different kinds of errors that might result from an io
operation. The variant we’re interested in 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 in order for that arm’s code to get run;
otherwise, the pattern matching will move on to consider the next arm in the
match. The ref in the pattern is needed so that error is not moved into
the guard condition but is merely referenced by it. The reason ref is used to
take 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 us its value, but ref matches a value and gives us 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, since 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 that the program panics on any error
besides the missing file error.
Shortcuts for Panic on Error: unwrap and expect
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 things. One of those methods, called unwrap, is a
shortcut method that is implemented just like the match statement we wrote in
Listing 9-3. 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.
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" } }',
/stable-dist-rustc/build/src/libcore/result.rs:868
There’s another method similar to unwrap that lets us also choose the
panic! error message: expect. 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:
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 that expect uses in its call to
panic! will be the parameter that we pass to expect instead of 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" } }',
/stable-dist-rustc/build/src/libcore/result.rs:868
Propagating Errors
When writing a function whose implementation calls something that might fail, instead of handling the error within this function, you can choose to let your caller know about the error so they 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 9-5 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:
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 9-5: 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 that 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
caller of 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 caller of 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 9-3, 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
caller 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 in order 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. We don’t need to explicitly say return, however, since 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 caller will do with those values. If they get an Err
value, they could choose to call panic! and crash their 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 caller is actually trying
to do, so we propagate all the success or error information upwards for them to
handle as they see fit.
This pattern of propagating errors is so common in Rust that there is dedicated
syntax to make this easier: ?.
A Shortcut for Propagating Errors: ?
Listing 9-6 shows an implementation of read_username_from_file that has the
same functionality as it had in Listing 9-5, but this implementation uses the
question mark operator:
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 9-6: A function that returns errors to the
calling code using ?
The ? placed after a Result value is defined to work the exact same way as
the match expressions we defined to handle the Result values in Listing
9-5. 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 that the error value gets
propagated to the caller.
In the context of Listing 9-6, 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 our
caller. The same thing applies to the ? at the end of the read_to_string
call.
The ? 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 ?:
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)
}
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 9-5 and
Listing 9-6, this is just a different, more ergonomic way to write it.
? Can Only Be Used in Functions That Return Result
The ? can only be used in functions that have a return type of Result,
since it is defined to work in exactly the same way as the match expression
we defined in Listing 9-5. 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, we get the following error message:
error[E0308]: mismatched types
-->
|
3 | let f = File::open("hello.txt")?;
| ^^^^^^^^^^^^^^^^^^^^^^^^^ expected (), found enum
`std::result::Result`
|
= note: expected type `()`
= note: found type `std::result::Result<_, _>`
This error is pointing out that we have mismatched types: the main function
has a return type of (), but the ? might return a 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 it,
instead of using ? to potentially propagate the error to the caller.
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.