New upstream version 1.63.0+dfsg1

[rustc.git] / src / doc / book / src / ch09-02-recoverable-errors-with-result.md

## 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 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”][handle_failure]<!-- ignore --> in Chapter 2 that the `Result` enum is
defined as having two variants, `Ok` and `Err`, as follows:

```rust
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 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.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch09-error-handling/listing-09-03/src/main.rs}}
```

<span class="caption">Listing 9-3: Opening a file</span>

The return type of `File::open` is a `Result<T, E>`. The generic parameter `T`
has been filled in by the implementation of `File::open` 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
`greeting_file_result` will be an instance of `Ok` that contains a file handle.
In the case where it fails, the value in `greeting_file_result` 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.

<span class="filename">Filename: src/main.rs</span>

```rust,should_panic
{{#rustdoc_include ../listings/ch09-error-handling/listing-09-04/src/main.rs}}
```

<span class="caption">Listing 9-4: Using a `match` expression to handle the
`Result` variants that might be returned</span>

Note that, like the `Option` enum, the `Result` enum and its variants have been
brought into scope by the prelude, so we don’t need to specify `Result::`
before the `Ok` and `Err` variants in the `match` arms.

When the result is `Ok`, this code will return the inner `file` value out of
the `Ok` variant, and we then assign that file handle value to the variable
`greeting_file`. 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:

```console
{{#include ../listings/ch09-error-handling/listing-09-04/output.txt}}
```

As usual, this output tells us exactly what has gone wrong.

### Matching on Different Errors

The code in Listing 9-4 will `panic!` no matter why `File::open` failed.
However, we want to 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. For this we
add an inner `match` expression, shown in Listing 9-5.

<span class="filename">Filename: src/main.rs</span>

<!-- ignore this test because otherwise it creates hello.txt which causes other
tests to fail lol -->

```rust,ignore
{{#rustdoc_include ../listings/ch09-error-handling/listing-09-05/src/main.rs}}
```

<span class="caption">Listing 9-5: Handling different kinds of errors in
different ways</span>

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. So we match on
`greeting_file_result`, but we also have an inner match on `error.kind()`.

The condition we want to check in the inner match 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 a second arm in the inner `match` expression. When the
file can’t be created, a different error message is printed. The second arm of
the outer `match` stays the same, so the program panics on any error besides
the missing file error.

> ### Alternatives to Using `match` with `Result<T, E>`
>
> That’s a lot of `match`! The `match` expression is very useful but also very
> much a primitive. In Chapter 13, you’ll learn about closures, which are used
> with many of the methods defined on `Result<T, E>`. These methods can be more
> concise than using `match` when handling `Result<T, E>` values in your code.
>
> For example, here’s another way to write the same logic as shown in Listing
> 9-5, this time using closures and the `unwrap_or_else` method:
>
> <!-- CAN'T EXTRACT SEE https://github.com/rust-lang/mdBook/issues/1127 -->
>
> ```rust,ignore
> use std::fs::File;
> use std::io::ErrorKind;
>
> fn main() {
>     let greeting_file = File::open("hello.txt").unwrap_or_else(|error| {
>         if error.kind() == ErrorKind::NotFound {
>             File::create("hello.txt").unwrap_or_else(|error| {
>                 panic!("Problem creating the file: {:?}", error);
>             })
>         } else {
>             panic!("Problem opening the file: {:?}", error);
>         }
>     });
> }
> ```
>
> Although this code has the same behavior as Listing 9-5, it doesn’t contain
> any `match` expressions and is cleaner to read. Come back to this example
> after you’ve read Chapter 13, and look up the `unwrap_or_else` method in the
> standard library documentation. Many more of these methods can clean up huge
> nested `match` expressions when you’re dealing with errors.

### 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, more specific tasks. The `unwrap` method is a
shortcut method implemented just like the `match` expression 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:

<span class="filename">Filename: src/main.rs</span>

```rust,should_panic
{{#rustdoc_include ../listings/ch09-error-handling/no-listing-04-unwrap/src/main.rs}}
```

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:

<!-- manual-regeneration
cd listings/ch09-error-handling/no-listing-04-unwrap
cargo run
copy and paste relevant text
-->

```text
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os {
code: 2, kind: NotFound, message: "No such file or directory" }',
src/main.rs:4:49
```

Similarly, the `expect` method 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:

<span class="filename">Filename: src/main.rs</span>

```rust,should_panic
{{#rustdoc_include ../listings/ch09-error-handling/no-listing-05-expect/src/main.rs}}
```

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:

<!-- manual-regeneration
cd listings/ch09-error-handling/no-listing-05-expect
cargo run
copy and paste relevant text
-->

```text
thread 'main' panicked at 'hello.txt should be included in this project: Os {
code: 2, kind: NotFound, message: "No such file or directory" }',
src/main.rs:5:10
```

In production-quality code, most Rustaceans choose `expect` rather than
`unwrap` and give more context about why the operation is expected to always
succeed. That way, if your assumptions are ever proven wrong, you have more
information to use in debugging.

### Propagating Errors

When a function’s implementation calls something that might fail, instead of
handling the error within the function itself, 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 the function.

<span class="filename">Filename: src/main.rs</span>

<!-- Deliberately not using rustdoc_include here; the `main` function in the
file panics. We do want to include it for reader experimentation purposes, but
don't want to include it for rustdoc testing purposes. -->

```rust
{{#include ../listings/ch09-error-handling/listing-09-06/src/main.rs:here}}
```

<span class="caption">Listing 9-6: A function that returns errors to the
calling code using `match`</span>

This function can be written in a much shorter way, but we’re going to start by
doing a lot of it manually in order to explore error handling; at the end,
we’ll show the shorter way. 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
calling code 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 with a `match` similar to the `match` in Listing 9-4.
If `File::open` succeeds, the file handle in the pattern variable `file`
becomes the value in the mutable variable `username_file` and the function
continues. In the `Err` case, instead of calling `panic!`, we use the `return`
keyword to return early out of the function entirely and pass the error value
from `File::open`, now in the pattern variable `e`, back to the calling code as
this function’s error value.

So if we have a file handle in `username_file`, the function then creates a new
`String` in variable `username` and calls the `read_to_string` method on
the file handle in `username_file` to read the contents of the file into
`username`. 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 `username`
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`. It’s
up to the calling code to decide what to 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.

#### A Shortcut for Propagating Errors: the `?` Operator

Listing 9-7 shows an implementation of `read_username_from_file` that has the
same functionality as in Listing 9-6, but this implementation uses the
`?` operator.

<span class="filename">Filename: src/main.rs</span>

<!-- Deliberately not using rustdoc_include here; the `main` function in the
file panics. We do want to include it for reader experimentation purposes, but
don't want to include it for rustdoc testing purposes. -->

```rust
{{#include ../listings/ch09-error-handling/listing-09-07/src/main.rs:here}}
```

<span class="caption">Listing 9-7: A function that returns errors to the
calling code using the `?` operator</span>

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 `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 does
and what the `?` operator does: error values that have the `?` operator called
on them go through the `from` function, defined in the `From` trait in the
standard library, which is used to convert values from one type into another.
When the `?` operator 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.

For example, we could change the `read_username_from_file` function in Listing
9-7 to return a custom error type named `OurError` that we define. If we also
define `impl From<io::Error> for OurError` to construct an instance of
`OurError` from an `io::Error`, then the `?` operator calls in the body of
`read_username_from_file` will call `from` and convert the error types without
needing to add any more code to the function.

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 `username_file`. If an error
occurs, the `?` operator 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.

<span class="filename">Filename: src/main.rs</span>

<!-- Deliberately not using rustdoc_include here; the `main` function in the
file panics. We do want to include it for reader experimentation purposes, but
don't want to include it for rustdoc testing purposes. -->

```rust
{{#include ../listings/ch09-error-handling/listing-09-08/src/main.rs:here}}
```

<span class="caption">Listing 9-8: Chaining method calls after the `?`
operator</span>

We’ve moved the creation of the new `String` in `username` to the beginning of
the function; that part hasn’t changed. Instead of creating a variable
`username_file`, 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 `username`
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.

Listing 9-9 shows a way to make this even shorter using `fs::read_to_string`.

<span class="filename">Filename: src/main.rs</span>

<!-- Deliberately not using rustdoc_include here; the `main` function in the
file panics. We do want to include it for reader experimentation purposes, but
don't want to include it for rustdoc testing purposes. -->

```rust
{{#include ../listings/ch09-error-handling/listing-09-09/src/main.rs:here}}
```

<span class="caption">Listing 9-9: Using `fs::read_to_string` instead of
opening and then reading the file</span>

Reading a file into a string is a fairly common operation, so the standard
library provides the convenient `fs::read_to_string` function that opens the
file, creates a new `String`, reads the contents of the file, puts the contents
into that `String`, and returns it. Of course, using `fs::read_to_string`
doesn’t give us the opportunity to explain all the error handling, so we did it
the longer way first.

#### Where The `?` Operator Can Be Used

The `?` operator can only be used in functions whose return type is compatible
with the value the `?` is used on. This is because the `?` operator is defined
to perform an early return of a value out of the function, in the same manner
as the `match` expression we defined in Listing 9-6. In Listing 9-6, the
`match` was using a `Result` value, and the early return arm returned an
`Err(e)` value. The return type of the function has to be a `Result` so that
it’s compatible with this `return`.

In Listing 9-10, let’s look at the error we’ll get if we use the `?` operator
in a `main` function with a return type incompatible with the type of the value
we use `?` on:

<span class="filename">Filename: src/main.rs</span>

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch09-error-handling/listing-09-10/src/main.rs}}
```

<span class="caption">Listing 9-10: Attempting to use the `?` in the `main`
function that returns `()` won’t compile</span>

This code opens a file, which might fail. The `?` operator follows the `Result`
value returned by `File::open`, but this `main` function has the return type of
`()`, not `Result`. When we compile this code, we get the following error
message:

```console
{{#include ../listings/ch09-error-handling/listing-09-10/output.txt}}
```

This error points out that we’re only allowed to use the `?` operator in a
function that returns `Result`, `Option`, or another type that implements
`FromResidual`.

To fix the error, you have two choices. One choice is to change the return type
of your function to be compatible with the value you’re using the `?` operator
on as long as you have no restrictions preventing that. The other technique is
to use a `match` or one of the `Result<T, E>` methods to handle the `Result<T,
E>` in whatever way is appropriate.

The error message also mentioned that `?` can be used with `Option<T>` values
as well. As with using `?` on `Result`, you can only use `?` on `Option` in a
function that returns an `Option`. The behavior of the `?` operator when called
on an `Option<T>` is similar to its behavior when called on a `Result<T, E>`:
if the value is `None`, the `None` will be returned early from the function at
that point. If the value is `Some`, the value inside the `Some` is the
resulting value of the expression and the function continues. Listing 9-11 has
an example of a function that finds the last character of the first line in the
given text:

```rust
{{#rustdoc_include ../listings/ch09-error-handling/listing-09-11/src/main.rs:here}}
```

<span class="caption">Listing 9-11: Using the `?` operator on an `Option<T>`
value</span>

This function returns `Option<char>` because it’s possible that there is a
character there, but it’s also possible that there isn’t. This code takes the
`text` string slice argument and calls the `lines` method on it, which returns
an iterator over the lines in the string. Because this function wants to
examine the first line, it calls `next` on the iterator to get the first value
from the iterator. If `text` is the empty string, this call to `next` will
return `None`, in which case we use `?` to stop and return `None` from
`last_char_of_first_line`. If `text` is not the empty string, `next` will
return a `Some` value containing a string slice of the first line in `text`.

The `?` extracts the string slice, and we can call `chars` on that string slice
to get an iterator of its characters. We’re interested in the last character in
this first line, so we call `last` to return the last item in the iterator.
This is an `Option` because it’s possible that the first line is the empty
string, for example if `text` starts with a blank line but has characters on
other lines, as in `"\nhi"`. However, if there is a last character on the first
line, it will be returned in the `Some` variant. The `?` operator in the middle
gives us a concise way to express this logic, allowing us to implement the
function in one line. If we couldn’t use the `?` operator on `Option`, we’d
have to implement this logic using more method calls or a `match` expression.

Note that you can use the `?` operator on a `Result` in a function that returns
`Result`, and you can use the `?` operator on an `Option` in a function that
returns `Option`, but you can’t mix and match. The `?` operator won’t
automatically convert a `Result` to an `Option` or vice versa; in those cases,
you can use methods like the `ok` method on `Result` or the `ok_or` method on
`Option` to do the conversion explicitly.

So far, all the `main` functions we’ve used return `()`. The `main` function is
special because it’s the entry and exit point of executable programs, and there
are restrictions on what its return type can be for the programs to behave as
expected.

Luckily, `main` can also return a `Result<(), E>`. Listing 9-12 has the
code from Listing 9-10 but we’ve changed the return type of `main` to be
`Result<(), Box<dyn Error>>` and added a return value `Ok(())` to the end. This
code will now compile:

```rust,ignore
{{#rustdoc_include ../listings/ch09-error-handling/listing-09-12/src/main.rs}}
```

<span class="caption">Listing 9-12: Changing `main` to return `Result<(), E>`
allows the use of the `?` operator on `Result` values</span>

The `Box<dyn Error>` type is a *trait object*, which we’ll talk about in the
[“Using Trait Objects that Allow for Values of Different
Types”][trait-objects]<!-- ignore --> section in Chapter 17. For now, you can
read `Box<dyn Error>` to mean “any kind of error.” Using `?` on a `Result`
value in a `main` function with the error type `Box<dyn Error>` is allowed,
because it allows any `Err` value to be returned early. Even though the body of
this `main` function will only ever return errors of type `std::io::Error`, by
specifying `Box<dyn Error>`, this signature will continue to be correct even if
more code that returns other errors is added to the body of `main`.

When a `main` function returns a `Result<(), E>`, the executable will
exit with a value of `0` if `main` returns `Ok(())` and will exit with a
nonzero value if `main` returns an `Err` value. Executables written in C return
integers when they exit: programs that exit successfully return the integer
`0`, and programs that error return some integer other than `0`. Rust also
returns integers from executables to be compatible with this convention.

The `main` function may return any types that implement [the
`std::process::Termination` trait][termination]<!-- ignore -->, which contains
a function `report` that returns an `ExitCode` Consult the standard library
documentation for more information on implementing the `Termination` trait for
your own types.

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.

[handle_failure]: ch02-00-guessing-game-tutorial.html#handling-potential-failure-with-the-result-type
[trait-objects]: ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types
[termination]: ../std/process/trait.Termination.html