book/nostarch/chapter09.md

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[TOC]

# Error Handling

Errors are a fact of life in software, so Rust has a number of features for
handling situations in which something goes wrong. In many cases, Rust requires
you to acknowledge the possibility of an error and take some action before your
code will compile. This requirement makes your program more robust by ensuring
that you’ll discover errors and handle them appropriately before you’ve
deployed your code to production!

Rust groups errors into two major categories: *recoverable* and *unrecoverable*
errors. For a recoverable error, such as a *file not found* error, we most
likely just want to report the problem to the user and retry the operation.
Unrecoverable errors are always symptoms of bugs, such as trying to access a
location beyond the end of an array, and so we want to immediately stop the
program.

Most languages don’t distinguish between these two kinds of errors and handle
both in the same way, using mechanisms such as exceptions. Rust doesn’t have
exceptions. Instead, it has the type `Result<T, E>` for recoverable errors and
the `panic!` macro that stops execution when the program encounters an
unrecoverable error. This chapter covers calling `panic!` first and then talks
about returning `Result<T, E>` values. Additionally, we’ll explore
considerations when deciding whether to try to recover from an error or to stop
execution.

## Unrecoverable Errors with panic!

Sometimes bad things happen in your code, and there’s nothing you can do about
it. In these cases, Rust has the `panic!` macro. There are two ways to cause a
panic in practice: by taking an action that causes our code to panic (such as
accessing an array past the end) or by explicitly calling the `panic!` macro.
In both cases, we cause a panic in our program. By default, these panics will
print a failure message, unwind, clean up the stack, and quit. Via an
environment variable, you can also have Rust display the call stack when a
panic occurs to make it easier to track down the source of the panic.

> ### Unwinding the Stack or Aborting in Response to a Panic
>
> By default, when a panic occurs the program starts *unwinding*, which means
Rust walks back up the stack and cleans up the data from each function it
encounters. However, walking back and cleaning up is a lot of work. Rust,
therefore, allows you to choose the alternative of immediately *aborting*,
which ends the program without cleaning up.
>
> Memory that the program was using will then need to be cleaned up by the
operating system. If in your project you need to make the resultant binary as
small as possible, you can switch from unwinding to aborting upon a panic by
adding `panic = 'abort'` to the appropriate `[profile]` sections in your
*Cargo.toml* file. For example, if you want to abort on panic in release mode,
add this:
>
> ```
> [profile.release]
> panic = 'abort'
> ```

Let’s try calling `panic!` in a simple program:

Filename: src/main.rs

```
fn main() {
    panic!("crash and burn");
}
```

When you run the program, you’ll see something like this:

```
thread 'main' panicked at 'crash and burn', src/main.rs:2:5
note: run with `RUST_BACKTRACE=1` environment variable to display
a backtrace
```

The call to `panic!` causes the error message contained in the last two lines.
The first line shows our panic message and the place in our source code where
the panic occurred: *src/main.rs:2:5* indicates that it’s the second line,
fifth character of our *src/main.rs* file.

In this case, the line indicated is part of our code, and if we go to that
line, we see the `panic!` macro call. In other cases, the `panic!` call might
be in code that our code calls, and the filename and line number reported by
the error message will be someone else’s code where the `panic!` macro is
called, not the line of our code that eventually led to the `panic!` call.

We can use the backtrace of the functions the `panic!` call came from to figure
out the part of our code that is causing the problem. To understand how to use
a `panic!` backtrace, let’s look at another example and see what it’s like when
a `panic!` call comes from a library because of a bug in our code instead of
from our code calling the macro directly. Listing 9-1 has some code that
attempts to access an index in a vector beyond the range of valid indexes.

Filename: src/main.rs

```
fn main() {
    let v = vec![1, 2, 3];

    v[99];
}
```

Listing 9-1: Attempting to access an element beyond the end of a vector, which
will cause a call to `panic!`

Here, we’re attempting to access the 100th element of our vector (which is at
index 99 because indexing starts at zero), but the vector has only three
elements. In this situation, Rust will panic. Using `[]` is supposed to return
an element, but if you pass an invalid index, there’s no element that Rust
could return here that would be correct.

In C, attempting to read beyond the end of a data structure is undefined
behavior. You might get whatever is at the location in memory that would
correspond to that element in the data structure, even though the memory
doesn’t belong to that structure. This is called a *buffer overread* and can
lead to security vulnerabilities if an attacker is able to manipulate the index
in such a way as to read data they shouldn’t be allowed to that is stored after
the data structure.

To protect your program from this sort of vulnerability, if you try to read an
element at an index that doesn’t exist, Rust will stop execution and refuse to
continue. Let’s try it and see:

```
thread 'main' panicked at 'index out of bounds: the len is 3 but the index is
99', src/main.rs:4:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

This error points at line 4 of our *main.rs* where we attempt to access `index`.

The `note:` line tells us that we can set the `RUST_BACKTRACE` environment
variable to get a backtrace of exactly what happened to cause the error. A
*backtrace* is a list of all the functions that have been called to get to this
point. Backtraces in Rust work as they do in other languages: the key to
reading the backtrace is to start from the top and read until you see files you
wrote. That’s the spot where the problem originated. The lines above that spot
are code that your code has called; the lines below are code that called your
code. These before-and-after lines might include core Rust code, standard
library code, or crates that you’re using. Let’s try getting a backtrace by
setting the `RUST_BACKTRACE` environment variable to any value except `0`.
Listing 9-2 shows output similar to what you’ll see.

```
$ RUST_BACKTRACE=1 cargo run
thread 'main' panicked at 'index out of bounds: the len is 3 but the index is
99', src/main.rs:4:5
stack backtrace:
   0: rust_begin_unwind
             at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/std
/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core
/src/panicking.rs:142:14
   2: core::panicking::panic_bounds_check
             at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core
/src/panicking.rs:84:5
   3: <usize as core::slice::index::SliceIndex<[T]>>::index
             at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core
/src/slice/index.rs:242:10
   4: core::slice::index::<impl core::ops::index::Index<I> for [T]>::index
             at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core
/src/slice/index.rs:18:9
   5: <alloc::vec::Vec<T,A> as core::ops::index::Index<I>>::index
             at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/alloc
/src/vec/mod.rs:2591:9
   6: panic::main
             at ./src/main.rs:4:5
   7: core::ops::function::FnOnce::call_once
             at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core
/src/ops/function.rs:248:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose
backtrace.
```

Listing 9-2: The backtrace generated by a call to `panic!` displayed when the
environment variable `RUST_BACKTRACE` is set

That’s a lot of output! The exact output you see might be different depending
on your operating system and Rust version. In order to get backtraces with this
information, debug symbols must be enabled. Debug symbols are enabled by
default when using `cargo build` or `cargo run` without the `--release` flag,
as we have here.

In the output in Listing 9-2, line 6 of the backtrace points to the line in our
project that’s causing the problem: line 4 of *src/main.rs*. If we don’t want
our program to panic, we should start our investigation at the location pointed
to by the first line mentioning a file we wrote. In Listing 9-1, where we
deliberately wrote code that would panic, the way to fix the panic is to not
request an element beyond the range of the vector indexes. When your code
panics in the future, you’ll need to figure out what action the code is taking
with what values to cause the panic and what the code should do instead.

We’ll come back to `panic!` and when we should and should not use `panic!` to
handle error conditions in “To panic! or Not to panic!” on page XX. Next, we’ll
look at how to recover from an error using `Result`.

## 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 Result” on page XX 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 defined on it in
many different situations where the success 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 greeting_file_result = File::open("hello.txt");
}
```

Listing 9-3: Opening a file

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
occurred.

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 greeting_file_result = File::open("hello.txt");

    let greeting_file = match greeting_file_result {
        Ok(file) => file,
        Err(error) => {
            panic!("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
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:

```
thread 'main' panicked at 'Problem opening the file: Os { code:
 2, kind: NotFound, message: "No such file or directory" }',
src/main.rs:8:23
```

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 it did in Listing 9-4. For this, we
add an inner `match` expression, shown in Listing 9-5.

Filename: src/main.rs

```
use std::fs::File;
use std::io::ErrorKind;

fn main() {
    let greeting_file_result = File::open("hello.txt");

    let greeting_file = match greeting_file_result {
        Ok(file) => file,
        Err(error) => match error.kind() {
            ErrorKind::NotFound => {
                match File::create("hello.txt") {
                    Ok(fc) => fc,
                    Err(e) => panic!(
                        "Problem creating the file: {:?}",
                        e
                    ),
                }
            }
            other_error => {
                panic!(
                    "Problem opening the file: {:?}",
                    other_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. 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:

```
// src/main.rs
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:

Filename: src/main.rs

```
use std::fs::File;

fn main() {
    let greeting_file = 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: 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:

Filename: src/main.rs

```
use std::fs::File;

fn main() {
    let greeting_file = File::open("hello.txt")
        .expect("hello.txt should be included in this project");
}
```

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 '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.

Filename: src/main.rs

```
use std::fs::File;
use std::io::{self, Read};

1 fn read_username_from_file() -> Result<String, io::Error> {
  2 let username_file_result = File::open("hello.txt");

  3 let mut username_file = match username_file_result {
      4 Ok(file) => file,
      5 Err(e) => return Err(e),
    };

  6 let mut username = String::new();

  7 match username_file.read_to_string(&mut username) {
      8 Ok(_) => Ok(username),
      9 Err(e) => Err(e),
    }
}
```

Listing 9-6: A function that returns errors to the calling code using `match`

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>` [1]. 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 [8]. 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 [2]
and the `read_to_string` method [7].

The body of the function starts by calling the `File::open` function [2]. 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`
[4] becomes the value in the mutable variable `username_file` [3] 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 [5].

So, if we have a file handle in `username_file`, the function then creates a
new `String` in variable `username` [6] and calls the `read_to_string` method
on the file handle in `username_file` to read the contents of the file into
`username` [7]. 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 [9].

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.

Filename: src/main.rs

```
use std::fs::File;
use std::io::{self, Read};

fn read_username_from_file() -> Result<String, io::Error> {
    let mut username_file = File::open("hello.txt")?;
    let mut username = String::new();
    username_file.read_to_string(&mut username)?;
    Ok(username)
}
```

Listing 9-7: A function that returns errors to the calling code using the `?`
operator

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.

Filename: src/main.rs

```
use std::fs::File;
use std::io::{self, Read};

fn read_username_from_file() -> Result<String, io::Error> {
    let mut username = String::new();

    File::open("hello.txt")?.read_to_string(&mut username)?;

    Ok(username)
}
```

Listing 9-8: Chaining method calls after the `?` operator

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`.

Filename: src/main.rs

```
use std::fs;
use std::io;

fn read_username_from_file() -> Result<String, io::Error> {
    fs::read_to_string("hello.txt")
}
```

Listing 9-9: Using `fs::read_to_string` instead of opening and then reading the
file

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 that is incompatible with the type of
the value we use `?` on.

Filename: src/main.rs

```
use std::fs::File;

fn main() {
    let greeting_file = File::open("hello.txt")?;
}
```

Listing 9-10: Attempting to use the `?` in the `main` function that returns
`()` won’t compile.

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:

```
error[E0277]: the `?` operator can only be used in a function that returns
`Result` or `Option` (or another type that implements `FromResidual`)
 --> src/main.rs:4:48
  |
3 | / fn main() {
4 | |     let greeting_file = File::open("hello.txt")?;
  | |                                                ^ cannot use the `?`
operator in a function that returns `()`
5 | | }
  | |_- this function should return `Result` or `Option` to accept `?`
  |
  = help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not
implemented for `()`
```

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 choice 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
resultant 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.

```
fn last_char_of_first_line(text: &str) -> Option<char> {
    text.lines().next()?.chars().last()
}
```

Listing 9-11: Using the `?` operator on an `Option<T>` value

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 point and exit point of an executable program,
and there are restrictions on what its return type can be for the program 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.

Filename: src/main.rs

```
use std::error::Error;
use std::fs::File;

fn main() -> Result<(), Box<dyn Error>> {
    let greeting_file = File::open("hello.txt")?;

    Ok(())
}
```

Listing 9-12: Changing `main` to return `Result<(), E>` allows the use of the
`?` operator on `Result` values.

The `Box<dyn Error>` type is a *trait object*, which we’ll talk about in “Using
Trait Objects That Allow for Values of Different Types” on page XX. 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, 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.

## To panic! or Not to panic!

So how do you decide when you should call `panic!` and when you should return
`Result`? When code panics, there’s no way to recover. You could call `panic!`
for any error situation, whether there’s a possible way to recover or not, but
then you’re making the decision that a situation is unrecoverable on behalf of
the calling code. When you choose to return a `Result` value, you give the
calling code options. The calling code could choose to attempt to recover in a
way that’s appropriate for its situation, or it could decide that an `Err`
value in this case is unrecoverable, so it can call `panic!` and turn your
recoverable error into an unrecoverable one. Therefore, returning `Result` is a
good default choice when you’re defining a function that might fail.

In situations such as examples, prototype code, and tests, it’s more
appropriate to write code that panics instead of returning a `Result`. Let’s
explore why, then discuss situations in which the compiler can’t tell that
failure is impossible, but you as a human can. The chapter will conclude with
some general guidelines on how to decide whether to panic in library code.

### Examples, Prototype Code, and Tests

When you’re writing an example to illustrate some concept, also including
robust error-handling code can make the example less clear. In examples, it’s
understood that a call to a method like `unwrap` that could panic is meant as a
placeholder for the way you’d want your application to handle errors, which can
differ based on what the rest of your code is doing.

Similarly, the `unwrap` and `expect` methods are very handy when prototyping,
before you’re ready to decide how to handle errors. They leave clear markers in
your code for when you’re ready to make your program more robust.

If a method call fails in a test, you’d want the whole test to fail, even if
that method isn’t the functionality under test. Because `panic!` is how a test
is marked as a failure, calling `unwrap` or `expect` is exactly what should
happen.

### Cases in Which You Have More Information Than the Compiler

It would also be appropriate to call `unwrap` or `expect` when you have some
other logic that ensures the `Result` will have an `Ok` value, but the logic
isn’t something the compiler understands. You’ll still have a `Result` value
that you need to handle: whatever operation you’re calling still has the
possibility of failing in general, even though it’s logically impossible in
your particular situation. If you can ensure by manually inspecting the code
that you’ll never have an `Err` variant, it’s perfectly acceptable to call
`unwrap`, and even better to document the reason you think you’ll never have an
`Err` variant in the `expect` text. Here’s an example:

```
use std::net::IpAddr;

let home: IpAddr = "127.0.0.1"
    .parse()
    .expect("Hardcoded IP address should be valid");
```

We’re creating an `IpAddr` instance by parsing a hardcoded string. We can see
that `127.0.0.1` is a valid IP address, so it’s acceptable to use `expect`
here. However, having a hardcoded, valid string doesn’t change the return type
of the `parse` method: we still get a `Result` value, and the compiler will
still make us handle the `Result` as if the `Err` variant is a possibility
because the compiler isn’t smart enough to see that this string is always a
valid IP address. If the IP address string came from a user rather than being
hardcoded into the program and therefore *did* have a possibility of failure,
we’d definitely want to handle the `Result` in a more robust way instead.
Mentioning the assumption that this IP address is hardcoded will prompt us to
change `expect` to better error-handling code if, in the future, we need to get
the IP address from some other source instead.

### Guidelines for Error Handling

It’s advisable to have your code panic when it’s possible that your code could
end up in a bad state. In this context, a *bad state* is when some assumption,
guarantee, contract, or invariant has been broken, such as when invalid values,
contradictory values, or missing values are passed to your code—plus one or
more of the following:

* The bad state is something that is unexpected, as opposed to something that
will likely happen occasionally, like a user entering data in the wrong format.
* Your code after this point needs to rely on not being in this bad state,
rather than checking for the problem at every step.
* There’s not a good way to encode this information in the types you use. We’ll
work through an example of what we mean in “Encoding States and Behavior as
Types” on page XX.

If someone calls your code and passes in values that don’t make sense, it’s
best to return an error if you can so the user of the library can decide what
they want to do in that case. However, in cases where continuing could be
insecure or harmful, the best choice might be to call `panic!` and alert the
person using your library to the bug in their code so they can fix it during
development. Similarly, `panic!` is often appropriate if you’re calling
external code that is out of your control and it returns an invalid state that
you have no way of fixing.

However, when failure is expected, it’s more appropriate to return a `Result`
than to make a `panic!` call. Examples include a parser being given malformed
data or an HTTP request returning a status that indicates you have hit a rate
limit. In these cases, returning a `Result` indicates that failure is an
expected possibility that the calling code must decide how to handle.

When your code performs an operation that could put a user at risk if it’s
called using invalid values, your code should verify the values are valid first
and panic if the values aren’t valid. This is mostly for safety reasons:
attempting to operate on invalid data can expose your code to vulnerabilities.
This is the main reason the standard library will call `panic!` if you attempt
an out-of-bounds memory access: trying to access memory that doesn’t belong to
the current data structure is a common security problem. Functions often have
*contracts*: their behavior is only guaranteed if the inputs meet particular
requirements. Panicking when the contract is violated makes sense because a
contract violation always indicates a caller-side bug, and it’s not a kind of
error you want the calling code to have to explicitly handle. In fact, there’s
no reasonable way for calling code to recover; the calling *programmers* need
to fix the code. Contracts for a function, especially when a violation will
cause a panic, should be explained in the API documentation for the function.

However, having lots of error checks in all of your functions would be verbose
and annoying. Fortunately, you can use Rust’s type system (and thus the type
checking done by the compiler) to do many of the checks for you. If your
function has a particular type as a parameter, you can proceed with your code’s
logic knowing that the compiler has already ensured you have a valid value. For
example, if you have a type rather than an `Option`, your program expects to
have *something* rather than *nothing*. Your code then doesn’t have to handle
two cases for the `Some` and `None` variants: it will only have one case for
definitely having a value. Code trying to pass nothing to your function won’t
even compile, so your function doesn’t have to check for that case at runtime.
Another example is using an unsigned integer type such as `u32`, which ensures
the parameter is never negative.

### Creating Custom Types for Validation

Let’s take the idea of using Rust’s type system to ensure we have a valid value
one step further and look at creating a custom type for validation. Recall the
guessing game in Chapter 2 in which our code asked the user to guess a number
between 1 and 100. We never validated that the user’s guess was between those
numbers before checking it against our secret number; we only validated that
the guess was positive. In this case, the consequences were not very dire: our
output of “Too high” or “Too low” would still be correct. But it would be a
useful enhancement to guide the user toward valid guesses and have different
behavior when the user guesses a number that’s out of range versus when the
user types, for example, letters instead.

One way to do this would be to parse the guess as an `i32` instead of only a
`u32` to allow potentially negative numbers, and then add a check for the
number being in range, like so:

Filename: src/main.rs

```
loop {
    --snip--

    let guess: i32 = match guess.trim().parse() {
        Ok(num) => num,
        Err(_) => continue,
    };

    if guess < 1 || guess > 100 {
        println!("The secret number will be between 1 and 100.");
        continue;
    }

    match guess.cmp(&secret_number) {
        --snip--
}
```

The `if` expression checks whether our value is out of range, tells the user
about the problem, and calls `continue` to start the next iteration of the loop
and ask for another guess. After the `if` expression, we can proceed with the
comparisons between `guess` and the secret number knowing that `guess` is
between 1 and 100.

However, this is not an ideal solution: if it were absolutely critical that the
program only operated on values between 1 and 100, and it had many functions
with this requirement, having a check like this in every function would be
tedious (and might impact performance).

Instead, we can make a new type and put the validations in a function to create
an instance of the type rather than repeating the validations everywhere. That
way, it’s safe for functions to use the new type in their signatures and
confidently use the values they receive. Listing 9-13 shows one way to define a
`Guess` type that will only create an instance of `Guess` if the `new` function
receives a value between 1 and 100.

Filename: src/lib.rs

```
1 pub struct Guess {
    value: i32,
}

impl Guess {
  2 pub fn new(value: i32) -> Guess {
      3 if value < 1 || value > 100 {
          4 panic!(
                "Guess value must be between 1 and 100, got {}.",
                value
            );
        }

      5 Guess { value }
    }

  6 pub fn value(&self) -> i32 {
        self.value
    }
}
```

Listing 9-13: A `Guess` type that will only continue with values between 1 and
100

First we define a struct named `Guess` that has a field named `value` that
holds an `i32` [1]. This is where the number will be stored.

Then we implement an associated function named `new` on `Guess` that creates
instances of `Guess` values [2]. The `new` function is defined to have one
parameter named `value` of type `i32` and to return a `Guess`. The code in the
body of the `new` function tests `value` to make sure it’s between 1 and 100
[3]. If `value` doesn’t pass this test, we make a `panic!` call [4], which will
alert the programmer who is writing the calling code that they have a bug they
need to fix, because creating a `Guess` with a `value` outside this range would
violate the contract that `Guess::new` is relying on. The conditions in which
`Guess::new` might panic should be discussed in its public-facing API
documentation; we’ll cover documentation conventions indicating the possibility
of a `panic!` in the API documentation that you create in Chapter 14. If
`value` does pass the test, we create a new `Guess` with its `value` field set
to the `value` parameter and return the `Guess` [5].

Next, we implement a method named `value` that borrows `self`, doesn’t have any
other parameters, and returns an `i32` [6]. This kind of method is sometimes
called a *getter* because its purpose is to get some data from its fields and
return it. This public method is necessary because the `value` field of the
`Guess` struct is private. It’s important that the `value` field be private so
code using the `Guess` struct is not allowed to set `value` directly: code
outside the module *must* use the `Guess::new` function to create an instance
of `Guess`, thereby ensuring there’s no way for a `Guess` to have a `value`
that hasn’t been checked by the conditions in the `Guess::new` function.

A function that has a parameter or returns only numbers between 1 and 100 could
then declare in its signature that it takes or returns a `Guess` rather than an
`i32` and wouldn’t need to do any additional checks in its body.

## Summary

Rust’s error-handling features are designed to help you write more robust code.
The `panic!` macro signals that your program is in a state it can’t handle and
lets you tell the process to stop instead of trying to proceed with invalid or
incorrect values. The `Result` enum uses Rust’s type system to indicate that
operations might fail in a way that your code could recover from. You can use
`Result` to tell code that calls your code that it needs to handle potential
success or failure as well. Using `panic!` and `Result` in the appropriate
situations will make your code more reliable in the face of inevitable problems.

Now that you’ve seen useful ways that the standard library uses generics with
the `Option` and `Result` enums, we’ll talk about how generics work and how you
can use them in your code.