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[TOC]
Error Handling
Rust's commitment to reliability extends to 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 will require you to acknowledge the possibility of an error occurring and take some action before your code will compile. This makes your program more robust by ensuring that you won't only discover errors after you've deployed your code to production.
Rust groups errors into two major categories: recoverable and unrecoverable errors. Recoverable errors are situations when it's usually reasonable to report the problem to the user and retry the operation, like a file not being found. Unrecoverable errors are always symptoms of bugs, like trying to access a location beyond the end of an array.
Most languages don't distinguish between the two kinds of errors, and handle
both in the same way using mechanisms like exceptions. Rust doesn't have
exceptions. Instead, it has the value Result<T, E>
for recoverable errors and
the panic!
macro that stops execution when it encounters unrecoverable
errors. This chapter will cover calling panic!
first, then talk about
returning Result<T, E>
values. Finally, we'll discuss considerations to take
into account when deciding whether to try to recover from an error or to stop
execution.
Unrecoverable Errors with panic!
Sometimes, bad things happen, and there's nothing that you can do about it. For
these cases, Rust has the panic!
macro. When this macro executes, your
program will print a failure message, unwind and clean up the stack, and then
quit. The most common situation this occurs in is when a bug of some kind has
been detected and it's not clear to the programmer how to handle the error.
Unwinding
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, but this walking and cleanup is a lot of work. The alternative is to immediatelyabort
, 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 program you need to make the resulting binary as small as possible, you can switch from unwinding to aborting on panic by addingpanic = 'abort'
to the appropriate[profile]
sections in yourCargo.toml
. For example, if you want to abort on panic in release mode:profile.release panic = 'abort'
Let's try calling panic!()
with a simple program:
Filename: src/main.rs
fn main() {
panic!("crash and burn");
}
If you run it, you'll see something like this:
$ cargo run
Compiling panic v0.1.0 (file:///projects/panic)
Finished debug [unoptimized + debuginfo] target(s) in 0.25 secs
Running `target/debug/panic`
thread 'main' panicked at 'crash and burn', src/main.rs:2
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: Process didn't exit successfully: `target/debug/panic` (exit code: 101)
The last three lines contain the error message caused by the call to panic!
.
The first line shows our panic message and the place in our source code where
the panic occurred: src/main.rs:2
indicates that it's the second like of our
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. 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!
. We can use the backtrace
of the functions the panic!
call came from to figure this out.
Using a panic!
Backtrace
Let's look at another example to 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:
Filename: src/main.rs
fn main() {
let v = vec![1, 2, 3];
v[100];
}
We're attempting to access the hundredth element of our vector, but it only has
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.
Other languages like C will attempt to give you exactly what you asked for in this situation, even though it isn't what you want: you'll get whatever is at the location in memory that would correspond to that element in the vector, even though the memory doesn't belong to the vector. This is called a buffer overread, and can lead to security vulnerabilities if an attacker can manipulate the index in such a way as to read data they shouldn't be allowed to that is stored after the array.
In order 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:
$ cargo run
Compiling panic v0.1.0 (file:///projects/panic)
Finished debug [unoptimized + debuginfo] target(s) in 0.27 secs
Running `target/debug/panic`
thread 'main' panicked at 'index out of bounds: the len is 3 but the index is
100', ../src/libcollections/vec.rs:1265
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: Process didn't exit successfully: `target/debug/panic` (exit code: 101)
This points at a file we didn't write, ../src/libcollections/vec.rs. That's
the implementation of Vec<T>
in the standard library. The code that gets run
when we use []
on our vector v
is in ../src/libcollections/vec.rs, and
that is where the panic!
is actually happening.
The next 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. Let's
try that. Listing 9-1 shows the output:
That's a lot of output! Line 11 of the backtrace points to the line in our
project causing the problem: src/main.rs
, line four. A backtrace is a list of
all the functions that have been called to get to this point. Backtraces in
Rust work like 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 the lines mentioning your files
are code that your code called; the lines below are code that called your code.
These lines might include core Rust code, standard library code, or crates that
you're using.
If we don't want our program to panic, the location pointed to by the first line mentioning a file we wrote is where we should start investigating in order to figure out how we got to this location with values that caused the panic. In our example where we deliberately wrote code that would panic in order to demonstrate how to use backtraces, the way to fix the panic is to not try to request an element at index 100 from a vector that only contains three items. When your code panics in the future, you'll need to figure out for your particular case what action the code is taking with what values that causes the panic and what the code should do instead.
We'll come back to panic!
and when we should and should not use these methods
later in the chapter. Next, we'll now look at how to recover from an error with
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 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.
How do we know File::open
returns a Result
? We could look at the standard
library API documentation. 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.
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
:
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 the 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 XX. In short, in the context of a pattern, &
matches a reference
and give 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" } }',
../src/libcore/result.rs:837
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 ofexpect
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" } }', ../src/libcore/result.rs:837
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:
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:
The ?
placed after a Result
value is defined to work the exact same way as
thematch
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 binding 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 use try!
in the main
function, which you'll
recall has a return type of ()
:
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.
To panic!
or Not To panic!
So how do you decide when you should panic!
and when you should return
Result
? When code panics, there's no way to recover. You could choose to call
panic!
for any error situation, whether there's a possible way to recover or
not, but then you're making the decision for your callers that a situation is
unrecoverable. When you choose to return a Result
value, you give your caller
options, rather than making the decision for them. They could choose to attempt
to recover in a way that's appropriate for their situation, or they could
decide that actually, an Err
value in this case is unrecoverable, so they 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.
There are a few situations in which it's more appropriate to write code that
panics instead of returning a Result
, but they are less common. Let's discuss
why it's appropriate to panic in examples, prototype code, and tests, then
situations where you as a human can know a method won't fail that the compiler
can't reason about, and conclude with some general guidelines on how to decide
whether to panic in library code.
Examples, Prototype Code, and Tests: Perfectly Fine to Panic
When you're writing an example to illustrate some concept, having robust error
handling code in the example as well 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 that you'd actually like 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, we'd want the whole test to fail, even if that
method isn't the functionality under test. Because panic!
is how a test gets
marked as a failure, calling unwrap
or expect
is exactly what makes sense to
do.
Cases When You Have More Information Than The Compiler
It would also be appropriate to call unwrap
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 is perfectly acceptable to call unwrap
. Here's an
example:
use std::net::IpAddr;
let home = "127.0.0.1".parse::<IpAddr>().unwrap();
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 unwrap
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 still a possibility
since 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 instead of 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.
Guidelines for Error Handling
It's advisable to have your code panic!
when it's possible that you could end
up in a bad state---in this context, 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 not something that's expected to happen occasionally
- Your code after this point needs to rely on not being in this bad state
- There's not a good way to encode this information in the types you use
If someone calls your code and passes in values that don't make sense, the best
thing might be to panic!
and alert the person using your library to the bug
in their code so that 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.
When a bad state is reached, but it's expected to happen no matter how well you
write your code, it's still more appropriate to return a Result
rather than
calling panic!
. Examples of this 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, you should indicate that failure is an expected possibility by
returning a Result
in order to propagate these bad states upwards so that the
caller can decide how they would like to handle the problem. To panic!
wouldn't be the best way to handle these cases.
When your code performs operations on 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 that the standard library will
panic!
if you attempt an out-of-bounds array 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 is not a kind of error you want callers 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.
Having lots of error checks in all of your functions would be verbose and
annoying, though. Luckily, you can use Rust's type system (and thus the type
checking the compiler does) to do a lot of the checks for you. If your function
takes a particular type as an argument, 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 like u32
, which ensures the
argument value 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, where our code asked the user to guess a number between 1 and 100. We actually never validated that the user's guess was between those numbers before checking it against our secret number, only that it was positive. In this case, the consequences were not very dire: our output of "Too high" or "Too low" would still be correct. It would be a useful enhancement to guide the user towards valid guesses, though, and have different behavior when a user guesses a number that's out of range versus when a 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, then add a check for the number
being in range:
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 to see if 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 was absolutely critical that the program only operated on values between 1 and 100, and it had many functions with this requirement, it would be tedious (and potentially impact performance) to have a check like this in every function.
Instead, we can make a new type and put the validations in the type's
constructor rather than repeating them. That way, it's safe for functions to
use the new type in their signatures and confidently use the values they
receive. Listing 9-8 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:
First, we define a struct named Guess
that has a field named value
that
holds a u32
. This is where the number will be stored.
Then we implement an associated function named new
on Guess
that is a
constructor of Guess
values. The new
function takes one argument named
value
of type u32
and returns a Guess
. The code in the body of the new
function tests the value
argument to make sure it is between 1 and 100. If
value
doesn't pass this test, we call panic!
, which will alert the
programmer who is calling this code that they have a bug they need to fix,
since 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 around 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
argument, and return the Guess
.
Next, we implement a method named value
that borrows self
, doesn't take any
other arguments, and returns a u32
. This is a kind of method sometimes called
a getter, since 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 is private so that
code using the Guess
struct is not allowed to set value
directly: callers
must use the Guess::new
constructor function to create an instance of
Guess
, which ensures there's no way for a Guess
to have a value
that
hasn't been checked by the conditions in the constructor.
A function that takes as an argument or returns only numbers between 1 and 100
could then declare in its signature that it takes a Guess
rather than a
u32
, 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 we've seen useful ways that the standard library uses generics with
the Option
and Result
enums, let's talk about how generics work and how you
can make use of them in your code.