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<h2 id="to-panic-or-not-to-panic"><a class="header" href="#to-panic-or-not-to-panic">To <code>panic!</code> or Not to <code>panic!</code></a></h2>
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<p>So, how do you decide when you should call <code>panic!</code> and when you should return
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<code>Result</code>? When code panics, there’s no way to recover. You could call <code>panic!</code>
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for any error situation, whether there’s a possible way to recover or not, but
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then you’re making the decision that a situation is unrecoverable on behalf of
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the calling code. When you choose to return a <code>Result</code> value, you give the
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calling code options. The calling code could choose to attempt to recover in a
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way that’s appropriate for its situation, or it could decide that an <code>Err</code>
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value in this case is unrecoverable, so it can call <code>panic!</code> and turn your
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recoverable error into an unrecoverable one. Therefore, returning <code>Result</code> is a
|
||
good default choice when you’re defining a function that might fail.</p>
|
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<p>In situations such as examples, prototype code, and tests, it’s more
|
||
appropriate to write code that panics instead of returning a <code>Result</code>. Let’s
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explore why, then discuss situations in which the compiler can’t tell that
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failure is impossible, but you as a human can. The chapter will conclude with
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||
some general guidelines on how to decide whether to panic in library code.</p>
|
||
<h3 id="examples-prototype-code-and-tests"><a class="header" href="#examples-prototype-code-and-tests">Examples, Prototype Code, and Tests</a></h3>
|
||
<p>When you’re writing an example to illustrate some concept, also including
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||
robust error-handling code can make the example less clear. In examples, it’s
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||
understood that a call to a method like <code>unwrap</code> 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.</p>
|
||
<p>Similarly, the <code>unwrap</code> and <code>expect</code> methods are very handy when you’re
|
||
prototyping and you’re not yet 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.</p>
|
||
<p>If a method call fails in a test, you’d want the whole test to fail, even if
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||
that method isn’t the functionality under test. Because <code>panic!</code> is how a test
|
||
is marked as a failure, calling <code>unwrap</code> or <code>expect</code> is exactly what should
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||
happen.</p>
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||
<!-- Old headings. Do not remove or links may break. -->
|
||
<p><a id="cases-in-which-you-have-more-information-than-the-compiler"></a></p>
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||
<h3 id="when-you-have-more-information-than-the-compiler"><a class="header" href="#when-you-have-more-information-than-the-compiler">When You Have More Information Than the Compiler</a></h3>
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||
<p>It would also be appropriate to call <code>expect</code> when you have some other logic
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||
that ensures that the <code>Result</code> will have an <code>Ok</code> value, but the logic isn’t
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||
something the compiler understands. You’ll still have a <code>Result</code> value that you
|
||
need to handle: Whatever operation you’re calling still has the possibility of
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||
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
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||
have an <code>Err</code> variant, it’s perfectly acceptable to call <code>expect</code> and document
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the reason you think you’ll never have an <code>Err</code> variant in the argument text.
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Here’s an example:</p>
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> use std::net::IpAddr;
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let home: IpAddr = "127.0.0.1"
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.parse()
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.expect("Hardcoded IP address should be valid");
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<span class="boring">}</span></code></pre>
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<p>We’re creating an <code>IpAddr</code> instance by parsing a hardcoded string. We can see
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that <code>127.0.0.1</code> is a valid IP address, so it’s acceptable to use <code>expect</code>
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here. However, having a hardcoded, valid string doesn’t change the return type
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||
of the <code>parse</code> method: We still get a <code>Result</code> value, and the compiler will
|
||
still make us handle the <code>Result</code> as if the <code>Err</code> variant is a possibility
|
||
because the compiler isn’t smart enough to see that this string is always a
|
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valid IP address. If the IP address string came from a user rather than being
|
||
hardcoded into the program and therefore <em>did</em> have a possibility of failure,
|
||
we’d definitely want to handle the <code>Result</code> in a more robust way instead.
|
||
Mentioning the assumption that this IP address is hardcoded will prompt us to
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change <code>expect</code> to better error-handling code if, in the future, we need to get
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||
the IP address from some other source instead.</p>
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<h3 id="guidelines-for-error-handling"><a class="header" href="#guidelines-for-error-handling">Guidelines for Error Handling</a></h3>
|
||
<p>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 <em>bad state</em> 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:</p>
|
||
<ul>
|
||
<li>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.</li>
|
||
<li>Your code after this point needs to rely on not being in this bad state,
|
||
rather than checking for the problem at every step.</li>
|
||
<li>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 <a href="ch18-03-oo-design-patterns.html#encoding-states-and-behavior-as-types">“Encoding States and Behavior as
|
||
Types”</a><!-- ignore --> in Chapter 18.</li>
|
||
</ul>
|
||
<p>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 that 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 <code>panic!</code> and alert the
|
||
person using your library to the bug in their code so that they can fix it
|
||
during development. Similarly, <code>panic!</code> is often appropriate if you’re calling
|
||
external code that is out of your control and returns an invalid state that you
|
||
have no way of fixing.</p>
|
||
<p>However, when failure is expected, it’s more appropriate to return a <code>Result</code>
|
||
than to make a <code>panic!</code> 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 <code>Result</code> indicates that failure is an
|
||
expected possibility that the calling code must decide how to handle.</p>
|
||
<p>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 <code>panic!</code> 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
|
||
<em>contracts</em>: 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 <em>programmers</em> 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.</p>
|
||
<p>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 that you have a valid
|
||
value. For example, if you have a type rather than an <code>Option</code>, your program
|
||
expects to have <em>something</em> rather than <em>nothing</em>. Your code then doesn’t have
|
||
to handle two cases for the <code>Some</code> and <code>None</code> 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
|
||
<code>u32</code>, which ensures that the parameter is never negative.</p>
|
||
<!-- Old headings. Do not remove or links may break. -->
|
||
<p><a id="creating-custom-types-for-validation"></a></p>
|
||
<h3 id="custom-types-for-validation"><a class="header" href="#custom-types-for-validation">Custom Types for Validation</a></h3>
|
||
<p>Let’s take the idea of using Rust’s type system to ensure that 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.</p>
|
||
<p>One way to do this would be to parse the guess as an <code>i32</code> instead of only a
|
||
<code>u32</code> to allow potentially negative numbers, and then add a check for the
|
||
number being in range, like so:</p>
|
||
<figure class="listing">
|
||
<span class="file-name">Filename: src/main.rs</span>
|
||
<pre><code class="language-rust ignore"><span class="boring">use rand::Rng;
|
||
</span><span class="boring">use std::cmp::Ordering;
|
||
</span><span class="boring">use std::io;
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn main() {
|
||
</span><span class="boring"> println!("Guess the number!");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let secret_number = rand::thread_rng().gen_range(1..=100);
|
||
</span><span class="boring">
|
||
</span> loop {
|
||
// --snip--
|
||
|
||
<span class="boring"> println!("Please input your guess.");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let mut guess = String::new();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> io::stdin()
|
||
</span><span class="boring"> .read_line(&mut guess)
|
||
</span><span class="boring"> .expect("Failed to read line");
|
||
</span><span class="boring">
|
||
</span> 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--
|
||
<span class="boring"> Ordering::Less => println!("Too small!"),
|
||
</span><span class="boring"> Ordering::Greater => println!("Too big!"),
|
||
</span><span class="boring"> Ordering::Equal => {
|
||
</span><span class="boring"> println!("You win!");
|
||
</span><span class="boring"> break;
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> }
|
||
</span> }
|
||
<span class="boring">}</span></code></pre>
|
||
</figure>
|
||
<p>The <code>if</code> expression checks whether our value is out of range, tells the user
|
||
about the problem, and calls <code>continue</code> to start the next iteration of the loop
|
||
and ask for another guess. After the <code>if</code> expression, we can proceed with the
|
||
comparisons between <code>guess</code> and the secret number knowing that <code>guess</code> is
|
||
between 1 and 100.</p>
|
||
<p>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).</p>
|
||
<p>Instead, we can make a new type in a dedicated module 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 <code>Guess</code> type that will only create an instance of
|
||
<code>Guess</code> if the <code>new</code> function receives a value between 1 and 100.</p>
|
||
<figure class="listing" id="listing-9-13">
|
||
<span class="file-name">Filename: src/guessing_game.rs</span>
|
||
<pre class="playground"><code class="language-rust edition2024"><span class="boring">#![allow(unused)]
|
||
</span><span class="boring">fn main() {
|
||
</span>pub struct Guess {
|
||
value: i32,
|
||
}
|
||
|
||
impl Guess {
|
||
pub fn new(value: i32) -> Guess {
|
||
if value < 1 || value > 100 {
|
||
panic!("Guess value must be between 1 and 100, got {value}.");
|
||
}
|
||
|
||
Guess { value }
|
||
}
|
||
|
||
pub fn value(&self) -> i32 {
|
||
self.value
|
||
}
|
||
}
|
||
<span class="boring">}</span></code></pre>
|
||
<figcaption><a href="#listing-9-13">Listing 9-13</a>: A <code>Guess</code> type that will only continue with values between 1 and 100</figcaption>
|
||
</figure>
|
||
<p>Note that this code in <em>src/guessing_game.rs</em> depends on adding a module
|
||
declaration <code>mod guessing_game;</code> in <em>src/lib.rs</em> that we haven’t shown here.
|
||
Within this new module’s file, we define a struct named <code>Guess</code> that has a
|
||
field named <code>value</code> that holds an <code>i32</code>. This is where the number will be
|
||
stored.</p>
|
||
<p>Then, we implement an associated function named <code>new</code> on <code>Guess</code> that creates
|
||
instances of <code>Guess</code> values. The <code>new</code> function is defined to have one
|
||
parameter named <code>value</code> of type <code>i32</code> and to return a <code>Guess</code>. The code in the
|
||
body of the <code>new</code> function tests <code>value</code> to make sure it’s between 1 and 100.
|
||
If <code>value</code> doesn’t pass this test, we make a <code>panic!</code> call, which will alert
|
||
the programmer who is writing the calling code that they have a bug they need
|
||
to fix, because creating a <code>Guess</code> with a <code>value</code> outside this range would
|
||
violate the contract that <code>Guess::new</code> is relying on. The conditions in which
|
||
<code>Guess::new</code> might panic should be discussed in its public-facing API
|
||
documentation; we’ll cover documentation conventions indicating the possibility
|
||
of a <code>panic!</code> in the API documentation that you create in Chapter 14. If
|
||
<code>value</code> does pass the test, we create a new <code>Guess</code> with its <code>value</code> field set
|
||
to the <code>value</code> parameter and return the <code>Guess</code>.</p>
|
||
<p>Next, we implement a method named <code>value</code> that borrows <code>self</code>, doesn’t have any
|
||
other parameters, and returns an <code>i32</code>. This kind of method is sometimes called
|
||
a <em>getter</em> because its purpose is to get some data from its fields and return
|
||
it. This public method is necessary because the <code>value</code> field of the <code>Guess</code>
|
||
struct is private. It’s important that the <code>value</code> field be private so that
|
||
code using the <code>Guess</code> struct is not allowed to set <code>value</code> directly: Code
|
||
outside the <code>guessing_game</code> module <em>must</em> use the <code>Guess::new</code> function to
|
||
create an instance of <code>Guess</code>, thereby ensuring that there’s no way for a
|
||
<code>Guess</code> to have a <code>value</code> that hasn’t been checked by the conditions in the
|
||
<code>Guess::new</code> function.</p>
|
||
<p>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 <code>Guess</code> rather than an
|
||
<code>i32</code> and wouldn’t need to do any additional checks in its body.</p>
|
||
<h2 id="summary"><a class="header" href="#summary">Summary</a></h2>
|
||
<p>Rust’s error-handling features are designed to help you write more robust code.
|
||
The <code>panic!</code> 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 <code>Result</code> enum uses Rust’s type system to indicate that
|
||
operations might fail in a way that your code could recover from. You can use
|
||
<code>Result</code> to tell code that calls your code that it needs to handle potential
|
||
success or failure as well. Using <code>panic!</code> and <code>Result</code> in the appropriate
|
||
situations will make your code more reliable in the face of inevitable problems.</p>
|
||
<p>Now that you’ve seen useful ways that the standard library uses generics with
|
||
the <code>Option</code> and <code>Result</code> enums, we’ll talk about how generics work and how you
|
||
can use them in your code.</p>
|
||
|
||
</main>
|
||
|
||
<nav class="nav-wrapper" aria-label="Page navigation">
|
||
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<template id=fa-eye><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 576 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M288 32c-80.8 0-145.5 36.8-192.6 80.6C48.6 156 17.3 208 2.5 243.7c-3.3 7.9-3.3 16.7 0 24.6C17.3 304 48.6 356 95.4 399.4C142.5 443.2 207.2 480 288 480s145.5-36.8 192.6-80.6c46.8-43.5 78.1-95.4 93-131.1c3.3-7.9 3.3-16.7 0-24.6c-14.9-35.7-46.2-87.7-93-131.1C433.5 68.8 368.8 32 288 32zM432 256c0 79.5-64.5 144-144 144s-144-64.5-144-144s64.5-144 144-144s144 64.5 144 144zM288 192c0 35.3-28.7 64-64 64c-11.5 0-22.3-3-31.6-8.4c-.2 2.8-.4 5.5-.4 8.4c0 53 43 96 96 96s96-43 96-96s-43-96-96-96c-2.8 0-5.6 .1-8.4 .4c5.3 9.3 8.4 20.1 8.4 31.6z"/></svg></span></template>
|
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<template id=fa-eye-slash><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 640 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M38.8 5.1C28.4-3.1 13.3-1.2 5.1 9.2S-1.2 34.7 9.2 42.9l592 464c10.4 8.2 25.5 6.3 33.7-4.1s6.3-25.5-4.1-33.7L525.6 386.7c39.6-40.6 66.4-86.1 79.9-118.4c3.3-7.9 3.3-16.7 0-24.6c-14.9-35.7-46.2-87.7-93-131.1C465.5 68.8 400.8 32 320 32c-68.2 0-125 26.3-169.3 60.8L38.8 5.1zM223.1 149.5C248.6 126.2 282.7 112 320 112c79.5 0 144 64.5 144 144c0 24.9-6.3 48.3-17.4 68.7L408 294.5c5.2-11.8 8-24.8 8-38.5c0-53-43-96-96-96c-2.8 0-5.6 .1-8.4 .4c5.3 9.3 8.4 20.1 8.4 31.6c0 10.2-2.4 19.8-6.6 28.3l-90.3-70.8zm223.1 298L373 389.9c-16.4 6.5-34.3 10.1-53 10.1c-79.5 0-144-64.5-144-144c0-6.9 .5-13.6 1.4-20.2L83.1 161.5C60.3 191.2 44 220.8 34.5 243.7c-3.3 7.9-3.3 16.7 0 24.6c14.9 35.7 46.2 87.7 93 131.1C174.5 443.2 239.2 480 320 480c47.8 0 89.9-12.9 126.2-32.5z"/></svg></span></template>
|
||
<template id=fa-copy><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M502.6 70.63l-61.25-61.25C435.4 3.371 427.2 0 418.7 0H255.1c-35.35 0-64 28.66-64 64l.0195 256C192 355.4 220.7 384 256 384h192c35.2 0 64-28.8 64-64V93.25C512 84.77 508.6 76.63 502.6 70.63zM464 320c0 8.836-7.164 16-16 16H255.1c-8.838 0-16-7.164-16-16L239.1 64.13c0-8.836 7.164-16 16-16h128L384 96c0 17.67 14.33 32 32 32h47.1V320zM272 448c0 8.836-7.164 16-16 16H63.1c-8.838 0-16-7.164-16-16L47.98 192.1c0-8.836 7.164-16 16-16H160V128H63.99c-35.35 0-64 28.65-64 64l.0098 256C.002 483.3 28.66 512 64 512h192c35.2 0 64-28.8 64-64v-32h-47.1L272 448z"/></svg></span></template>
|
||
<template id=fa-play><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 384 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M73 39c-14.8-9.1-33.4-9.4-48.5-.9S0 62.6 0 80V432c0 17.4 9.4 33.4 24.5 41.9s33.7 8.1 48.5-.9L361 297c14.3-8.7 23-24.2 23-41s-8.7-32.2-23-41L73 39z"/></svg></span></template>
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