Refactoring to Improve Modularity and Error Handling
To improve our program, we’ll fix four problems that have to do with the
program’s structure and how it’s handling potential errors. First, our main
function now performs two tasks: it parses arguments and reads files. As our
program grows, the number of separate tasks the main function handles will
increase. As a function gains responsibilities, it becomes more difficult to
reason about, harder to test, and harder to change without breaking one of its
parts. It’s best to separate functionality so each function is responsible for
one task.
This issue also ties into the second problem: although query and file_path
are configuration variables to our program, variables like contents are used
to perform the program’s logic. The longer main becomes, the more variables
we’ll need to bring into scope; the more variables we have in scope, the harder
it will be to keep track of the purpose of each. It’s best to group the
configuration variables into one structure to make their purpose clear.
The third problem is that we’ve used expect to print an error message when
reading the file fails, but the error message just prints Should have been able to read the file. Reading a file can fail in a number of ways: for
example, the file could be missing, or we might not have permission to open it.
Right now, regardless of the situation, we’d print the same error message for
everything, which wouldn’t give the user any information!
Fourth, we use expect repeatedly to handle different errors, and if the user
runs our program without specifying enough arguments, they’ll get an index out of bounds error from Rust that doesn’t clearly explain the problem. It would
be best if all the error-handling code were in one place so future maintainers
had only one place to consult the code if the error-handling logic needed to
change. Having all the error-handling code in one place will also ensure that
we’re printing messages that will be meaningful to our end users.
Let’s address these four problems by refactoring our project.
Separation of Concerns for Binary Projects
The organizational problem of allocating responsibility for multiple tasks to
the main function is common to many binary projects. As a result, the Rust
community has developed guidelines for splitting the separate concerns of a
binary program when main starts getting large. This process has the following
steps:
- Split your program into a main.rs and a lib.rs and move your program’s logic to lib.rs.
- As long as your command line parsing logic is small, it can remain in main.rs.
- When the command line parsing logic starts getting complicated, extract it from main.rs and move it to lib.rs.
The responsibilities that remain in the main function after this process
should be limited to the following:
- Calling the command line parsing logic with the argument values
- Setting up any other configuration
- Calling a
runfunction in lib.rs - Handling the error if
runreturns an error
This pattern is about separating concerns: main.rs handles running the
program, and lib.rs handles all the logic of the task at hand. Because you
can’t test the main function directly, this structure lets you test all of
your program’s logic by moving it into functions in lib.rs. The code that
remains in main.rs will be small enough to verify its correctness by reading
it. Let’s rework our program by following this process.
Extracting the Argument Parser
We’ll extract the functionality for parsing arguments into a function that
main will call to prepare for moving the command line parsing logic to
src/lib.rs. Listing 12-5 shows the new start of main that calls a new
function parse_config, which we’ll define in src/main.rs for the moment.
Filename: src/main.rs
use std::env;
use std::fs;
fn main() {
let args: Vec<String> = env::args().collect();
let (query, file_path) = parse_config(&args);
// --snip--
println!("Searching for {}", query);
println!("In file {}", file_path);
let contents = fs::read_to_string(file_path)
.expect("Should have been able to read the file");
println!("With text:\n{contents}");
}
fn parse_config(args: &[String]) -> (&str, &str) {
let query = &args[1];
let file_path = &args[2];
(query, file_path)
}
Listing 12-5: Extracting a parse_config function from
main
We’re still collecting the command line arguments into a vector, but instead of
assigning the argument value at index 1 to the variable query and the
argument value at index 2 to the variable file_path within the main
function, we pass the whole vector to the parse_config function. The
parse_config function then holds the logic that determines which argument
goes in which variable and passes the values back to main. We still create
the query and file_path variables in main, but main no longer has the
responsibility of determining how the command line arguments and variables
correspond.
This rework may seem like overkill for our small program, but we’re refactoring in small, incremental steps. After making this change, run the program again to verify that the argument parsing still works. It’s good to check your progress often, to help identify the cause of problems when they occur.
Grouping Configuration Values
We can take another small step to improve the parse_config function further.
At the moment, we’re returning a tuple, but then we immediately break that
tuple into individual parts again. This is a sign that perhaps we don’t have
the right abstraction yet.
Another indicator that shows there’s room for improvement is the config part
of parse_config, which implies that the two values we return are related and
are both part of one configuration value. We’re not currently conveying this
meaning in the structure of the data other than by grouping the two values into
a tuple; we’ll instead put the two values into one struct and give each of the
struct fields a meaningful name. Doing so will make it easier for future
maintainers of this code to understand how the different values relate to each
other and what their purpose is.
Listing 12-6 shows the improvements to the parse_config function.
Filename: src/main.rs
use std::env;
use std::fs;
fn main() {
let args: Vec<String> = env::args().collect();
let config = parse_config(&args);
println!("Searching for {}", config.query);
println!("In file {}", config.file_path);
let contents = fs::read_to_string(config.file_path)
.expect("Should have been able to read the file");
// --snip--
println!("With text:\n{contents}");
}
struct Config {
query: String,
file_path: String,
}
fn parse_config(args: &[String]) -> Config {
let query = args[1].clone();
let file_path = args[2].clone();
Config { query, file_path }
}
Listing 12-6: Refactoring parse_config to return an
instance of a Config struct
We’ve added a struct named Config defined to have fields named query and
file_path. The signature of parse_config now indicates that it returns a
Config value. In the body of parse_config, where we used to return
string slices that reference String values in args, we now define Config
to contain owned String values. The args variable in main is the owner of
the argument values and is only letting the parse_config function borrow
them, which means we’d violate Rust’s borrowing rules if Config tried to take
ownership of the values in args.
There are a number of ways we could manage the String data; the easiest,
though somewhat inefficient, route is to call the clone method on the values.
This will make a full copy of the data for the Config instance to own, which
takes more time and memory than storing a reference to the string data.
However, cloning the data also makes our code very straightforward because we
don’t have to manage the lifetimes of the references; in this circumstance,
giving up a little performance to gain simplicity is a worthwhile trade-off.
The Trade-Offs of Using
cloneThere’s a tendency among many Rustaceans to avoid using
cloneto fix ownership problems because of its runtime cost. In Chapter 13, you’ll learn how to use more efficient methods in this type of situation. But for now, it’s okay to copy a few strings to continue making progress because you’ll make these copies only once and your file path and query string are very small. It’s better to have a working program that’s a bit inefficient than to try to hyperoptimize code on your first pass. As you become more experienced with Rust, it’ll be easier to start with the most efficient solution, but for now, it’s perfectly acceptable to callclone.
We’ve updated main so it places the instance of Config returned by
parse_config into a variable named config, and we updated the code that
previously used the separate query and file_path variables so it now uses
the fields on the Config struct instead.
Now our code more clearly conveys that query and file_path are related and
that their purpose is to configure how the program will work. Any code that
uses these values knows to find them in the config instance in the fields
named for their purpose.
Creating a Constructor for Config
So far, we’ve extracted the logic responsible for parsing the command line
arguments from main and placed it in the parse_config function. Doing so
helped us to see that the query and file_path values were related and that
relationship should be conveyed in our code. We then added a Config struct to
name the related purpose of query and file_path and to be able to return the
values’ names as struct field names from the parse_config function.
So now that the purpose of the parse_config function is to create a Config
instance, we can change parse_config from a plain function to a function
named new that is associated with the Config struct. Making this change
will make the code more idiomatic. We can create instances of types in the
standard library, such as String, by calling String::new. Similarly, by
changing parse_config into a new function associated with Config, we’ll
be able to create instances of Config by calling Config::new. Listing 12-7
shows the changes we need to make.
Filename: src/main.rs
use std::env;
use std::fs;
fn main() {
let args: Vec<String> = env::args().collect();
let config = Config::new(&args);
println!("Searching for {}", config.query);
println!("In file {}", config.file_path);
let contents = fs::read_to_string(config.file_path)
.expect("Should have been able to read the file");
println!("With text:\n{contents}");
// --snip--
}
// --snip--
struct Config {
query: String,
file_path: String,
}
impl Config {
fn new(args: &[String]) -> Config {
let query = args[1].clone();
let file_path = args[2].clone();
Config { query, file_path }
}
}
Listing 12-7: Changing parse_config into
Config::new
We’ve updated main where we were calling parse_config to instead call
Config::new. We’ve changed the name of parse_config to new and moved it
within an impl block, which associates the new function with Config. Try
compiling this code again to make sure it works.
Fixing the Error Handling
Now we’ll work on fixing our error handling. Recall that attempting to access
the values in the args vector at index 1 or index 2 will cause the program to
panic if the vector contains fewer than three items. Try running the program
without any arguments; it will look like this:
$ cargo run
Compiling minigrep v0.1.0 (file:///projects/minigrep)
Finished dev [unoptimized + debuginfo] target(s) in 0.0s
Running `target/debug/minigrep`
thread 'main' panicked at 'index out of bounds: the len is 1 but the index is 1', src/main.rs:27:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
The line index out of bounds: the len is 1 but the index is 1 is an error
message intended for programmers. It won’t help our end users understand what
they should do instead. Let’s fix that now.
Improving the Error Message
In Listing 12-8, we add a check in the new function that will verify that the
slice is long enough before accessing index 1 and 2. If the slice isn’t long
enough, the program panics and displays a better error message.
Filename: src/main.rs
use std::env;
use std::fs;
fn main() {
let args: Vec<String> = env::args().collect();
let config = Config::new(&args);
println!("Searching for {}", config.query);
println!("In file {}", config.file_path);
let contents = fs::read_to_string(config.file_path)
.expect("Should have been able to read the file");
println!("With text:\n{contents}");
}
struct Config {
query: String,
file_path: String,
}
impl Config {
// --snip--
fn new(args: &[String]) -> Config {
if args.len() < 3 {
panic!("not enough arguments");
}
// --snip--
let query = args[1].clone();
let file_path = args[2].clone();
Config { query, file_path }
}
}
Listing 12-8: Adding a check for the number of arguments
This code is similar to the Guess::new function we wrote in Listing
9-13, where we called panic! when the
value argument was out of the range of valid values. Instead of checking for
a range of values here, we’re checking that the length of args is at least 3
and the rest of the function can operate under the assumption that this
condition has been met. If args has fewer than three items, this condition
will be true, and we call the panic! macro to end the program immediately.
With these extra few lines of code in new, let’s run the program without any
arguments again to see what the error looks like now:
$ cargo run
Compiling minigrep v0.1.0 (file:///projects/minigrep)
Finished dev [unoptimized + debuginfo] target(s) in 0.0s
Running `target/debug/minigrep`
thread 'main' panicked at 'not enough arguments', src/main.rs:26:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
This output is better: we now have a reasonable error message. However, we also
have extraneous information we don’t want to give to our users. Perhaps using
the technique we used in Listing 9-13 isn’t the best to use here: a call to
panic! is more appropriate for a programming problem than a usage problem,
as discussed in Chapter 9. Instead,
we’ll use the other technique you learned about in Chapter 9—returning a
Result that indicates either success or an error.
Returning a Result Instead of Calling panic!
We can instead return a Result value that will contain a Config instance in
the successful case and will describe the problem in the error case. We’re also
going to change the function name from new to build because many
programmers expect new functions to never fail. When Config::build is
communicating to main, we can use the Result type to signal there was a
problem. Then we can change main to convert an Err variant into a more
practical error for our users without the surrounding text about thread 'main' and RUST_BACKTRACE that a call to panic! causes.
Listing 12-9 shows the changes we need to make to the return value of the
function we’re now calling Config::build and the body of the function needed
to return a Result. Note that this won’t compile until we update main as
well, which we’ll do in the next listing.
Filename: src/main.rs
use std::env;
use std::fs;
fn main() {
let args: Vec<String> = env::args().collect();
let config = Config::new(&args);
println!("Searching for {}", config.query);
println!("In file {}", config.file_path);
let contents = fs::read_to_string(config.file_path)
.expect("Should have been able to read the file");
println!("With text:\n{contents}");
}
struct Config {
query: String,
file_path: String,
}
impl Config {
fn build(args: &[String]) -> Result<Config, &'static str> {
if args.len() < 3 {
return Err("not enough arguments");
}
let query = args[1].clone();
let file_path = args[2].clone();
Ok(Config { query, file_path })
}
}
Listing 12-9: Returning a Result from
Config::build
Our build function returns a Result with a Config instance in the success
case and a &'static str in the error case. Our error values will always be
string literals that have the 'static lifetime.
We’ve made two changes in the body of the function: instead of calling panic!
when the user doesn’t pass enough arguments, we now return an Err value, and
we’ve wrapped the Config return value in an Ok. These changes make the
function conform to its new type signature.
Returning an Err value from Config::build allows the main function to
handle the Result value returned from the build function and exit the
process more cleanly in the error case.
Calling Config::build and Handling Errors
To handle the error case and print a user-friendly message, we need to update
main to handle the Result being returned by Config::build, as shown in
Listing 12-10. We’ll also take the responsibility of exiting the command line
tool with a nonzero error code away from panic! and instead implement it by
hand. A nonzero exit status is a convention to signal to the process that
called our program that the program exited with an error state.
Filename: src/main.rs
use std::env;
use std::fs;
use std::process;
fn main() {
let args: Vec<String> = env::args().collect();
let config = Config::build(&args).unwrap_or_else(|err| {
println!("Problem parsing arguments: {err}");
process::exit(1);
});
// --snip--
println!("Searching for {}", config.query);
println!("In file {}", config.file_path);
let contents = fs::read_to_string(config.file_path)
.expect("Should have been able to read the file");
println!("With text:\n{contents}");
}
struct Config {
query: String,
file_path: String,
}
impl Config {
fn build(args: &[String]) -> Result<Config, &'static str> {
if args.len() < 3 {
return Err("not enough arguments");
}
let query = args[1].clone();
let file_path = args[2].clone();
Ok(Config { query, file_path })
}
}
Listing 12-10: Exiting with an error code if building a
Config fails
In this listing, we’ve used a method we haven’t covered in detail yet:
unwrap_or_else, which is defined on Result<T, E> by the standard library.
Using unwrap_or_else allows us to define some custom, non-panic! error
handling. If the Result is an Ok value, this method’s behavior is similar
to unwrap: it returns the inner value Ok is wrapping. However, if the value
is an Err value, this method calls the code in the closure, which is an
anonymous function we define and pass as an argument to unwrap_or_else. We’ll
cover closures in more detail in Chapter 13. For now,
you just need to know that unwrap_or_else will pass the inner value of the
Err, which in this case is the static string "not enough arguments" that we
added in Listing 12-9, to our closure in the argument err that appears
between the vertical pipes. The code in the closure can then use the err
value when it runs.
We’ve added a new use line to bring process from the standard library into
scope. The code in the closure that will be run in the error case is only two
lines: we print the err value and then call process::exit. The
process::exit function will stop the program immediately and return the
number that was passed as the exit status code. This is similar to the
panic!-based handling we used in Listing 12-8, but we no longer get all the
extra output. Let’s try it:
$ cargo run
Compiling minigrep v0.1.0 (file:///projects/minigrep)
Finished dev [unoptimized + debuginfo] target(s) in 0.48s
Running `target/debug/minigrep`
Problem parsing arguments: not enough arguments
Great! This output is much friendlier for our users.
Extracting Logic from main
Now that we’ve finished refactoring the configuration parsing, let’s turn to
the program’s logic. As we stated in “Separation of Concerns for Binary
Projects”, we’ll
extract a function named run that will hold all the logic currently in the
main function that isn’t involved with setting up configuration or handling
errors. When we’re done, main will be concise and easy to verify by
inspection, and we’ll be able to write tests for all the other logic.
Listing 12-11 shows the extracted run function. For now, we’re just making
the small, incremental improvement of extracting the function. We’re still
defining the function in src/main.rs.
Filename: src/main.rs
use std::env;
use std::fs;
use std::process;
fn main() {
// --snip--
let args: Vec<String> = env::args().collect();
let config = Config::build(&args).unwrap_or_else(|err| {
println!("Problem parsing arguments: {err}");
process::exit(1);
});
println!("Searching for {}", config.query);
println!("In file {}", config.file_path);
run(config);
}
fn run(config: Config) {
let contents = fs::read_to_string(config.file_path)
.expect("Should have been able to read the file");
println!("With text:\n{contents}");
}
// --snip--
struct Config {
query: String,
file_path: String,
}
impl Config {
fn build(args: &[String]) -> Result<Config, &'static str> {
if args.len() < 3 {
return Err("not enough arguments");
}
let query = args[1].clone();
let file_path = args[2].clone();
Ok(Config { query, file_path })
}
}
Listing 12-11: Extracting a run function containing the
rest of the program logic
The run function now contains all the remaining logic from main, starting
from reading the file. The run function takes the Config instance as an
argument.
Returning Errors from the run Function
With the remaining program logic separated into the run function, we can
improve the error handling, as we did with Config::build in Listing 12-9.
Instead of allowing the program to panic by calling expect, the run
function will return a Result<T, E> when something goes wrong. This will let
us further consolidate the logic around handling errors into main in a
user-friendly way. Listing 12-12 shows the changes we need to make to the
signature and body of run.
Filename: src/main.rs
use std::env;
use std::fs;
use std::process;
use std::error::Error;
// --snip--
fn main() {
let args: Vec<String> = env::args().collect();
let config = Config::build(&args).unwrap_or_else(|err| {
println!("Problem parsing arguments: {err}");
process::exit(1);
});
println!("Searching for {}", config.query);
println!("In file {}", config.file_path);
run(config);
}
fn run(config: Config) -> Result<(), Box<dyn Error>> {
let contents = fs::read_to_string(config.file_path)?;
println!("With text:\n{contents}");
Ok(())
}
struct Config {
query: String,
file_path: String,
}
impl Config {
fn build(args: &[String]) -> Result<Config, &'static str> {
if args.len() < 3 {
return Err("not enough arguments");
}
let query = args[1].clone();
let file_path = args[2].clone();
Ok(Config { query, file_path })
}
}
Listing 12-12: Changing the run function to return
Result
We’ve made three significant changes here. First, we changed the return type of
the run function to Result<(), Box<dyn Error>>. This function previously
returned the unit type, (), and we keep that as the value returned in the
Ok case.
For the error type, we used the trait object Box<dyn Error> (and we’ve
brought std::error::Error into scope with a use statement at the top).
We’ll cover trait objects in Chapter 17. For now, just
know that Box<dyn Error> means the function will return a type that
implements the Error trait, but we don’t have to specify what particular type
the return value will be. This gives us flexibility to return error values that
may be of different types in different error cases. The dyn keyword is short
for “dynamic.”
Second, we’ve removed the call to expect in favor of the ? operator, as we
talked about in Chapter 9. Rather than
panic! on an error, ? will return the error value from the current function
for the caller to handle.
Third, the run function now returns an Ok value in the success case.
We’ve declared the run function’s success type as () in the signature,
which means we need to wrap the unit type value in the Ok value. This
Ok(()) syntax might look a bit strange at first, but using () like this is
the idiomatic way to indicate that we’re calling run for its side effects
only; it doesn’t return a value we need.
When you run this code, it will compile but will display a warning:
$ cargo run the poem.txt
Compiling minigrep v0.1.0 (file:///projects/minigrep)
warning: unused `Result` that must be used
--> src/main.rs:19:5
|
19 | run(config);
| ^^^^^^^^^^^^
|
= note: `#[warn(unused_must_use)]` on by default
= note: this `Result` may be an `Err` variant, which should be handled
warning: `minigrep` (bin "minigrep") generated 1 warning
Finished dev [unoptimized + debuginfo] target(s) in 0.71s
Running `target/debug/minigrep the poem.txt`
Searching for the
In file poem.txt
With text:
I'm nobody! Who are you?
Are you nobody, too?
Then there's a pair of us - don't tell!
They'd banish us, you know.
How dreary to be somebody!
How public, like a frog
To tell your name the livelong day
To an admiring bog!
Rust tells us that our code ignored the Result value and the Result value
might indicate that an error occurred. But we’re not checking to see whether or
not there was an error, and the compiler reminds us that we probably meant to
have some error-handling code here! Let’s rectify that problem now.
Handling Errors Returned from run in main
We’ll check for errors and handle them using a technique similar to one we used
with Config::build in Listing 12-10, but with a slight difference:
Filename: src/main.rs
use std::env;
use std::error::Error;
use std::fs;
use std::process;
fn main() {
// --snip--
let args: Vec<String> = env::args().collect();
let config = Config::build(&args).unwrap_or_else(|err| {
println!("Problem parsing arguments: {err}");
process::exit(1);
});
println!("Searching for {}", config.query);
println!("In file {}", config.file_path);
if let Err(e) = run(config) {
println!("Application error: {e}");
process::exit(1);
}
}
fn run(config: Config) -> Result<(), Box<dyn Error>> {
let contents = fs::read_to_string(config.file_path)?;
println!("With text:\n{contents}");
Ok(())
}
struct Config {
query: String,
file_path: String,
}
impl Config {
fn build(args: &[String]) -> Result<Config, &'static str> {
if args.len() < 3 {
return Err("not enough arguments");
}
let query = args[1].clone();
let file_path = args[2].clone();
Ok(Config { query, file_path })
}
}
We use if let rather than unwrap_or_else to check whether run returns an
Err value and call process::exit(1) if it does. The run function doesn’t
return a value that we want to unwrap in the same way that Config::build
returns the Config instance. Because run returns () in the success case,
we only care about detecting an error, so we don’t need unwrap_or_else to
return the unwrapped value, which would only be ().
The bodies of the if let and the unwrap_or_else functions are the same in
both cases: we print the error and exit.
Splitting Code into a Library Crate
Our minigrep project is looking good so far! Now we’ll split the
src/main.rs file and put some code into the src/lib.rs file. That way we
can test the code and have a src/main.rs file with fewer responsibilities.
Let’s move all the code that isn’t the main function from src/main.rs to
src/lib.rs:
- The
runfunction definition - The relevant
usestatements - The definition of
Config - The
Config::buildfunction definition
The contents of src/lib.rs should have the signatures shown in Listing 12-13 (we’ve omitted the bodies of the functions for brevity). Note that this won’t compile until we modify src/main.rs in Listing 12-14.
Filename: src/lib.rs
use std::error::Error;
use std::fs;
pub struct Config {
pub query: String,
pub file_path: String,
}
impl Config {
pub fn build(args: &[String]) -> Result<Config, &'static str> {
// --snip--
if args.len() < 3 {
return Err("not enough arguments");
}
let query = args[1].clone();
let file_path = args[2].clone();
Ok(Config { query, file_path })
}
}
pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
// --snip--
let contents = fs::read_to_string(config.file_path)?;
println!("With text:\n{contents}");
Ok(())
}
Listing 12-13: Moving Config and run into
src/lib.rs
We’ve made liberal use of the pub keyword: on Config, on its fields and its
build method, and on the run function. We now have a library crate that has
a public API we can test!
Now we need to bring the code we moved to src/lib.rs into the scope of the binary crate in src/main.rs, as shown in Listing 12-14.
Filename: src/main.rs
use std::env;
use std::process;
use minigrep::Config;
fn main() {
// --snip--
let args: Vec<String> = env::args().collect();
let config = Config::build(&args).unwrap_or_else(|err| {
println!("Problem parsing arguments: {err}");
process::exit(1);
});
println!("Searching for {}", config.query);
println!("In file {}", config.file_path);
if let Err(e) = minigrep::run(config) {
// --snip--
println!("Application error: {e}");
process::exit(1);
}
}
Listing 12-14: Using the minigrep library crate in
src/main.rs
We add a use minigrep::Config line to bring the Config type from the
library crate into the binary crate’s scope, and we prefix the run function
with our crate name. Now all the functionality should be connected and should
work. Run the program with cargo run and make sure everything works
correctly.
Whew! That was a lot of work, but we’ve set ourselves up for success in the future. Now it’s much easier to handle errors, and we’ve made the code more modular. Almost all of our work will be done in src/lib.rs from here on out.
Let’s take advantage of this newfound modularity by doing something that would have been difficult with the old code but is easy with the new code: we’ll write some tests!