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## 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` to handle an error, 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, many Rust programmers find it useful to split up the separate concerns of a binary program when the `main` function starts getting large. This process has the following steps:

- Split your program into a main.rs file and a lib.rs file and move your program’s logic to lib.rs.
- As long as your command line parsing logic is small, it can remain in the `main` function.
- When the command line parsing logic starts getting complicated, extract it from the `main` function into other functions or types.

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 `run` function in lib.rs
- Handling the error if `run` returns 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 out of the `main` function. The code that remains in the `main` function 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. Listing 12-5 shows the new start of the `main` function that calls a new function `parse_config`, which we’ll define in src/main.rs.

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 `clone`

There’s a tendency among many Rustaceans to avoid using `clone` to 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 call `clone`.

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 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` profile [unoptimized + debuginfo] target(s) in 0.0s
         Running `target/debug/minigrep`

    thread 'main' panicked at src/main.rs:27:21:
    index out of bounds: the len is 1 but the index is 1
    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 index 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` profile [unoptimized + debuginfo] target(s) in 0.0s
         Running `target/debug/minigrep`

    thread 'main' panicked at src/main.rs:26:13:
    not enough arguments
    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 the technique we used in Listing 9-13 isn’t the best one 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 string literal 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 that `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` profile [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 the `main` Function

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, the `main` function 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 small, incremental improvement of extracting a `run` function.

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 18. 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: this `Result` may be an `Err` variant, which should be handled
       = note: `#[warn(unused_must_use)]` on by default
    help: use `let _ = ...` to ignore the resulting value
       |
    19 |     let _ = run(config);
       |     +++++++

    warning: `minigrep` (bin "minigrep") generated 1 warning
        Finished `dev` profile [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 to 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 define the code responsible for searching text in src/lib.rs rather than in src/main.rs, which will let us (or anyone else using our `minigrep` library) call the searching function from more contexts than our `minigrep` binary.

First, let’s define the `search` function signature in src/lib.rs as shown in Listing 12-13, with a body that calls the `unimplemented!` macro. We’ll explain the signature in more detail when we fill in the implementation.

Filename: src/lib.rs

    pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
        unimplemented!();
    }

Listing 12-13: Defining the `search` function in src/lib.rs

We’ve used the `pub` keyword on the function definition to designate `search` as part of our library crate’s public API. We now have a library crate that we can use from our binary crate and that we can test!

Now we need to bring the code defined in src/lib.rs into the scope of the binary crate in src/main.rs and call it, as shown in Listing 12-14.

Filename: src/main.rs

    use std::env;
    use std::error::Error;
    use std::fs;
    use std::process;

    // --snip--
    use minigrep::search;

    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);
        });

        if let Err(e) = run(config) {
            println!("Application error: {e}");
            process::exit(1);
        }
    }

    // --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 })
        }
    }

    fn run(config: Config) -> Result<(), Box<dyn Error>> {
        let contents = fs::read_to_string(config.file_path)?;

        for line in search(&config.query, &contents) {
            println!("{line}");
        }

        Ok(())
    }

Listing 12-14: Using the `minigrep` library crate’s `search` function in src/main.rs

We add a `use minigrep::search` line to bring the `search` function from the library crate into the binary crate’s scope. Then, in the `run` function, rather than printing out the contents of the file, we call the `search` function and pass the `config.query` value and `contents` as arguments. Then `run` will use a `for` loop to print each line returned from `search` that matched the query. This is also a good time to remove the `println!` calls in the `main` function that displayed the query and the file path so that our program only prints the search results (if no errors occur).

Note that the search function will be collecting all the results into a vector it returns before any printing happens. This implementation could be slow to display results when searching large files because results aren’t printed as they’re found; we’ll discuss a possible way to fix this using iterators in Chapter 13.

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!