Refactoring to Improve Modularity and Error Handling - The Rust Programming Language

## 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 opens files. For such a small function, this isn’t a major problem. However, if we continue to grow our program inside `main`, 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 `filename` 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 opening the file fails, but the error message just prints `something went wrong`. Opening 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 `something went wrong` error message, 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 in 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 a process to use as a guideline for splitting the separate concerns of a binary program when `main` starts getting large. The 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 `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 into functions in lib.rs. The only 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

    fn main() {
        let args: Vec<String> = env::args().collect();

        let (query, filename) = parse_config(&args);

        // --snip--
    }

    fn parse_config(args: &[String]) -> (&str, &str) {
        let query = &args[1];
        let filename = &args[2];

        (query, filename)
    }

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 `filename` 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 `filename` 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 could 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.

> Note: Some people call this anti-pattern of using primitive values when a complex type would be more appropriate primitive obsession.

Listing 12-6 shows the addition of a struct named `Config` defined to have fields named `query` and `filename`. We’ve also changed the `parse_config` function to return an instance of the `Config` struct and updated `main` to use the struct fields rather than having separate variables:

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.filename);

        let contents = fs::read_to_string(config.filename)
            .expect("Something went wrong reading the file");

        // --snip--
    }

    struct Config {
        query: String,
        filename: String,
    }

    fn parse_config(args: &[String]) -> Config {
        let query = args[1].clone();
        let filename = args[2].clone();

        Config { query, filename }
    }

Listing 12-6: Refactoring `parse_config` to return an instance of a `Config` struct

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`.

We could manage the `String` data in a number of different ways, but 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 filename 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 `filename` variables so it now uses the fields on the `Config` struct instead.

Now our code more clearly conveys that `query` and `filename` 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 `filename` 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 `filename` 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;
    #
    fn main() {
        let args: Vec<String> = env::args().collect();

        let config = Config::new(&args);

        // --snip--
    }

    # struct Config {
    #     query: String,
    #     filename: String,
    # }
    #
    // --snip--

    impl Config {
        fn new(args: &[String]) -> Config {
            let query = args[1].clone();
            let filename = args[2].clone();

            Config { query, filename }
        }
    }

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 0.0 secs
         Running `target/debug/minigrep`
    thread 'main' panicked at 'index out of bounds: the len is 1
    but the index is 1', src/main.rs:29:21
    note: Run with `RUST_BACKTRACE=1` for 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 happened and 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 than the `index out of bounds` message.

Filename: src/main.rs

    // --snip--
    fn new(args: &[String]) -> Config {
        if args.len() < 3 {
            panic!("not enough arguments");
        }
        // --snip--

Listing 12-8: Adding a check for the number of arguments

This code is similar to the `Guess::new` function we wrote in Listing 9-9, 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 0.0 secs
         Running `target/debug/minigrep`
    thread 'main' panicked at 'not enough arguments', src/main.rs:30:12
    note: Run with `RUST_BACKTRACE=1` for 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 9-9 isn’t the best to use here: a call to `panic!` is more appropriate for a programming problem rather than a usage problem, as discussed in Chapter 9. Instead, we can use the other technique you learned about in Chapter 9—returning a `Result` that indicates either success or an error.

#### Returning a `Result` from `new` 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. When `Config::new` 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 `Config::new` 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

    impl Config {
        fn new(args: &[String]) -> Result<Config, &'static str> {
            if args.len() < 3 {
                return Err("not enough arguments");
            }

            let query = args[1].clone();
            let filename = args[2].clone();

            Ok(Config { query, filename })
        }
    }

Listing 12-9: Returning a `Result` from `Config::new`

Our `new` function now returns a `Result` with a `Config` instance in the success case and a `&'static str` in the error case. Recall from “The Static Lifetime” section in Chapter 10 that `&'static str` is the type of string literals, which is our error message type for now.

We’ve made two changes in the body of the `new` 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::new` allows the `main` function to handle the `Result` value returned from the `new` function and exit the process more cleanly in the error case.

#### Calling `Config::new` 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::new`, as shown in Listing 12-10. We’ll also take the responsibility of exiting the command line tool with a nonzero error code from `panic!` and 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::process;

    fn main() {
        let args: Vec<String> = env::args().collect();

        let config = Config::new(&args).unwrap_or_else(|err| {
            println!("Problem parsing arguments: {}", err);
            process::exit(1);
        });

        // --snip--

Listing 12-10: Exiting with an error code if creating a new `Config` fails

In this listing, we’ve used a method we haven’t covered before: `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 import `process` from the standard library. 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 0.48 secs
         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

    fn main() {
        // --snip--

        println!("Searching for {}", config.query);
        println!("In file {}", config.filename);

        run(config);
    }

    fn run(config: Config) {
        let contents = fs::read_to_string(config.filename)
            .expect("something went wrong reading the file");

        println!("With text:\n{}", contents);
    }

    // --snip--

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::new` 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 into `main` the logic around handling errors 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::error::Error;

    // --snip--

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

        println!("With text:\n{}", contents);

        Ok(())
    }

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. This is what the `dyn` means, it's short for "dynamic."

Second, we’ve removed the call to `expect` in favor of `?`, 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:

    warning: unused `std::result::Result` which must be used
      --> src/main.rs:18:5
       |
    18 |     run(config);
       |     ^^^^^^^^^^^^
    = note: #[warn(unused_must_use)] on by default

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::new` in Listing 12-10, but with a slight difference:

Filename: src/main.rs

    fn main() {
        // --snip--

        println!("Searching for {}", config.query);
        println!("In file {}", config.filename);

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

            process::exit(1);
        }
    }

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::new` 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 because it 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 so we can test it 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 `run` function definition
- The relevant `use` statements
- The definition of `Config`
- The `Config::new` function 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 the listing after this one.

Filename: src/lib.rs

    use std::error::Error;
    use std::fs;

    pub struct Config {
        pub query: String,
        pub filename: String,
    }

    impl Config {
        pub fn new(args: &[String]) -> Result<Config, &'static str> {
            // --snip--
        }
    }

    pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
        // --snip--
    }

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 `new` method, and on the `run` function. We now have a library crate that has a public API that 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

    extern crate minigrep;

    use std::env;
    use std::process;

    use minigrep::Config;

    fn main() {
        // --snip--
        if let Err(e) = minigrep::run(config) {
            // --snip--
        }
    }

Listing 12-14: Bringing the `minigrep` crate into the scope of src/main.rs

To bring the library crate into the binary crate, we use `extern crate minigrep`. Then we add a `use minigrep::Config` line to bring the `Config` type into 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!