Refactoring to Improve Modularity and Error Handling
There are four problems that we’d like to fix to improve our program, and they have to do with the way the program is structured and how it’s handling potential errors.
First, our main function now performs two tasks: it parses arguments and
opens up files. For such a small function, this isn’t a huge problem. However,
if we keep growing our program inside of main, the number of separate tasks
the main function handles will grow. As a function gains responsibilities, it
gets harder to reason about, harder to test, and harder to change without
breaking one of its parts. It’s better to separate out functionality so that
each function is responsible for one task.
This also ties into our second problem: while query and filename are
configuration variables to our program, variables like f and contents are
used to perform our program’s logic. The longer main gets, the more variables
we’re going to need to bring into scope; the more variables we have in scope,
the harder it is to keep track of the purpose of each. It’s better to group the
configuration variables into one structure to make their purpose clear.
The third problem is that we’ve used expect to print out an error message
when opening the file fails, but the error message only says file not found.
There are a number of ways that opening a file can fail besides the file being
missing: for example, the file might exist, but we might not have permission to
open it. Right now, if we’re in that situation, we’d print the file not found
error message that would give the user the wrong advice!
Fourth, we use expect repeatedly to deal with different errors, and if the
user runs our programs 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 better if all our error handling code was in one place so that
future maintainers only have one place to consult in the code if the error
handling logic needs to change. Having all the error handling code in one place
will also help us to ensure that we’re printing messages that will be
meaningful to our end users.
Let’s address these problems by refactoring our project.
Separation of Concerns for Binary Projects
The organizational problem of allocating responsibility for multiple tasks to
the main function responsible is common to many binary projects, so the Rust
community has developed a kind of guideline process for splitting up the
separate concerns of a binary program when main starts getting large. The
process has the following steps:
- Split your program into both a main.rs and a lib.rs and move your program’s logic into lib.rs.
- While 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 into lib.rs as well.
- The responsibilities that remain in the
mainfunction after this process should be limited to:- Calling the command line parsing logic with the argument values
- Setting up any other configuration
- Calling a
runfunction in lib.rs - If
runreturns an error, handling that error
This pattern is all about separating concerns: main.rs handles running the
program, and lib.rs handles all of the logic of the task at hand. Because we
can’t test the main function directly, this structure lets us test all of our
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 re-work our program by following this process.
Extracting the Argument Parser
First, we’ll extract the functionality for parsing arguments. Listing 12-5
shows the new start of main that calls a new function parse_config, which
we’re still going to 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: Extract 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 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, as that will help you identify the cause of problems when they occur.
Grouping Configuration Values
We can take another small step to improve this function further. At the moment, we’re returning a tuple, but then we immediately break that tuple up into individual parts again. This is a sign that perhaps we don’t have the right abstraction yet.
Another indicator that 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 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. This 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::File; # 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 mut f = File::open(config.filename).expect("file not found"); // ...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’ve now chosen to 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, though, 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 different ways we could manage the String data, and 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 does take more time and memory than storing a reference to the
string data. However, cloning the data also makes our code very straightforward
since we don’t have to manage the lifetimes of the references, so in this
circumstance giving up a little performance to gain simplicity is a worthwhile
trade-off.
The Tradeoffs of Using
cloneThere’s a tendency among many Rustaceans to avoid using
cloneto fix ownership problems because of its runtime cost. In Chapter 13 on iterators, you’ll learn how to use more efficient methods in this kind of situation, but for now, it’s okay to copy a few strings to keep making progress since we’ll only make these copies once, and our filename and query string are both very small. It’s better to have a working program that’s a bit inefficient than try to hyper-optimize code on your first pass. As you get more experienced with Rust, it’ll be easier to go straight to the desirable method, but for now it’s perfectly acceptable to callclone.
We’ve updated main so that it places the instance of Config returned by
parse_config into a variable named config, and updated the code that
previously used the separate query and filename variables so that it now
uses the fields on the Config struct instead.
Our code now more clearly conveys that query and filename are related and
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 into the parse_config function, which 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 being a plain function into a
function named new that is associated with the Config struct. Making this
change will make our code more idiomatic: we can create instances of types in
the standard library like String by calling String::new, and 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’ll 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 makes the new function associated with
Config. Try compiling this again to make sure it works.
Fixing the Error Handling
Now we’ll work on fixing our error handling. Recall that we mentioned 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 3 items. Try
running the program without any arguments; it will look like this:
$ cargo run
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', /stable-dist-rustc/build/src/libcollections/vec.rs:1307
note: Run with `RUST_BACKTRACE=1` for a backtrace.
The line that states index out of bounds: the len is 1 but the index is 1 is
an error message intended for programmers, and won’t really 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’re adding a check in the new function that will check
that the slice is long enough before accessing index 1 and 2. If the slice
isn’t long enough, the program panics, with 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 is similar to the Guess::new function we wrote in Listing 9-8, where
panic! was called 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 3
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 try running our program
without any arguments again and see what the error looks like now:
$ cargo run
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
Running `target/debug/minigrep`
thread 'main' panicked at 'not enough arguments', src/main.rs:29
note: Run with `RUST_BACKTRACE=1` for a backtrace.
This output is better, we now have a reasonable error message. However, we also
have a bunch of extra information we don’t want to give to our users. So
perhaps using the technique we used in Listing 9-8 isn’t the best to use here;
a call to panic! is more appropriate for a programming problem rather than a
usage problem, as we discussed in Chapter 9. Instead, we can use the other
technique you also learned about in Chapter 9: returning a Result that can
indicate either success or an error.
Returning a Result from new Instead of Calling panic!
We can choose to 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 that 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 you need to make to the return value of
Config::new and the body of the function needed to return a Result:
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: Return 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.
By having Config::new return an Err value, it 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
In order 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’re also going to 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 our program ended 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’re using 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
be covering closures in more detail in Chapter 13. What you need to know for
now is 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 get run in the error case is only two lines: we
print out the err value, 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, with the exception that 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 we’re done refactoring our configuration parsing; let’s turn to our
program’s logic. As we laid out in the “Separation of Concerns for Binary
Projects” section, we’re going to extract a function named run that will hold
all of the logic currently in the main function not involved with setting up
configuration or handling errors. Once we’re done, main will be concise and
easy to verify by inspection, and we’ll be able to write tests for all of the
other logic.
Listing 12-11 shows the extracted run function. For now, we’re making only
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 mut f = File::open(config.filename).expect("file not found");
let mut contents = String::new();
f.read_to_string(&mut contents)
.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 like 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 the logic around handling errors in a user-friendly way
into main. Listing 12-12 shows the changes you 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<Error>> {
let mut f = File::open(config.filename)?;
let mut contents = String::new();
f.read_to_string(&mut contents)?;
println!("With text:\n{}", contents);
Ok(())
}
Listing 12-12: Changing the run function to return
Result
We’ve made three big changes here. First, we’re changing the return type of the
run function to Result<(), Box<Error>>. This function previously returned
the unit type, (), and we keep that as the value returned in the Ok case.
For our error type, we’re using the trait object Box<Error> (and we’ve
brought std::error::Error into scope with a use statement at the top).
We’ll be covering trait objects in Chapter 17. For now, just know that
Box<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 second change we’re making is removing the calls to expect in favor of
?, like we talked about in Chapter 9. Rather than panic! on an error, this
will return the error value from the current function for the caller to handle.
Thirdly, this 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 may 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, it will compile, but with a warning:
warning: unused result which must be used, #[warn(unused_must_use)] on by
default
--> src/main.rs:39:5
|
39 | run(config);
| ^^^^^^^^^^^^
Rust is telling us that our code ignores the Result value, which might be
indicating that there was an error. We’re not checking to see if there was an
error or not, though, and the compiler is reminding us that we probably meant
to have some error handling code here! Let’s rectify that now.
Handling Errors Returned from run in main
We’ll check for errors and handle them using a similar technique to the way we
handled errors 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 to check whether run returns an Err value, rather than
unwrap_or_else, and call process::exit(1) if it does. run doesn’t return
a value that we want to unwrap like Config::new returns the Config
instance. Because run returns a () in the success case, we only care about
detecting an error, so we don’t need unwrap_or_else to return the unwrapped
value as it would only be ().
The bodies of the if let and the unwrap_or_else functions are the same in
both cases though: we print out the error and exit.
Splitting Code into a Library Crate
This is looking pretty good so far! Now we’re going to split the src/main.rs file up and put some code into src/lib.rs so that we can test it and have a src/main.rs file with fewer responsibilities.
Let’s move everything that isn't the main function from src/main.rs to a
new file, src/lib.rs:
- The
runfunction definition - The relevant
usestatements - The definition of
Config - The
Config::newfunction 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):
Filename: src/lib.rs
use std::error::Error;
use std::fs::File;
use std::io::prelude::*;
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<Error>> {
// ...snip...
}
Listing 12-13: Moving Config and run into
src/lib.rs
We’ve made liberal use of pub here: on Config, 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 by using extern crate minigrep. Then we’ll add
a use minigrep::Config line to bring the Config type into scope, and prefix
the run function with our crate name 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’ll add a use minigrep``::Config line to bring the
Config type into scope, and we'll prefix the run function with our crate
name. With that, all the functionality should be connected and should work.
Give it a cargo run and make sure everything is wired up 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 our 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 hard with our old code, but is easy with our new code: write some tests!