Developing the Library’s Functionality with Test Driven Development

Now that we’ve extracted the logic into src/lib.rs and left the argument collecting and error handling in src/main.rs, it’s much easier for us to write tests for the core functionality of our code. We can call our functions directly with various arguments and check return values without having to call our binary from the command line. Feel free to write some tests for the functionality in the Config::new and run functions on your own if you’d like.

In this section, we’re going to move on to adding the searching logic of minigrep by following the Test Driven Development (TDD) process. This is a software development technique that follows this set of steps:

  • Write a test that fails, and run it to make sure it fails for the reason you expected.
  • Write or modify just enough code to make the new test pass.
  • Refactor the code you just added or changed, and make sure the tests continue to pass.
  • Repeat!

This is just one of many ways to write software, but TDD can help drive the design of code. Writing the test before you write the code that makes the test pass helps to maintain high test coverage throughout the process.

We’re going to test drive the implementation of the functionality that will actually do the searching for the query string in the file contents and produce a list of lines that match the query. We’re going to add this functionality in a function called search.

Writing a Failing Test

First, since we don’t really need them any more, let’s remove the println! statements from both src/lib.rs and src/main.rs. Then we’ll add a test module with a test function like we did in Chapter 11. The test function specifies the behavior we’d like the search function to have: it will take a query and the text to search for the query in, and will return only the lines from the text that contain the query. Listing 12-15 shows this test:

Filename: src/lib.rs

# fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
#      vec![]
# }
#
#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn one_result() {
        let query = "duct";
        let contents = "\
Rust:
safe, fast, productive.
Pick three.";

        assert_eq!(
            vec!["safe, fast, productive."],
            search(query, contents)
        );
    }
}

Listing 12-15: Creating a failing test for the search function we wish we had

The string we are searching for is “duct” in this test. The text we’re searching is three lines, only one of which contains “duct”. We assert that the value returned from the search function contains only the line we expect.

We aren’t able to run this test and watch it fail though, since this test doesn’t even compile–the search function doesn’t exist yet! So now we’ll add just enough code to get the tests to compile and run: a definition of the search function that always returns an empty vector, as shown in Listing 12-16. Once we have this, the test should compile and fail because an empty vector doesn’t match a vector containing the line "safe, fast, productive.".

Filename: src/lib.rs

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

Listing 12-16: Defining just enough of the search function so that our test will compile

Notice that we need an explicit lifetime 'a defined in the signature of search and used with the contents argument and the return value. Remember from Chapter 10 that the lifetime parameters specify which argument lifetime is connected to the lifetime of the return value. In this case, we’re indicating that the returned vector should contain string slices that reference slices of the argument contents (rather than the argument query).

In other words, we’re telling Rust that the data returned by the search function will live as long as the data passed into the search function in the contents argument. This is important! The data referenced by a slice needs to be valid in order for the reference to be valid; if the compiler assumed we were making string slices of query rather than contents, it would do its safety checking incorrectly.

If we tried to compile this function without lifetimes, we would get this error:

error[E0106]: missing lifetime specifier
 --> src/lib.rs:5:47
  |
5 | fn search(query: &str, contents: &str) -> Vec<&str> {
  |                                               ^ expected lifetime parameter
  |
  = help: this function's return type contains a borrowed value, but the
  signature does not say whether it is borrowed from `query` or `contents`

Rust can’t possibly know which of the two arguments we need, so we need to tell it. Because contents is the argument that contains all of our text and we want to return the parts of that text that match, we know contents is the argument that should be connected to the return value using the lifetime syntax.

Other programming languages don’t require you to connect arguments to return values in the signature, so this may still feel strange, but will get easier over time. You may want to compare this example with the Lifetime Syntax section in Chapter 10.

Now let’s try running our test:

$ cargo test
...warnings...
    Finished dev [unoptimized + debuginfo] target(s) in 0.43 secs
     Running target/debug/deps/minigrep-abcabcabc

running 1 test
test test::one_result ... FAILED

failures:

---- test::one_result stdout ----
    thread 'test::one_result' panicked at 'assertion failed: `(left == right)`
(left: `["safe, fast, productive."]`, right: `[]`)', src/lib.rs:16
note: Run with `RUST_BACKTRACE=1` for a backtrace.


failures:
    test::one_result

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured

error: test failed

Great, our test fails, exactly as we expected. Let’s get the test to pass!

Writing Code to Pass the Test

Currently, our test is failing because we always return an empty vector. To fix that and implement search, our program needs to follow these steps:

  • Iterate through each line of the contents.
  • Check if the line contains our query string.
  • If it does, add it to the list of values we’re returning.
  • If it doesn’t, do nothing.
  • Return the list of results that match.

Let’s take each step at a time, starting with iterating through lines.

Iterating Through Lines with the lines Method

Rust has a helpful method to handle line-by-line iteration of strings, conveniently named lines, that works as shown in Listing 12-17:

Filename: src/lib.rs

pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
    for line in contents.lines() {
        // do something with line
    }
}

Listing 12-17: Iterating through each line in contents

The lines method returns an iterator. We’ll be talking about iterators in depth in Chapter 13, but we’ve already seen this way of using an iterator in Listing 3-6, where we used a for loop with an iterator to run some code on each item in a collection.

Searching Each Line for the Query

Next, we’ll add functionality to check if the current line contains the query string. Luckily, strings have another helpful method named contains that does this for us! Add a call to the contains method in the search function as shown in Listing 12-18:

Filename: src/lib.rs

pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
    for line in contents.lines() {
        if line.contains(query) {
            // do something with line
        }
    }
}

Listing 12-18: Adding functionality to see if the line contains the string in query

Storing Matching Lines

Finally, we need a way to store the lines that contain our query string. For that, we can make a mutable vector before the for loop and call the push method to store a line in the vector. After the for loop, we return the vector, as shown in Listing 12-19:

Filename: src/lib.rs

pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
    let mut results = Vec::new();

    for line in contents.lines() {
        if line.contains(query) {
            results.push(line);
        }
    }

    results
}

Listing 12-19: Storing the lines that match so that we can return them

Now the search function should be returning only the lines that contain query, and our test should pass. Let’s run the tests:

$ cargo test
running 1 test
test test::one_result ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

Our test passed, great, it works!

Now that our test is passing, we could consider opportunities for refactoring the implementation of the search function while keeping the code that passes the tests, in order to maintain the same functionality. The code in the search function isn’t too bad, but it isn’t taking advantage of some useful features of iterators. We’ll be coming back to this example in Chapter 13 where we’ll explore iterators in detail and see how to improve it.

Using the search Function in the run Function

Now that we have the search function working and tested, we need to actually call search from our run function. We need to pass the config.query value and the contents that run read from the file to the search function. Then run will print out each line returned from search:

Filename: src/lib.rs

pub 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)?;

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

    Ok(())
}

We’re still using a for loop to get each line returned from search and print it out.

Now our whole program should be working! Let’s try it out, first with a word that should return exactly one line from the Emily Dickinson poem, “frog”:

$ cargo run frog poem.txt
   Compiling minigrep v0.1.0 (file:///projects/minigrep)
    Finished dev [unoptimized + debuginfo] target(s) in 0.38 secs
     Running `target/debug/minigrep frog poem.txt`
How public, like a frog

Cool! Next, how about a word that will match multiple lines, like “the”:

$ cargo run the poem.txt
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/minigrep the poem.txt`
Then there’s a pair of us — don’t tell!
To tell your name the livelong day

And finally, let’s make sure that we don’t get any lines when we search for a word that isn’t anywhere in the poem, like “monomorphization”:

$ cargo run monomorphization poem.txt
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/minigrep monomorphization poem.txt`

Excellent! We’ve built our own mini version of a classic tool, and learned a lot about how to structure applications. We’ve also learned a bit about file input and output, lifetimes, testing, and command line parsing.

To round out this project chapter, we’re going to briefly demonstrate how to work with environment variables and how to print to standard error, both of which are useful when writing command line programs. Feel free to move on to Chapter 13 if you’d like at this point.

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