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URLs, Match and Result

In this exercise you will complete a number of mini exercises to learn about Error Handling. The final result will be a url parser that reads lines from a text file and distinguishes between URLs and non-urls.

In this exercise, you will learn how to

  • handle occurring Result-types with match for basic error handling.

  • when to use the .unwrap() method.

  • propagate an error with the ? operator

  • return the Option-type.

  • do some elementary file processing (opening, reading to buffer, counting, reading line by line).

  • navigate the Rust stdlib documentation

  • add external dependencies to your project

Task

Find the exercise template here ../../exercise-templates/urls-match-result

Find the solution to the exercise here ../../exercise-solutions/urls-match-result. You can run them with the following command: cargo run --example step_x, where x is the number of the step.

  1. Fix the runtime error in the template code by correcting the file path. Then, handle the Result type that is returned from the std::fs::read_to_string() with a match block, instead of using .unwrap().

  2. Take the code from Step 1 and instead of using a match, propagate the Error with ? out of fn main(). Note that your main function will now need to return something when it reaches the end.

  3. Take the code from Step 2, and split the String into lines using the lines() method. Use this to count how many lines there are.

  4. Change the code from Step 3 to filter out empty lines using is_empty and print the non-empty ones.

  5. Take your code from Step 4 and write a function like fn parse_url(input: &str) -> Option<url::Url> which checks if the given input: &str is a Url, or not. The function should return Some(url) where url is of type Url, which is from the url crate. Use this function to convert each line and use the returned value to print either Is a URL: <url> or Not a URL.

    The url crate has already been added as a dependency so you can just use url::Url::parse

Knowledge

Option and Result

Both Option and Result are similar in a way. Both have two variants, and depending on what those variants are, the program may continue in a different way.

The Option type can have the variant Some(T) or None. T is a type parameter that means some type should go here, we'll decide which one later. The Option type is used when you have to handle optional values. For example if you want to be able to leave a field of a struct empty, you use the Option type for that field. If the field has a value, it is Some(<value>), if it is empty, it is None.

The variants of the Result type are Ok(t) and Err(e). It is used to handle errors. If an operation was successful, Ok(t) is returned. In Ok(t), t can be the empty tuple or some other value. In Err(e), e contains an error message that can usually be printed with println!("Err: {:?}", e);.

Both types can be used with the match keyword. The received value is matched on patterns, each leads to the execution of one of a number of different expressions depending on which arm matches first.

How to use match

match is a way of controlling flow based on pattern matching. A pattern on the left results in the expression on the right.

let value = true;

match value {
   true => println!("This is true!"),
   false => println!("This is false!"),
}

Unlike with if/else, every case has to be handled explicitly, although you can use a catch-all pattern to cover 'everything else':

let value = 50_u32;

match value {
    1 => println!("This is one."),
    50 => println!("This is fifty!"),
    _ => println!("This is any other number from 0 to 4,294,967,295."),
}

There are different ways to use match:

The return values of the expression can be bound to a variable:

enum Season {
    Spring,
    Summer,
    Fall,
    Winter
}

fn which_season_is_now(season: Season) -> String {

    let return_value = match season {
        Season::Spring => String::from("It's spring!"),
        Season::Summer => String::from("It's summer!."),
        Season::Fall => String::from("It's Fall!"),
        Season::Winter => String::from("Brrr. It's Winter."),
    };

    return_value
}

In case of a Result<T, E>, match statements can be used to get to the inner value.

use std::fs::File;

fn main() {
    let file_result = File::open("hello.txt");

    let _file_itself = match file_result {
        Ok(file) => file,
        Err(error) => panic!("Error opening the file: {:?}", error),
    };
}

All arms of the match tree have to either result in the same type, or they have to diverge (that is, panic the program or return early from the function)!

Template

Start your VSCode in the proper root folder to have Rust-Analyzer working properly.

../../exercise-templates/urls-match-result/

The template builds, but has a runtime error, as the location of the file is wrong. This is intentional.

Your code will use the example data found in

../../exercise-templates/urls-match-result/src/data

Step-by-Step Solution

Step 1: Handle the Result instead of unwrapping it

std::fs::read_to_string returns a Result<T, E> kind of type, a quick way to get to inner type T is to use the .unwrap() method on the Result<T, E>. The cost is that the program panics if the Error variant occurs and the Error can not be propagated. It should only be used when the error does not need to be propagated and would result in a panic anyways. It’s often used as a quick fix before implementing proper error handling.

✅ Check the documentation for the exact type std::fs::read_to_string returns.

✅ Handle the Result using match to get to the inner type. Link the two possible patterns, Ok(some_string) and Err(e) to an an appropriate code block, for example: println!("File opened and read") and println!("Problem opening the file: {:?}", e).

✅ Fix the path of the file so that the program no longer prints an error.

Click to see the code using the fixed path 
{{#include ../../exercise-solutions/urls-match-result/examples/step_1.rs}}

TIP: IDEs often provide a "quick fix" to roll out all match arms quickly

Step 2: Returning a Result from main

✅ Add Result<(), Error> as return type to fn main() and Ok(()) as the last line of fn main().

✅ Delete the existing match block and add a ? after the call to std::fs::read_to_string(...).

✅ Print something after the std::fs::read_to_string but before the Ok(()) so you can see that your program did run. Try changing the file path back to the wrong value to see what happens if there is an error.

Click to see the solution 
{{#include ../../exercise-solutions/urls-match-result/examples/step_2.rs}}

Step 3: Count the number of lines

✅ Take a look at the documentation of std::lines. It returns a struct Lines which is an iterator.

✅ Add a block like for line in my_contents.lines() { }

✅ Declare a mutable integer, initialized to zero. Increment that integer inside the for loop.

✅ Print the number of lines the file contains.

Click to see the solution 
{{#include ../../exercise-solutions/urls-match-result/examples/step_3.rs}}

Step 4: Filter out empty lines

✅ Filter out the empty lines, and only print the the others. The is_empty method can help you here.

Click to see the solution 
{{#include ../../exercise-solutions/urls-match-result/examples/step_4.rs}}

Step 5: Check if a string is a URL, and return with Option<T>

✅ Write a function that takes (input: &str), parses each line and returns Option<url::Url> (using the url::Url). Search the docs for a method for this!

✅ If a line can be parsed successfully, return Some(url), and return None otherwise.

✅ In the main function, use your new function to only print value URLs.

✅ Test the fn parse_url().

Click me 
{{#include ../../exercise-solutions/urls-match-result/examples/step_5.rs}}

Help

Typing variables

Variables can be typed by using : and a type.

let my_value: String = String::from("test");