Line-by-line parsing

Now it’s time to actually do something with every file line that we read. To do that, we want to loop through the lines in the file that we are given with the reader variable. This reader variable, being assigned the value of the BufReader::new(file) call, now gives us a .lines() method that we can iterate over with a simple for-loop.

In Rust, a for-loop’s syntax is “for x in y”, like this:

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for line in reader.lines() {

Perfect: now each line buffered into reader can be parsed.

We’re not quite ready to read the line yet, though. Remember when we did that condensed and verbose example of File::open() because it returned a Result object? Well, line here is also a Result object! So how do we get the value of the line–the actual contents–out?

You might think we need to use a match block, like the verbose example for File::open(). While you can do it this way, there’s really nothing we care to do if the line is empty (i.e., if the Result object produces an Err()). Think about it: if the file was opened correctly, and the contents were buffered correctly, what could cause the line variable within this for-loop to be an error?

Well, it might be the end of the file! So instead of doing error checking on this line every time, we’ll just do a Rust trick called unwrapping. When you unwrap an Result object, you are telling Rust that you 1) expect the value to be available, and 2) don’t care if the value is garbage.

If you wanted to do it the verbose way, you might write:

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let line_contents = match line {
      Ok(contents) => contents,
      Err(e) => panic!("Garbage: {}", e.description())
};

Notice how we would have to match each of the Result elements (Ok() and Err()). We don’t really need all this because, frankly, it’s a bit overkill for just reading the contents of a line in a toy markdown compiler. So instead of manually unwrapping the Result object, we can just use Rust’s .unwrap() method:

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// Verbose way:
/*let line_contents = match line {
      Ok(contents) => contents,
      Err(e) => panic!("Garbage: {}", e.description())
};*/

// Condensed way:
let line_contents = line.unwrap();

Let’s look at what we have so far for the for-loop, complete with some comments:

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// Loop through the reader lines
for line in reader.lines() {
  // For each line, unwrap it
  let line_contents = line.unwrap();

You’re doing great! Get a drink of water, stretch your back and legs, and come back when you’re ready.