The match block

With the arguments collected into args, we now want to make sure that there are only two of them. Recall that our goal is to accept just one argument: the name of a Markdown file to parse. This means that we have two total elements in the args vector: the name of the program, and the name of the Markdown file.

We only handle invocation that includes two arguments, so the length of the args vector should be 2. To check this, we’ll use a match block to match the length of the vector with either 2 (the length we want) or not 2:

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fn main() {

    let args: Vec<String> = std::env::args().collect();

    match args.len() {
      // ...
    }

}

Here you can see that we grab the length of the vector by calling .len() on it. The result is an integer value. To match values in this match block, we declare them like this: left => right. If args.len() equals 2, then we want to call parse_markdown_file():

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match args.len() {
  2 => parse_markdown_file(),

You’ll notice the comma at the end; the match rule statements (left=>right) are comma separated. If we wanted to put one before, and call usage() if no file was passed, we might do something like this:

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match args.len() {
  1 => usage(),
  2 => parse_markdown_file(),
  // etc. etc. 

In this case, checking for one argument or more than two arguments is redundant; instead of checking twice for a number of arguments not equal to 2, we can use the default case for match, which is a _ (underscore):

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match args.len() {
  2 => parse_markdown_file(),
  _ => usage()
}

The default match case (_) will trigger if no other match case triggers.

The match cases don’t have to be a call to a single function. They can also be a block themselves:

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match args.len() {
  2 => parse_markdown_file(),
  _ => { 
    println!("[ ERROR ] Invalid invocation (you done goofed!)");
    usage();
  }
}

At this point, our main function can check whether there are two arguments–the name of the program (arg 1) and the Markdown file to be parsed (arg 2), and call the appropriate function (parse_markdown_file()) if there are only two arguments.