Use a default journal file to store tasks


It's common for some command-line applications to put their user-owned files, like dotfiles and config files, in their home directories. So we might want to place our default journal file there too.

Because home directories vary depending on the user's operating system, we'll rely on a third-party crate called home to determine the directory.

First, add it to the Cargo.toml file:

home = "0.5" # <--- Add `home` to our project dependencies.
serde_json = "1.0"
structopt = "0.3"

features = ["serde"]
version = "0.4"

features = ["derive"]
version = "1.0"

We can now update the file to use the home::home_dir() function. This function will look for the user's home directory and return it in an Option<PathBuf> type, just like the journal_file field from our CommandLineArgs type:

// ...
use std::path::PathBuf;

fn find_default_journal_file() -> Option<PathBuf> {
    home::home_dir().map(|mut path| {

fn main() {

    let CommandLineArgs {
    } = CommandLineArgs::from_args();

    let journal_file = journal_file
        .expect("Failed to find journal file.");

    // ...

We created a new function called find_default_journal_file. It takes no input arguments and returns an Option<PathBuf>.

Inside that function, we try to build the full path to our default journal file. We build the path by taking an Option type from the home::home_dir function output and calling its map method with an anonymous function that pushes the string ".rusty-journal.json" to the path. If the output of home::home_dir is None, no action is taken, because map will work only with a Some variant.

Then, in the main function, we shadow the journal_file variable to be updated with a call to find_default_journal_file only if its original value was None. The .or_else method does the opposite of the map method: it calls the function it holds only if the variant is None.

If the user hasn't provided a target journal file and find_default_journal_file can't find a suitable file, we cause the program to panic because it's impossible for it to do anything without a journal file.