Display friendly error messages

4 minutes

Right now, if we try to read from a journal file that doesn't exist, our program will panic with the following output:

    $ cargo run -- done 2

    thread 'main' panicked at 'Failed to perform action: Os { code: 2, kind: NotFound, message: "No such file or directory" }'

This error is a little verbose for our users, so we should make it more presentable. We could write lots of code to handle that task, but there's an excellent crate for displaying useful and pretty errors to users. It's called anyhow.

The logic behind the anyhow crate is that it provides its own error type. This type has pretty-printing properties and can easily be converted from other errors, like std::io::Error. It's easy to add anyhow to our project. All we have to do is place it as the return type of the main function.

First, declare it in the Cargo.toml file:

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

[dependencies.chrono]
features = ["serde"]
version = "0.4"

[dependencies.serde]
features = ["derive"]
version = "1.0"

Now update the main function signature so it returns the type anyhow::Result<()>:

use anyhow::anyhow;
use std::path::PathBuf;
use structopt::StructOpt;
mod cli;
mod tasks;

use cli::{Action::*, CommandLineArgs};
use tasks::Task;

fn find_default_journal_file() -> Option<PathBuf> {
    home::home_dir().map(|mut path| {
        path.push(".rust-journal.json");
        path
    })
}

fn main() -> anyhow::Result<()> {
    let CommandLineArgs {
        action,
        journal_file,
    } = CommandLineArgs::from_args();

    let journal_file = journal_file
        .or_else(find_default_journal_file)
        .ok_or(anyhow!("Failed to find journal file."))?;

    match action {
        Add { text } => tasks::add_task(journal_file, Task::new(text)),
        List => tasks::list_tasks(journal_file),
        Done { position } => tasks::complete_task(journal_file, position),
    }?;
    Ok(())
}

Because most error types can be converted to anyhow::Error, we can use ? syntax to remove the expect calls from our code. Also, note that we're using the anyhow! macro to produce an anyhow::Error on the fly that contains the provided error message.

Now every panic message caused by an I/O error being returned from within our program will be displayed to users like this:

    $ cargo run -- -j missing-journal done 2
    Error: No such file or directory (os error 2)

That's quite an improvement for a few extra lines of code.

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