A type for "non-empty" vector

The solution is leveraging the type system to enforce a newly established invariant. We can use a separate type for "a vector that cannot be empty"; in fact, such types already exist in several Rust crates - for example nonempty:

pub struct NonEmpty<T> {
    pub head: T,
    pub tail: Vec<T>,
}

This type has no constructor that permits "no elements"; its new takes one element, and its first method returns &T without an Option:

pub const fn new(e: T) -> Self {
    Self::singleton(e)
}

pub const fn singleton(head: T) -> Self {
    NonEmpty {
        head,
        tail: Vec::new(),
    }
}

pub const fn first(&self) -> &T {
    &self.head
}

The rest of the crate deals with making NonEmpty behave as close as possible to a normal Vec, by implementing many useful traits, as well as conversions like:

pub fn from_vec(mut vec: Vec<T>) -> Option<NonEmpty<T>> {
    if vec.is_empty() {
        None
    } else {
        let head = vec.remove(0);
        Some(NonEmpty { head, tail: vec })
    }
}

Let's see how our get_configuration_directories function would look if it returned a NonEmpty instead of a plain Vec:

fn get_configuration_directories() -> Result<NonEmpty<PathBuf>> {
    let value = env::var("CONFIG_DIRS").context("could not read CONFIG_DIRS")?;

    let directories = value
        .split(',')
        .map(str::trim)
        .map(PathBuf::from)
        .collect();

    let Some(directories) = NonEmpty::from_vec(directories) else {
        bail!("CONFIG_DIRS cannot be empty");
    };

    Ok(directories)
}

Note the use of NonEmpty::from_vec here - this is where the invariant is established. Now a successful result is NonEmpty, not just Vec. The client code looks like:

fn main() -> Result<()> {
    let config_dirs = get_configuration_directories()?;

    initialize_cache(config_dirs.first())?;
    Ok(())
}

There's no need to check if the returned value is empty again; this is enforced by the type system!

This is where the parse vs. validate terminology of the original article comes from. When get_configuration_directories returned a Vec, it simply validated it. But when it returns a NonEmpty - the vector is transformed into another entity which carries additional meaning. If we treat the concept of parsing in the most generic sense - "transforming data from one format to another", this fits.

To mention a less artificial example, the Rust rewrite of core POSIX utilities uses NonEmpty in several places . For example, when constructing a shell pipeline:

pub struct Pipeline {
    pub commands: NonEmpty<Command>,
    pub negate_status: bool,
}

The command parser's code:

fn parse_pipeline(&mut self, alias_table: &AliasTable) -> ParseResult<Option<Pipeline>> {
    // pipeline = "!" command ("|" linebreak command)*
    let negate_status = self.match_alternatives(&[CommandToken::Bang])?.is_some();
    let mut commands = if let Some(command) = self.parse_command(alias_table)? {
        NonEmpty::new(command)
    } else {
        return Ok(None);
    };

    // ...

A valid Pipeline is only returned if there are some commands in the parsed AST. Otherwise, it just returns None. Once this is done, the client code can use commands.first() without having to worry about the possibility of it returning None.

Gradual parsing and type refinement

A somewhat more interesting example can be found in the source code of rust-analyzer. This project has a type that represents an absolute filesystem path:

pub struct AbsPathBuf(Utf8PathBuf);

Instead of carrying around a regular path, the absoluteness is recorded in the type once the initial parsing and validation is done:

impl TryFrom<Utf8PathBuf> for AbsPathBuf {
    type Error = Utf8PathBuf;
    fn try_from(path_buf: Utf8PathBuf) -> Result<AbsPathBuf, Utf8PathBuf> {
        if !path_buf.is_absolute() {
            return Err(path_buf);
        }
        Ok(AbsPathBuf(path_buf))
    }
}

Subsequent code doesn't have to validate the the path is absolute. The type enforces it.

Note also that AbsPathBuf wraps Utf8PathBuf, not PathBuf. Utf8PathBuf is itself a custom, "parsed" type refinement from the camino crate. Regular paths in the Rust standard library aren't guaranteed to be valid UTF-8, so they cannot be easily converted to a String (which has to be valid UTF-8 in Rust); camino::Utf8PathBuf establishes validity on construction, and can then be converted to a string with just:

fn as_str(&self) -> &str {
  ...
}

So we have an example of gradual parsing and type refinement here:

std::path::PathBuf

      |
      |   prove UTF-8
      |
      V

camino::Utf8PathBuf

      |
      |   prove absolute
      |
      V

rust-analyzer's paths::AbsPathBuf

Non-zero integers

Rust has a generic type called NonZero, to describe unsigned numeric quantities that are known to be non-zero.

For example, thread::available_parallelism is defined as:

pub fn available_parallelism() -> Result<NonZero<usize>>

If the call is successful, it returns a NonZero<usize>, which is like a normal usize with the restriction that it's not zero. Client code doesn't have to keep checking whether the parallelism is 0 - it's enshrined in the type system.

Rust defines the division operator with NonZero<usize> in the denominator as an operation that "cannot panic".

NonZero has an additional advantage: zero is an invalid value for the type, so Rust can use the zero bit pattern to represent None. Consequently, Option<NonZeroUsize> is guaranteed to have the same size and alignment as NonZeroUsize itself (and as usize). This can avoid the extra storage that an Option<usize> would generally require.

Parsing JSON

A common example of the "parse, don't validate" idiom appears in deserializing data from a JSON string. Rust's serde crate enables us to do the parsing, with validated decisions encoded into the type system, e.g.:

#[derive(Debug, Deserialize)]
struct Config {
    name: String,
    workers: NonZeroUsize,
    mode: Mode,
}

#[derive(Debug, Deserialize)]
#[serde(rename_all = "snake_case")]
enum Mode {
    Fast,
    Safe,
}

And then later:

let input = r#"
     {
         "name": "compiler",
         "workers": 4,
         "mode": "fast"
     }
 "#;

 let config: Config = serde_json::from_str(input)?;

There is a lot happening behind the scenes:

• The types of all fields are enforced (e.g. "name" cannot be an array).
• The mode is validated to be one of the enum values of Mode.
• workers is validated to be a non-zero integer, because of the NonZeroUsize field type.

We take code like this for granted these days, but it's still a great example of the pattern discussed in this post. Once the parser converted mode into the Mode enum, no further validation is required.

In dynamic languages like Python and JS, the process is usually much more manual. Python's json.loads gives us a dictionary, and it's up to the user to validate its contents. Libraries like Pydantic permit an approach closer to Rust's, but they're not universally used.