13 releases (4 breaking)

0.6.0 Sep 2, 2021
0.5.1 Jul 11, 2021
0.4.5 Jul 5, 2021
0.3.2 Jul 2, 2021
0.2.1 Jul 2, 2021

#192 in Development tools

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crates.io version docs.rs crates.io license

This crate provides a convenient macro that allows you to generate type wrappers that promise to always uphold arbitrary invariants that you specified.


Let's create a Username type. It will be a wrapper around non-empty String:

prae::define!(pub Username: String ensure |u| !u.is_empty());

// We can't create an invalid username.

// But we can create a valid one!
let mut u = Username::new("valid name").unwrap();
assert_eq!(u.get(), "valid name");

// We can mutate it:
assert!(u.try_mutate(|u| *u = "new name".to_owned()).is_ok());
assert_eq!(u.get(), "new name"); // our name has changed!

// But we can't make it invalid:
assert!(u.try_mutate(|u| *u = "".to_owned()).is_err());
assert_eq!(u.get(), "new name"); // our name hasn't changed!

// Let's try this...
assert!(Username::new("  ").is_ok()); // looks kind of invalid though :(

As you can see, the last example treats " " as a valid username, but it's not. We can of course do something like Username::new(s.trim()) every time, but why should we do it ourselves? Let's automate it!

prae::define! {
    pub Username: String
    adjust |u| *u = u.trim().to_string()
    ensure |u| !u.is_empty()

let mut u = Username::new(" valid name \n\n").unwrap();
assert_eq!(u.get(), "valid name"); // now we're talking!

// This also works for mutations:
assert!(matches!(u.try_mutate(|u| *u = "   ".to_owned()), Err(prae::ValidationError { .. })))

Now our Username trims provided value automatically.

You might noticed that prae::ValidationError is returned by default when our construction/mutation fails. Altough it's convenient, there are situations when you might want to return a custom error. And prae can help with this:

struct UsernameError;

prae::define! {
    pub Username: String
    adjust   |u| *u = u.trim().to_string()
    validate |u| -> Option<UsernameError> {
        if u.is_empty() {
        } else {

assert!(matches!(Username::new("  "), Err(UsernameError)));

Perfect! Now you can integrate it in your code and don't write .map_err(...) everywhere.

Serde integration

You can enable serde integration with the serde feature. It will implement Serialize and Deserialize traits for the wrappers if the underlying type implements them. The deserialization will automatically fail if it contains invalid value. Here is an example:

use serde::{Deserialize, Serialize};

prae::define! {
    Username: String
    adjust   |u| *u = u.trim().to_string()
    validate |u| -> Option<&'static str> {
        if u.is_empty() {
            Some("username is empty")
        } else {

#[derive(Debug, Deserialize, Serialize)]
struct User {
    username: Username,

// Serialization works as expected.
let u = User {
    username: Username::new("  john doe  ").unwrap(),
let j = serde_json::to_string(&u).unwrap();
assert_eq!(j, r#"{"username":"john doe"}"#)

// Deserialization with invalid data fails.
let e = serde_json::from_str::<User>(r#"{ "username": "  " }"#).unwrap_err();
assert_eq!(e.to_string(), "username is empty at line 1 column 20");

// And here we get a nice adjusted value.
let u = serde_json::from_str::<User>(r#"{ "username": "  john doe  " }"#).unwrap();
assert_eq!(u.username.get(), "john doe");


Although proc macros are very powerful, they aren't free. In this case, you have to pull up additional dependencies such as syn and quote, and expect a slightly slower compile times.


This crate was highly inspired by the tightness crate. It's basically just a fork of tightness with a slightly different philosophy. See this issue for details.


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