compare_variables

A library for comparing the ordering of variables and producing useful error messages.

This library is based on the struct ComparisonError, which can be used to compare the partial ordering of two to three variables of any type T which implements the PartialOrd trait. If the comparison evaluates to false, ComparisonError can be formatted into a nice error message. To simplify the usage, the procedural macro compare_variables is provided via the feature flag proc_macro (enabled by default).

The full API documentation is available at https://docs.rs/compare_variables/0.2.0/compare_variables/.

Examples

The type annotation of the error can be omitted and is only written out in these examples for clarity.

use compare_variables::{compare_variables, ComparisonError};

let x = 1.0;
assert!(compare_variables!(0.0 < x <= 1.0).is_ok());

let err: ComparisonError<f64> = compare_variables!(x > 1.5).unwrap_err();
assert_eq!(err.to_string(), "`x (value: 1.0) > 1.5` is false");

assert!(compare_variables!(1.0 == x < 5.0).is_ok());

// Named and unnamed struct fields can also be used:
struct NamedField {
   x: usize
}
let n = NamedField {x: 1};
assert!(compare_variables!(n.x > 0).is_ok());
let err: ComparisonError<usize> = compare_variables!(n.x > 1).unwrap_err();
assert_eq!(err.to_string(), "`n.x (value: 1) > 1` is false");

struct AnonymousField(i32);
let a = AnonymousField(-5);
let err: ComparisonError<i32> = compare_variables!(a.0 > 1).unwrap_err();
assert_eq!(err.to_string(), "`a.0 (value: -5) > 1` is false");

// It is also possible to customize the error message via `as` (providing an alias) and `val` (omit the variable name):
let x: u16 = 1;
let y: u16 = 2;
let z: u16 = 3;
let err: ComparisonError<u16> = compare_variables!(x as arg > val y > z).unwrap_err();
assert_eq!(err.to_string(), "`arg (value: 1) > 2 > z (value: 3)` is false");

Usage without the procedural macro

In order to minimize dependencies, it is possible to use this crate without the proc_macro feature flag. Please see the docstring of ComparisonError for details.

use compare_variables::{compare_variables, ComparisonError, ComparisonValue, ComparisonOperator};

let x = 1;
let err_macro = compare_variables!(x > 2).unwrap_err();
let err_no_macro = ComparisonError::new(
    ComparisonValue::new(1, Some("x")),
    ComparisonOperator::Greater,
    ComparisonValue::new(2, None),
    ComparisonOperator::Equal,
    None,
).unwrap_err();
assert_eq!(err_macro.to_string(), err_no_macro.to_string());