Simple equation parser and equation validator.

I made this to learn nom, the Rust parser combinators library inspired by Haskell's parsec.

This library introduces an Equation type with an Expression and expected result. The Equation and expressions are parsed from a string slice using nom parser combinators. This allows for a lot of flexibility in the input format. For example from the unit tests.

  fn test_parser(input: &str, correct: bool) {
    let equation = input.parse::<Equation>().unwrap();
    assert_eq!(equation.is_correct(), correct);
  }

  #[test]
  fn test_parse() {
    test_parser("5 + -2 = 3", true);
    test_parser("10 + 10 = 20", true);
    test_parser("10+10=20", true);
    test_parser("10+10-10=10", true);
    test_parser("5*5= 25", true);
    test_parser("100 / 10 = 10", true);
    test_parser("5 * 5 = 25", true);
    test_parser("-25 + -25 = -50", true);
    test_parser("-1 + 2 = 1", true);
    test_parser("1 + 1 = 0", false);
    test_parser("(1 + 1) * 5 = 10", true);
    test_parser("1 + 1 * 5 = 6", true);
    test_parser("5^2 * 2 = 50", true);
    test_parser("5 * 2 + 3 = 13", true);
    test_parser("1.5 + 2.5 = 4.0", true);
    test_parser("   1.5 +  2.5 = 4.0", true);
    test_parser("   1.5 +  2.5 * 5 = 14  ", true);
  }

Every one of these equations can be correctly parsed and validated.

This example expands on the Basic Calculator example.