## RQL filter

### Operators

Implemented:

* Relational
  - exists(property)


* Comparison
  - eq(property,value)
  - ne(property,value)
  - lt(property,value)
  - gt(property,value)
  - le(property,value)
  - ge(property,value)


* Search
  - like(property,pattern)


* List
  - in(property,(value1,...))
  - out(property,(value1,...))


* Logical
  - not(query)
  - and(query1,query2,...)
  - or(query1,query2,...)

## Basic usage

```rust
use xyz_validator::{RqlValidator, ValidatorInterface};

fn main() {
    let valid_rql_statement = "or(and(eq(name,John),eq(surname,Doe)),eq(surname,Smith))".to_owned();
    let rql_validator: Box<dyn ValidatorInterface> = Box::new(RqlValidator::new(None));
    assert!(rql_validator.is_valid(valid_rql_statement));

    //to view errors we should define a callback function for `String` argument
    fn your_handle_error_function(your_var: String) {
        eprintln!("{}", your_var);
    }

    let rql_validator: Box<dyn ValidatorInterface> =
        Box::new(RqlValidator::new(Some(your_handle_error_function)));

    let invalid_rql_statement = "and(eq(name,John))".to_owned();
    assert!(!rql_validator.is_valid(invalid_rql_statement));
    //Operator `and` should have at least 2 nested queries
}
```