How to test a function fn(&str, &str) -> bool?

[febeling]

This is a question issue, and I hope that's an acceptable use in this project.

I try to use quickcheck to write tests for function accepting two strings. I haven't been able to adapt the example test to my use case. This is what I have. Is quickcheck able to provide random parameters for strings (or &strs)?

extern crate quickcheck;
// at least difference of the sizes of a and b
#[test]
fn prop_test_() {
    use quickcheck::quickcheck;

    fn at_least_size_difference(a: &str, b: &str) -> bool {
        let size_a = a.chars().count() as i32;
        let size_b = b.chars().count() as i32;
        let diff = (size_a - size_b).abs();

        edit_distance::edit_distance(a, b) > diff;
    }

    quickcheck(at_least_size_difference as fn(a: &str, b: &str) -> bool);
}

[BurntSushi]

No. QuickCheck doesn't generate values with type &str. It is much more convenient to limit QuickCheck to owned values (or non-borrowed types that implement Copy, like ints). I don't think it's possible to write a useful implementation of Arbitrary for &str (because it needs to be able to manipulate the strings to do shrinking).

With that said, given your code here, I don't see any reason why you can't use String. If you have a String, then you can get an &str. It will cost you an allocation, but it's usually not a huge deal in this context.

I think if you s/&str/String and change edit_distance(a, b) to edit_distance(&a, &b), then it should work (assuming edit_distance accepts two values with type &str).

[febeling]

Thank you! It makes sense the way you explain it, and I'm just in the process of forming an idea what the difference between &str and String entails. With this change it works beautifully.