Function contrafact::lens

pub fn lens<O, T, F, L, S>(
    label: S, 
    lens: L, 
    inner_fact: F
) -> LensFact<O, T, F> where
    O: Bounds,
    T: Bounds,
    S: ToString,
    F: Fact<T>,
    L: 'static + Fn(&mut O) -> &mut T, 

Lifts a Fact about a subset of some data into a Fact about the superset.

In other words, if type O contains a T, and you have a Fact<T>, LensFact lets you lift that fact into a Fact<O>.

The lens closure provides a mutable view into the subset of data. There must be a way to specify a mutable reference to the subset of data. If this is not always possible, consider using prism() instead.

This is a lazy way to provide a lens in the traditional optics sense. We may consider using a true lens library for this in the future.

use contrafact::*;
use arbitrary::*;

#[derive(Debug, Clone, PartialEq, Arbitrary)]
struct S {
    x: u32,
    y: u32,
}

let mut fact = lens("S::x", |s: &mut S| &mut s.x, eq("must be 1", &1));

assert!(fact.check(&S {x: 1, y: 333}).is_ok());
assert!(fact.check(&S {x: 2, y: 333}).is_err());

let mut u = Unstructured::new(&[0; 9999]);
let a = fact.build(&mut u);
assert_eq!(a.x, 1);