Function contrafact::lambda

pub fn lambda<'a, S, T>(
    label: impl ToString,
    state: S,
    f: impl 'a + Send + Sync + Fn(&mut Generator<'a>, &mut S, T) -> Mutation<T>
) -> Lambda<'a, S, T>where
    S: State,
    T: Target<'a>,

Create a fact from a bare function which specifies the mutation. Can be quicker to experiment with ideas this way than to have to directly implement the Fact trait

use contrafact::*;
let mut g = utils::random_generator();

let mut fact = vec_of_length(
    4,
    lambda("geometric series", 2, move |g, s, mut v| {
        g.set(&mut v, s, || "value is not geometrically increasing by 2")?;
        *s *= 2;
        Ok(v)
    }),
);

let list = fact.clone().build(&mut g);
fact.check(&list).unwrap();
assert_eq!(list, vec![2, 4, 8, 16]);