sample-std

Core sampling definitions and strategies. Designed for use with sample-test.

lib.rs:

Core sampling strategies, along with useful implementations for samplers on types from [std] and the quickcheck crate.

The core of this library is the Sample trait, which uses Random to generate arbitrary values from a sampled Output type with a custom "sampling strategy". It also defines a procedure for "shrinking" generated values, which can be used to generate simple counterexamples against expected properties.

This library is generally intended for usage alongside the sample_test crate. See that crate for macros and examples for using samplers within unit tests.

Sampling Strategies

The simplest Sample implementation is for Range. It is a sampler that generates values uniformly from the given range, and attempts to shrink down to the start of the range:

use sample_std::{Sample, Random};

let mut s = 10..100;
let v = s.generate(&mut Random::new());
assert!(s.contains(&v));
let mut shrunk = s.shrink(v);
assert_eq!(shrunk.next(), Some(s.start));
if v > s.start {
    assert!(shrunk.next().unwrap() < v)
}

Samplers are defined for tuples of samplers up to size 8, which can be used in concert with Sample::try_convert to combine samplers into a sampler for a larger type:

use sample_std::{Chance, Sample, VecSampler, choice};

struct Large {
    values: Vec<usize>,
    is_blue: bool,
    name: String,
}

let sampler = (
    VecSampler { length: 0..10, el: 5..20 },
    Chance(0.5),
    choice(["cora".to_string(), "james".to_string()])
).try_convert(
    |(values, is_blue, name)| Large { values, is_blue, name },
    |large| Some((large.values, large.is_blue, large.name))
);

For an example of sampling an enum, see sampler_choice.

Prior Work

This crate is heavily inspired by quickcheck. It builds upon it, in particular by defining samplers for Arbitrary (see arbitrary). Many methods and structs in here were derived from their quickcheck counterparts.

It attempts to iterate and improve on the quickcheck core idea:

• Allow definition of multiple sampling strategies for the same type.
• No need to define newtypes for custom sampling strategies.

There is still some cruft and weirdness from this early attempt to combine these worldviews:

• The concept of size isn't really necessary in a world with sampling strategies.
• The Random struct could probably just become a type definition around the underlying rng.

The core idea for sampling "strategies" comes from proptest, which uses macros instead of combinators for composition, and has more complex shrinking functionality.