Transforming Strategies

Suppose you have a function that takes a string which needs to be the Display format of an arbitrary u32. A first attempt to providing this argument might be to use a regular expression, like so:

use proptest::prelude::*;

fn do_stuff(v: String) {
    let i: u32 = v.parse().unwrap();
    let s = i.to_string();
    assert_eq!(s, v);
}

proptest! {
    #[test]
    fn test_do_stuff(v in "[1-9][0-9]{0,8}") {
        do_stuff(v);
    }
}
fn main() { test_do_stuff(); }

This kind of works, but it has problems. For one, it does not explore the whole u32 space. It is possible to write a regular expression that does, but such an expression is rather long, and also results in a pretty odd distribution of values. The input also doesn’t shrink correctly, since proptest tries to shrink it in terms of a string rather than an integer.

What you really want to do is generate a u32 and then pass in its string representation. One way to do this is to just take u32 as an input to the test and then transform it to a string within the test code. This approach works fine, but isn’t reusable or composable. Ideally, we could get a strategy that does this.

The thing we’re looking for is the first strategy combinator, prop_map. We need to ensure Strategy is in scope to use it.

// Grab `Strategy`, shorter namespace prefix, and the macros
use proptest::prelude::*;

fn do_stuff(v: String) {
    let i: u32 = v.parse().unwrap();
    let s = i.to_string();
    assert_eq!(s, v);
}

proptest! {
    #[test]
    fn test_do_stuff(v in any::<u32>().prop_map(|v| v.to_string())) {
        do_stuff(v);
    }
}
fn main() { test_do_stuff(); }

Calling prop_map on a Strategy creates a new strategy which transforms every generated value using the provided function. Proptest retains the relationship between the original Strategy and the transformed one; as a result, shrinking occurs in terms of u32, even though we’re generating a String.

prop_map is also the principal way to define strategies for new types, since most types are simply composed of other, simpler values.

Let’s update our code so it takes a more interesting structure.

use proptest::prelude::*;

#[derive(Clone, Debug)]
struct Order {
  id: String,
  // Some other fields, though the test doesn't do anything with them
  item: String,
  quantity: u32,
}

fn do_stuff(order: Order) {
    let i: u32 = order.id.parse().unwrap();
    let s = i.to_string();
    assert_eq!(s, order.id);
}

proptest! {
    #[test]
    fn test_do_stuff(
        order in
        (any::<u32>().prop_map(|v| v.to_string()),
         "[a-z]*", 1..1000u32).prop_map(
             |(id, item, quantity)| Order { id, item, quantity })
    ) {
        do_stuff(order);
    }
}
fn main() { test_do_stuff(); }

Notice how we were able to take the output from prop_map and put it in a tuple, then call prop_map on that tuple to produce yet another value.

But that’s quite a mouthful in the argument list. Fortunately, strategies are normal values, so we can extract it to a function.

use proptest::prelude::*;

// snip

#[derive(Clone, Debug)]
struct Order {
  id: String,
  // Some other fields, though the test doesn't do anything with them
  item: String,
  quantity: u32,
}

fn do_stuff(order: Order) {
    let i: u32 = order.id.parse().unwrap();
    let s = i.to_string();
    assert_eq!(s, order.id);
}

fn arb_order(max_quantity: u32) -> BoxedStrategy<Order> {
    (any::<u32>().prop_map(|v| v.to_string()),
     "[a-z]*", 1..max_quantity)
    .prop_map(|(id, item, quantity)| Order { id, item, quantity })
    .boxed()
}

proptest! {
    #[test]
    fn test_do_stuff(order in arb_order(1000)) {
        do_stuff(order);
    }
}
fn main() { test_do_stuff(); }

We boxed() the strategy in the function since otherwise the type would not be nameable, and even if it were, it would be very hard to read or write. Boxing a Strategy turns both it and its ValueTrees into trait objects, which both makes the types simpler and can be used to mix heterogeneous Strategy types as long as they produce the same value types.

The arb_order() function is also parameterised, which is another advantage of extracting strategies to separate functions. In this case, if we have a test that needs an Order with no more than a dozen items, we can simply call arb_order(12) rather than needing to write out a whole new strategy.

We can also use -> impl Strategy<Value = Order> instead to avoid the overhead as in the following example. You should use -> impl Strategy<..> unless you need the dynamic dispatch.

use proptest::prelude::*;

// snip

#[derive(Clone, Debug)]
struct Order {
  id: String,
  // Some other fields, though the test doesn't do anything with them
  item: String,
  quantity: u32,
}


fn do_stuff(order: Order) {
    let i: u32 = order.id.parse().unwrap();
    let s = i.to_string();
    assert_eq!(s, order.id);
}

fn arb_order(max_quantity: u32) -> impl Strategy<Value = Order> {
    (any::<u32>().prop_map(|v| v.to_string()),
     "[a-z]*", 1..max_quantity)
    .prop_map(|(id, item, quantity)| Order { id, item, quantity })
}

proptest! {
    #[test]
    fn test_do_stuff(order in arb_order(1000)) {
        do_stuff(order);
    }
}

fn main() { test_do_stuff(); }