Feature: allow generating a single seed based value

[Aderinom]

Hi Proptest team,

We are using your library in our SCION packet parsing library, and the experience has been excellent.

To better bridge property-based testing and traditional unit testing, I would like to request support for extracting a single value from a strategy without having to manually set up a TestRunner.

This would make it easier to reuse strategies in conventional tests and small targeted test cases.

[cameron1024]

Hi, thanks for reaching out 😁

I feel hesitant about adding such functionality to proptest directly, since it's a bit of a footgun - one of the major benefits of a framework like proptest is the shrinking behaviour, which of course only makes sense when multiple values are being generated by the same test runner.

That said, you could try adding something like this to your codebase and using it in tests:

fn extract<T: Debug>(strategy: impl Strategy<Value = T>) -> T {
    let mut test_runner = TestRunner::new(ProptestConfig ::default());
    let value_tree = strategy.new_tree(&mut test_runner).unwrap();
    value_tree.current()
}

Note that you would also probably want some way to seed the RNG (which can be done using TestRunner::new_with_rng), otherwise your non-proptest tests will be non-deterministic.

Does that solve your issue?

[Aderinom]

Hey Cameron,

Thank you for the quick response! Yes, that solves the immediate issue.
That is the approach I ended up taking already, with the concern that the Test runner might do something in the background. :D

I understand the hesitation around exposing this directly in proptest, especially since it can bypass the usual shrinking-focused workflow.

As a Library Author, it would be useful to support both property tests and conventional tests from the same underlying strategy definitions for downstream users.

Right now, the practical options seem to be:

• add a method on each type implementing Arbitrary to produce a single seeded value
• add a helper function that takes a strategy and produces a single seeded value
• add an extension trait on Arbitrary to produce a single seeded value
• implement a separate Trait like rand

The first two options are not intrusive, but are either boilerplate-y or hard to discover, which is not ideal.
The extension trait is what I am doing currently, but it feels a bit intrusive because it effectively extends all Arbitrary types for this purpose.
The last one would avoid that, but would also mean duplicating generator logic that already exists in strategies, which seems undesirable from a maintenance and effort perspective.

It would be nice to be able to reuse the existing strategy logic for a lightweight deterministic “sample one value from this strategy” API.

Maybe this could be behind a feature flag? Or in a more 'low-level' function on the Strategy itself? This way it would be clear to users that this is not really the intended way to use proptest.