QuickCheck Mini (WIP)

[CAIMEOX]

• Introduce a mini property-based testing framework
• trait Shrink and instances for common used data types
• support simple property and filter

[Copilot]

Pull request overview

Adds core building blocks for a “QuickCheck Mini (WIP)” in moonbitlang/core, introducing a shrinker API and a size-aware generator API, plus tuple/list instances and corresponding package wiring.

Changes:

• Introduces @quickcheck/shrink with a Shrink trait, primitive/composite shrinkers, and tests.
• Adds @quickcheck/gen providing a Gen[T] generator type with common combinators and tests.
• Extends tuple/list packages with Shrink instances, updates tuple Arbitrary coverage to 8/9-tuples, and updates moon.pkg / generated .mbti interfaces accordingly.

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
tuple/tuple_shrink.mbt	Adds @shrink.Shrink impls for tuples (2–9).
tuple/tuple_shrink_test.mbt	Adds snapshot tests validating tuple shrinking behavior.
tuple/tuple_arbitrary.mbt	Adds @quickcheck.Arbitrary impls for 8- and 9-tuples.
tuple/pkg.generated.mbti	Exposes new tuple Shrink impls and imports quickcheck/shrink.
tuple/moon.pkg	Adds moonbitlang/core/quickcheck/shrink dependency.
tuple/extends.mbt	Adds deprecated extend entries to avoid promoting Shrink methods as inherent tuple methods.
quickcheck/shrink/utils.mbt	Adds shared helpers for shrinkers (array chunk removal + decimal shrinking).
quickcheck/shrink/shrink.mbt	Defines Shrink trait plus primitive shrinker impls and docs.
quickcheck/shrink/shrink_test.mbt	Adds tests for primitive shrinkers.
quickcheck/shrink/composite.mbt	Adds composite shrinkers for Option, Result, Array, and Iter.
quickcheck/shrink/composite_test.mbt	Adds tests for composite shrinkers.
quickcheck/shrink/pkg.generated.mbti	Generated interface for the new quickcheck/shrink package.
quickcheck/shrink/moon.pkg	Declares quickcheck/shrink package imports and warning config.
quickcheck/gen/gen.mbt	Implements Gen[T] and generator combinators (map, flat_map, one_of, frequency, etc.).
quickcheck/gen/gen_test.mbt	Adds tests for generator behavior and combinators.
quickcheck/gen/moon.pkg	Declares quickcheck/gen package imports.
quickcheck/gen/pkg.generated.mbti	Generated interface for the new quickcheck/gen package.
list/shrink.mbt	Adds @shrink.Shrink impl for List[T].
list/pkg.generated.mbti	Exposes new list Shrink impl and imports quickcheck/shrink.
list/moon.pkg	Adds moonbitlang/core/quickcheck/shrink dependency.
list/list_test.mbt	Adds snapshot test for list shrinking.
list/extends.mbt	Adds deprecated extend entry to avoid promoting Shrink methods as inherent list methods.

Comments suppressed due to low confidence (2)

list/list_test.mbt:1220

• After fixing list shrinking to include suffix removals, shrinking [1,2,3,4,5,6] should also include removing the last chunk of size 3 (<List: [1, 2, 3]>).

      #|  <List: [4, 5, 6]>,

quickcheck/shrink/composite_test.mbt:74

• After fixing array chunk removal to allow suffix removal, shrinking [1,2,3,4,5,6] should also include removing the last chunk of size 3 ([1, 2, 3]).

      #|  [0, 2, 3, 4, 5, 6],

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[bobzhang]

Feedback from migrating a real project to this package: moonbit-community/toml-parser#122 replaced all of its moonbitlang/quickcheck@0.14.0 usage (custom generators, custom shrinkers, a 2000-case round-trip property with coverage classification) with this package. The core design held up well — the typeclass-driven check/report driver is cleaner than explicit forall_shrink plumbing, deterministic seeding is snapshot-friendly, and the Shrink instances covered everything we used. That said, the migration surfaced some gaps worth addressing while the API is still WIP:

1. No coverage classification. Classic QuickCheck's classify/label is how you notice that a generator quietly stopped exercising the interesting regions of the input space. We had to hand-roll a fixed-seed sample tally to keep that safety net. Suggestion: a classify? : Array[(String, (A) -> Bool)] parameter on report whose counts come back in Passed(labels~).

2. QuickCheckReport is abstract outside the package. report returns a structured result that callers cannot destructure — the only way to consume it is via its Debug rendering, and even this package's own black-box tests resort to debug_inspect everywhere. Making the enum pub (matchable, not constructible) would let tests assert on Passed/Falsified structurally.

3. No way to attach counterexample context. The property can only return Bool. Attaching human-readable context (in our case, the rendered TOML document) requires raising fail(msg), which lands in Raised and conflates "property errored" with "property failed, with context". Either a counterexample-style mechanism or documenting the raise pattern as the blessed idiom would help.

4. Missing small combinators. char_range and applicative combines (zip/zip_with/zip_with3) are absent; every nontrivial generator ends up re-deriving them from flat_map/map (our migration needed them ~15 times).

5. Arbitrary for Char is ASCII-only, and String inherits that. For property testing, multi-byte and non-BMP characters are exactly where string-handling bugs hide; an ASCII-biased distribution that regularly emits arbitrary Unicode scalar values (never surrogates) would be a better default. Relatedly, the internal unsafe_to_char uses could be replaced by validated construction.

6. Minor: getting a Generator[T] from an Arbitrary instance (the old Gen::spawn) is possible via Generator(Arbitrary::arbitrary) but non-obvious — worth a documented helper.

A patch implementing 1, 2, 4, and 5 is prepared (all 7015 core tests pass with seeded snapshots updated) and will be submitted as a follow-up PR.

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