feat(quickcheck): Generator::spawn draws from an Arbitrary instance (#3994)

* feat(quickcheck): Generator::spawn draws from an Arbitrary instance

Hand-written generator suites routinely mix Arbitrary-based generation
into custom combinators (e.g. arbitrary strings inside a structured
document generator); until now each project re-derived the same
one-liner. Generator::spawn() makes the bridge a core API.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* test: pin explicit seeds in the spawn docstring example and test

Review feedback: do not rely on Generator::samples' implicit default
seed, so the snapshots stay stable if the default ever changes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* refactor: make spawn a free function

Review feedback: the package's generator constructors (pure, one_of,
elements, frequency, int_range, char_range) are free functions; spawn
now follows the same convention.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: bobzhang

quickcheck/generator.mbt

@@ -29,6 +29,27 @@ pub fn[T] Generator::Generator(
   { generate, }
 }
 
+///|
+/// Creates a generator that draws values from the type's `Arbitrary`
+/// instance, for mixing `Arbitrary`-based generation into hand-written
+/// generators.
+///
+/// ```mbt check
+/// test "spawn draws from Arbitrary" {
+///   let strings : @quickcheck.Generator[String] = @quickcheck.spawn()
+///   let lengths = strings.samples(count=3, size=4, seed=37).map(s => s.length())
+///   debug_inspect(
+///     lengths,
+///     content=(
+///       #|[3, 2, 1]
+///     ),
+///   )
+/// }
+/// ```
+pub fn[T : Arbitrary] spawn() -> Generator[T] {
+  Generator(Arbitrary::arbitrary)
+}
+
 ///|
 /// Runs a generator with an explicit size and random state.
 pub fn[T] Generator::run(

quickcheck/generator_test.mbt

@@ -173,3 +173,16 @@ test "zip_with and zip_with3 combine generator outputs" {
     ),
   )
 }
+
+///|
+test "spawn composes with generator combinators" {
+  let entries = (@quickcheck.spawn() : @quickcheck.Generator[String])
+    .zip((@quickcheck.spawn() : @quickcheck.Generator[Int]))
+    .samples(count=3, size=4, seed=37)
+  debug_inspect(
+    entries,
+    content=(
+      #|[("", 3), ("Z\b", 3), ("", 0)]
+    ),
+  )
+}

quickcheck/pkg.generated.mbti

@@ -42,6 +42,8 @@ pub fn[X : Arbitrary] samples(Int) -> Array[X]
 
 pub fn[T] sized((Int) -> Generator[T]) -> Generator[T]
 
+pub fn[T : Arbitrary] spawn() -> Generator[T]
+
 // Errors
 
 // Types and methods