docs: fix FsCheck custom generators example for FsCheck 3.x API

TUnit.Example.FsCheck.TestProject/PropertyTests.cs

@@ -4,6 +4,19 @@
 
 namespace TUnit.Example.FsCheck.TestProject;
 
+/// <summary>
+/// Custom arbitrary that generates positive integers only.
+/// </summary>
+public class PositiveIntArbitrary
+{
+    public static Arbitrary<int> PositiveInt()
+    {
+        return ArbMap.Default.GeneratorFor<int>()
+            .Where(x => x > 0)
+            .ToArbitrary();
+    }
+}
+
 public class PropertyTests
 {
     [Test, FsCheckProperty]
@@ -88,4 +101,9 @@ public Property StringReversalProperty()
         });
     }
 
+    [Test, FsCheckProperty(Arbitrary = new[] { typeof(PositiveIntArbitrary) })]
+    public bool PositiveNumbersArePositive(int value)
+    {
+        return value > 0;
+    }
 }

docs/docs/examples/fscheck.md

@@ -122,14 +122,21 @@ public bool MyProperty(int value)
 
 ## Custom Generators
 
-You can provide custom `Arbitrary` implementations for generating test data:
+You can provide custom `Arbitrary` implementations for generating test data. FsCheck 3.x uses `ArbMap.Default` to access default arbitraries:
 
 ```csharp
+using FsCheck;
+using FsCheck.Fluent;
+
 public class PositiveIntArbitrary
 {
     public static Arbitrary<int> PositiveInt()
     {
-        return Arb.Default.Int32().Filter(x => x > 0);
+        // Use ArbMap.Default to get a generator for a type,
+        // then filter with Where() and convert to Arbitrary
+        return ArbMap.Default.GeneratorFor<int>()
+            .Where(x => x > 0)
+            .ToArbitrary();
     }
 }
 
@@ -140,6 +147,44 @@ public bool PositiveNumbersArePositive(int value)
 }
 ```
 
+### Alternative: Using Gen.Choose
+
+For simple ranges, use `Gen.Choose` directly:
+
+```csharp
+public class PositiveIntArbitrary
+{
+    public static Arbitrary<int> PositiveInt()
+    {
+        // Generate integers in a specific range
+        return Gen.Choose(1, int.MaxValue).ToArbitrary();
+    }
+}
+```
+
+### Custom types
+
+For custom types, compose generators using LINQ:
+
+```csharp
+public class Person
+{
+    public string Name { get; set; }
+    public int Age { get; set; }
+}
+
+public class PersonArbitrary
+{
+    public static Arbitrary<Person> Person()
+    {
+        var gen = from name in ArbMap.Default.GeneratorFor<string>()
+                  from age in Gen.Choose(0, 120)
+                  select new Person { Name = name, Age = age };
+        return gen.ToArbitrary();
+    }
+}
+```
+
 ## Limitations
 
 - **Native AOT**: TUnit.FsCheck is not compatible with Native AOT publishing because FsCheck requires reflection and dynamic code generation