flyingmutant · swist · Jul 28, 2025 · Jul 28, 2025
diff --git a/combinators.go b/combinators.go
@@ -76,14 +76,35 @@ func (g *deferredGen[V]) value(t *T) V {
 	return g.g.value(t)
 }

-func filter[V any](g *Generator[V], fn func(V) bool) *Generator[V] {
-	return newGenerator[V](&filteredGen[V]{
-		g:  g,
-		fn: fn,
-	})
+type filterOpts struct {
+	maxAttempts int
+}
+type filterOption func(*filterOpts) *filterOpts
+
+func filter[V any](g *Generator[V], fn func(V) bool, opts ...filterOption) *Generator[V] {
+	impl := &filteredGen[V]{
+		g:          g,
+		fn:         fn,
+		filterOpts: &filterOpts{},
+	}
+
+	impl.applyOptions(opts...)
+
+	return newGenerator[V](impl)
+}
+
+func WithMaxAttempts(max int) filterOption {
+	return func(o *filterOpts) *filterOpts {
+		if max <= 0 {
+			panic(invalidData("max attempts should be greater than 0"))
+		}
+		o.maxAttempts = max
+		return o
+	}
 }

 type filteredGen[V any] struct {
+	*filterOpts
 	g  *Generator[V]
 	fn func(V) bool
 }
@@ -93,7 +114,15 @@ func (g *filteredGen[V]) String() string {
 }

 func (g *filteredGen[V]) value(t *T) V {
-	return find(g.maybeValue, t, small)
+	tries := g.maxAttempts
+	if tries <= 0 {
+		// If no max attempts specified, use the default value.
+		tries = flags.filterMaxattempts
+
+	}
+
+	return find(g.maybeValue, t, tries)
+
 }

 func (g *filteredGen[V]) maybeValue(t *T) (V, bool) {
@@ -106,6 +135,12 @@ func (g *filteredGen[V]) maybeValue(t *T) (V, bool) {
 	}
 }

+func (g *filteredGen[V]) applyOptions(opts ...filterOption) {
+	for _, opt := range opts {
+		g.filterOpts = opt(g.filterOpts)
+	}
+}
+
 func find[V any](gen func(*T) (V, bool), t *T, tries int) V {
 	for n := 0; n < tries; n++ {
 		i := t.s.beginGroup(tryLabel, false)

diff --git a/engine.go b/engine.go
@@ -47,20 +47,22 @@ var (
 )

 type cmdline struct {
-	checks     int
-	steps      int
-	failfile   string
-	nofailfile bool
-	seed       uint64
-	log        bool
-	verbose    bool
-	debug      bool
-	debugvis   bool
-	shrinkTime time.Duration
+	checks            int
+	filterMaxattempts int
+	steps             int
+	failfile          string
+	nofailfile        bool
+	seed              uint64
+	log               bool
+	verbose           bool
+	debug             bool
+	debugvis          bool
+	shrinkTime        time.Duration
 }

 func init() {
 	flag.IntVar(&flags.checks, "rapid.checks", 100, "rapid: number of checks to perform")
+	flag.IntVar(&flags.filterMaxattempts, "rapid.filtermaxattempts", 1000, "rapid: maximum number of attempts to draw a valid value from a Filter generator")
 	flag.IntVar(&flags.steps, "rapid.steps", 30, "rapid: average number of Repeat actions to execute")
 	flag.StringVar(&flags.failfile, "rapid.failfile", "", "rapid: fail file to use to reproduce test failure")
 	flag.BoolVar(&flags.nofailfile, "rapid.nofailfile", false, "rapid: do not write fail files on test failures")

diff --git a/engine_test.go b/engine_test.go
@@ -40,6 +40,16 @@ func TestPanicTraceback(t *testing.T) {
 				return err
 			},
 		},
+		{
+			"impossible filter with options",
+			"pgregory.net/rapid.find[...]",
+			false,
+			func(t *T) *testError {
+				g := Bool().Filter(func(bool) bool { return false }, WithMaxAttempts(10))
+				_, err := recoverValue(g, t)
+				return err
+			},
+		},
 		{
 			"broken custom generator",
 			"pgregory.net/rapid.brokenGen",

diff --git a/generator.go b/generator.go
@@ -91,8 +91,8 @@ func (g *Generator[V]) Example(seed ...int) V {
 }

 // Filter creates a generator producing only values from g for which fn returns true.
-func (g *Generator[V]) Filter(fn func(V) bool) *Generator[V] {
-	return filter(g, fn)
+func (g *Generator[V]) Filter(fn func(V) bool, opts ...filterOption) *Generator[V] {
+	return filter(g, fn, opts...)
 }

 // AsAny creates a generator producing values from g converted to any.