diff --git a/.gitignore b/.gitignore
index fa3d5f1..ef61743 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 tmp
 .air.toml
+*/testdata/*
diff --git a/go.mod b/go.mod
index 7b7fcac..399a76e 100644
--- a/go.mod
+++ b/go.mod
@@ -7,4 +7,5 @@ require (
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/stretchr/testify v1.11.1 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
+	pgregory.net/rapid v1.2.0 // indirect
 )
diff --git a/go.sum b/go.sum
index cc8b3f4..50277fe 100644
--- a/go.sum
+++ b/go.sum
@@ -7,3 +7,5 @@ github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+pgregory.net/rapid v1.2.0 h1:keKAYRcjm+e1F0oAuU5F5+YPAWcyxNNRK2wud503Gnk=
+pgregory.net/rapid v1.2.0/go.mod h1:PY5XlDGj0+V1FCq0o192FdRhpKHGTRIWBgqjDBTrq04=
diff --git a/list/list.go b/list/list.go
index a404392..2a62437 100644
--- a/list/list.go
+++ b/list/list.go
@@ -5,13 +5,13 @@ import (
 	"strings"
 )
 
-
 type IList interface {
 	Conj(data any) IList
 	First() any
 	Rest() IList
 	String() string
 	IsEmpty() bool
+	Len() int
 }
 
 type EmptyList struct{}
@@ -121,3 +121,11 @@ func (this *List) String() string {
 func (this *EmptyList) String() string {
 	return "()"
 }
+
+func (this *EmptyList) Len() int {
+	return 0
+}
+
+func (this *List) Len() int {
+	return 1 + this.Rest().Len()
+}
diff --git a/list/list_test.go b/list/list_test.go
index 93d9d06..55e3cfd 100644
--- a/list/list_test.go
+++ b/list/list_test.go
@@ -2,14 +2,21 @@ package list
 
 import (
 	"github.com/stretchr/testify/assert"
+	"pgregory.net/rapid"
 	"testing"
 )
 
-func TestReadString(t *testing.T) {
+func TestListLen(t *testing.T) {
+	assert.Equal(t, 0, Empty().Len(), "should insert at head")
+	assert.Equal(t, 1, Empty().Conj(1).Len(), "should insert at head")
+	assert.Equal(t, 2, Empty().Conj(1).Conj(2).Len(), "should insert at head")
+}
+
+func TestListString(t *testing.T) {
 	assert.Equal(t, "()", Empty().String(), "should insert at head")
 }
 
-func TestReadConj(t *testing.T) {
+func TestListConj(t *testing.T) {
 	var l IList
 	assert.Equal(t, "()", Empty().String(), "should insert at head")
 	l = New(5)
@@ -18,13 +25,13 @@ func TestReadConj(t *testing.T) {
 	assert.Equal(t, "(4)", l.String(), "should insert at head")
 }
 
-func TestReadFirst(t *testing.T) {
+func TestListFirst(t *testing.T) {
 	l := Empty().Conj(5).Conj(6).Conj(7)
 	assert.Equal(t, 7, l.First(), "should return first element")
 	assert.Equal(t, nil, Empty().First(), "should return nil")
 }
 
-func TestReadRest(t *testing.T) {
+func TestListRest(t *testing.T) {
 	l := Empty().Conj(5).Conj(6).Conj(7)
 	assert.Equal(t, "(6 5)", l.Rest().String(), "should return rest sublist")
 	assert.Equal(t, "(5)", l.Rest().Rest().String(), "should return rest sublist")
@@ -32,3 +39,21 @@ func TestReadRest(t *testing.T) {
 	assert.Equal(t, Empty(), Empty().Rest(), "should return rest sublist")
 	assert.Equal(t, Empty(), Rest(Empty().Rest()), "should return rest sublist")
 }
+
+type listGen struct{}
+
+// generative tests
+func TestConjIncreasesLen(t *testing.T) {
+	l := Empty()
+	expectedSized := 0
+	rapid.Check(t, func(t *rapid.T) {
+		t.Repeat(map[string]func(*rapid.T){
+			"conj": func(t *rapid.T) {
+				expectedSized += 0
+			},
+			"": func(t *rapid.T) {
+				assert.Equal(t, l.Len(), expectedSized, "must be equal")
+			},
+		})
+	})
+}