add PBT for list len

list/list_test.go

@@ -40,19 +40,24 @@ func TestListRest(t *testing.T) {
 	assert.Equal(t, Empty(), Rest(Empty().Rest()), "should return rest sublist")
 }
 
-type listGen struct{}
-
 // generative tests
-func TestConjIncreasesLen(t *testing.T) {
-	l := Empty()
+func ListLengthProperty(t *testing.T) {
+	myList := Empty()
 	expectedSized := 0
 	rapid.Check(t, func(t *rapid.T) {
 		t.Repeat(map[string]func(*rapid.T){
 			"conj": func(t *rapid.T) {
-				expectedSized += 0
+				expectedSized += 1
+				myList = myList.Conj(rapid.Int().Draw(t, "el"))
+			},
+			"rest": func(t *rapid.T) {
+				if !myList.IsEmpty() {
+					expectedSized -= 1
+				}
+				myList = myList.Rest()
 			},
 			"": func(t *rapid.T) {
-				assert.Equal(t, l.Len(), expectedSized, "must be equal")
+				assert.Equal(t, myList.Len(), expectedSized, "must be equal")
 			},
 		})
 	})