support non-nil empty list

list/list.go

@@ -5,11 +5,17 @@ import (
 	"strings"
 )
 
+type EmptyList struct{}
+
 type List struct {
 	Value any
 	next  *List
 }
 
+func Empty() *EmptyList {
+	return new(EmptyList)
+}
+
 func New(val any) *List {
 	return new(List).Init(val)
 }
@@ -28,6 +34,10 @@ func (this *List) Conj(val any) *List {
 	return Conj(this, val)
 }
 
+func (this *EmptyList) Conj(val any) *List {
+	return Conj(nil, val)
+}
+
 func Conj(this *List, val any) *List {
 	if this == nil {
 		return New(val)
@@ -59,13 +69,25 @@ func (this *List) First() any {
 	return First(this)
 }
 
+func (this *EmptyList) First() any {
+	return First(nil)
+}
+
 func IsEmpty(this *List) bool {
 	return this == nil
 }
 
+func (this *List) IsEmpty() bool {
+	return this == nil
+}
+
+func (this *EmptyList) IsEmpty() bool {
+	return true
+}
+
 func String(this *List) string {
 	if this == nil {
-		return "{}"
+		return "()"
 	}
 	var sb strings.Builder
 	sb.WriteRune('(')

list/list_test.go

@@ -0,0 +1,25 @@
+package list
+
+import (
+	"github.com/stretchr/testify/assert"
+	"testing"
+)
+
+func TestReadConj(t *testing.T) {
+	l := Empty().Conj(5).Conj(6).Conj(7)
+	assert.Equal(t, "(7 6 5)", l.String(), "should insert at head")
+}
+
+func TestReadFirst(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")
+	assert.Equal(t, 5, New(5).First(), "should get first from New")
+}
+
+func TestReadRest(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")
+	assert.Equal(t, "()", l.Rest().Rest().Rest().String(), "should return rest sublist")
+}