start the tower of abstractions

list/list.go

@@ -5,57 +5,71 @@ import (
 	"strings"
 )
 
+type IList interface {
+	Conj(data any) IList
+	First() any
+	Rest() IList
+	String() string
+}
+
 type EmptyList struct{}
 
 type List struct {
 	Value any
-	next  *List
+	next  IList
 }
 
-func Empty() *EmptyList {
+func New() IList {
 	return new(EmptyList)
 }
 
-func New(val any) *List {
-	return new(List).Init(val)
-}
-
-func (this *List) Init(val any) *List {
-	return Init(this, val)
-}
-
-func Init(this *List, val any) *List {
+func single(val any) *List {
+	this := new(List)
 	this.Value = val
 	this.next = nil
 	return this
 }
 
-func (this *List) Conj(val any) *List {
-	return Conj(this, val)
+func (this *List) Conj(val any) IList {
+	if this == nil {
+		return single(val)
+	}
+	new_head := single(val)
+	new_head.next = this
+	return new_head
+
 }
 
-func (this *EmptyList) Conj(val any) *List {
+func (this *EmptyList) Conj(val any) IList {
 	return Conj(nil, val)
 }
 
-func Conj(this *List, val any) *List {
+func Conj(this IList, val any) IList {
+	var l IList
 	if this == nil {
-		return New(val)
+		l = single(val)
+	} else {
+		l = this
 	}
-	new_head := New(val)
-	new_head.next = this
-	return new_head
+	return l.Conj(val)
 }
 
-func Rest(this *List) *List {
+func Rest(this IList) IList {
+	if this == nil {
+		return nil
+	}
+	return this.Rest()
+}
+
+func (this *List) Rest() IList {
 	if this == nil {
 		return nil
 	}
 	return this.next
 }
 
-func (this *List) Rest() *List {
-	return Rest(this)
+func (this *EmptyList) Rest() IList {
+	return nil
 }
 
 func First(this *List) any {
@@ -73,7 +87,7 @@ func (this *EmptyList) First() any {
 	return First(nil)
 }
 
-func IsEmpty(this *List) bool {
+func IsEmpty(this IList) bool {
 	return this == nil
 }
 
@@ -92,9 +106,15 @@ func String(this *List) string {
 	var sb strings.Builder
 	sb.WriteRune('(')
 	// Iterate and print elements
-	for e := this; e != nil; e = Rest(e) {
-		sb.WriteString(fmt.Sprint(e.Value))
-		sb.WriteRune(' ')
+	var e IList
+	for e = this; e != nil; e = Rest(e) {
+		switch v := e.(type) {
+		case *List:
+			sb.WriteString(fmt.Sprint(v.Value))
+			sb.WriteRune(' ')
+		case *EmptyList:
+			break
+		}
 	}
 	return sb.String()[:sb.Len()-1] + ")"
 }
@@ -102,3 +122,7 @@ func String(this *List) string {
 func (this *List) String() string {
 	return String(this)
 }
+
+func (this *EmptyList) String() string {
+	return String(nil)
+}

list/list_test.go

@@ -6,20 +6,24 @@ import (
 )
 
 func TestReadConj(t *testing.T) {
-	l := Empty().Conj(5).Conj(6).Conj(7)
+	l := New().Conj(5).Conj(6).Conj(7)
 	assert.Equal(t, "(7 6 5)", l.String(), "should insert at head")
+	l = Conj(New(), 4)
+	assert.Equal(t, "(4 5)", l.String(), "should insert at head")
 }
 
 func TestReadFirst(t *testing.T) {
-	l := Empty().Conj(5).Conj(6).Conj(7)
+	l := New().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")
+	assert.Equal(t, nil, New().First(), "should return nil")
 }
 
 func TestReadRest(t *testing.T) {
-	l := Empty().Conj(5).Conj(6).Conj(7)
+	l := New().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")
+	var nilPtr *List = nil
+	assert.Equal(t, nilPtr, New().Rest(), "should return rest sublist")
+	assert.Equal(t, nilPtr, Rest(New().Rest()), "should return rest sublist")
 }