further improvements

internal/object/bool.go

@@ -4,10 +4,22 @@ import (
 	"fmt"
 )
 
+var (
+	TRUE  = Boolean{Value: true}
+	FALSE = Boolean{Value: false}
+)
+
 type Boolean struct {
 	Value bool
 }
 
+func NewBoolean(value bool) Boolean {
+	if value {
+		return TRUE
+	}
+	return FALSE
+}
+
 func (b Boolean) Bool() bool {
 	return b.Value
 }

internal/object/builtin.go

@@ -7,18 +7,18 @@ type Builtin struct {
 	Fn   BuiltinFunction
 }
 
-func (b *Builtin) Bool() bool {
+func (b Builtin) Bool() bool {
 	return true
 }
 
-func (b *Builtin) Type() Type {
+func (b Builtin) Type() Type {
 	return BuiltinType
 }
 
-func (b *Builtin) Inspect() string {
+func (b Builtin) Inspect() string {
 	return fmt.Sprintf("<built-in function %s>", b.Name)
 }
 
-func (b *Builtin) String() string {
+func (b Builtin) String() string {
 	return b.Inspect()
 }

internal/object/int.go

@@ -27,17 +27,21 @@ func (i Integer) String() string {
 }
 
 func (i Integer) Add(other Object) (Object, error) {
-	if !AssertTypes(other, IntegerType) {
+	switch obj := other.(type) {
+	case Integer:
+		return Integer{i.Value + obj.Value}, nil
+	default:
 		return nil, NewBinaryOpError(i, other, "+")
 	}
-	return Integer{i.Value + other.(Integer).Value}, nil
 }
 
 func (i Integer) Sub(other Object) (Object, error) {
-	if !AssertTypes(other, IntegerType) {
+	switch obj := other.(type) {
+	case Integer:
+		return Integer{i.Value - obj.Value}, nil
+	default:
 		return nil, NewBinaryOpError(i, other, "-")
 	}
-	return Integer{i.Value - other.(Integer).Value}, nil
 }
 
 func (i Integer) Mul(other Object) (Object, error) {
@@ -115,15 +119,16 @@ func (i Integer) Negate() Object {
 }
 
 func (i Integer) Compare(other Object) int {
-	if obj, ok := other.(Integer); ok {
-		switch {
-		case i.Value < obj.Value:
+	switch obj := other.(type) {
+	case Integer:
+		if i.Value < obj.Value {
 			return -1
-		case i.Value > obj.Value:
-			return 1
-		default:
-			return 0
 		}
+		if i.Value > obj.Value {
+			return 1
+		}
+		return 0
+	default:
+		return -1
 	}
-	return -1
 }

internal/object/null.go

@@ -1,5 +1,7 @@
 package object
 
+var NULL = Null{}
+
 type Null struct{}
 
 func (n Null) Bool() bool {