Refactor VM and operators

internal/object/str.go

@@ -2,6 +2,7 @@ package object
 
 import (
 	"fmt"
+	"strings"
 	"unicode/utf8"
 )
 
@@ -33,6 +34,33 @@ func (s *String) String() string {
 	return s.Value
 }
 
+func (s *String) Add(other Object) (Object, error) {
+	if !AssertTypes(other, StringType) {
+		return nil, NewBinaryOpError(s, other, "+")
+	}
+	return &String{s.Value + other.(*String).Value}, nil
+}
+
+func (s *String) Mul(other Object) (Object, error) {
+	if !AssertTypes(other, IntegerType) {
+		return nil, NewBinaryOpError(s, other, "*")
+	}
+	return &String{strings.Repeat(s.Value, int(other.(*Integer).Value))}, nil
+}
+
+func (s *String) Get(index Object) (Object, error) {
+	if !AssertTypes(index, IntegerType) {
+		return nil, fmt.Errorf("invalid type for string index, expected Integer got %s", index.Type())
+	}
+
+	i := int(index.(*Integer).Value)
+	if i < 0 || i >= len(s.Value) {
+		return &String{}, nil
+	}
+
+	return &String{string(s.Value[i])}, nil
+}
+
 func (s *String) Compare(other Object) int {
 	if obj, ok := other.(*String); ok {
 		switch {