VM:Add abstract stack item

pkg/vm/stack/stackitem.go

@@ -0,0 +1,42 @@
+package stack
+
+import (
+	"errors"
+)
+
+//Item is an interface which represents object that can be placed on the stack
+type Item interface {
+	Integer() (*Int, error)
+	Boolean() (*Boolean, error)
+	ByteArray() (*ByteArray, error)
+	Array() (*Array, error)
+}
+
+// Represents an `abstract` stack item
+// which will hold default values for stack items
+// this is intended to be embedded into types that you will use on the stack
+type abstractItem struct{}
+
+// Integer is the default implementation for a stackItem
+// Implements Item interface
+func (a *abstractItem) Integer() (*Int, error) {
+	return nil, errors.New("This stack item is not an Integer")
+}
+
+// Boolean is the default implementation for a stackItem
+// Implements Item interface
+func (a *abstractItem) Boolean() (*Boolean, error) {
+	return nil, errors.New("This stack item is not a Boolean")
+}
+
+// ByteArray is the default implementation for a stackItem
+// Implements Item interface
+func (a *abstractItem) ByteArray() (*ByteArray, error) {
+	return nil, errors.New("This stack item is not a byte array")
+}
+
+// Array is the default implementation for a stackItem
+// Implements Item interface
+func (a *abstractItem) Array() (*Array, error) {
+	return nil, errors.New("This stack item is not an array")
+}