add 'wrap' and 'unwrap' method to hold self error

expr_test.go

@@ -1348,7 +1348,7 @@ func TestCompile_exposed_error(t *testing.T) {
 
 	b, err := json.Marshal(err)
 	require.NoError(t, err)
-	require.Equal(t, `{"Line":1,"Column":2,"Message":"invalid operation: == (mismatched types int and bool)","Snippet":"\n | 1 == true\n | ..^"}`, string(b))
+	require.Equal(t, `{"Line":1,"Column":2,"Message":"invalid operation: == (mismatched types int and bool)","Snippet":"\n | 1 == true\n | ..^","Prev":null}`, string(b))
 }
 
 func TestCompile_deref(t *testing.T) {

file/error.go

@@ -10,6 +10,7 @@ type Error struct {
 	Location
 	Message string
 	Snippet string
+	Prev error
 }
 
 func (e *Error) Error() string {
@@ -44,6 +45,16 @@ func (e *Error) Bind(source *Source) *Error {
 	return e
 }
 
+
+func (e *Error) Unwrap() error {
+	return e.Prev
+}
+
+func (e *Error) Wrap(err error) {
+	e.Prev = err
+}
+
+
 func (e *Error) format() string {
 	if e.Location.Empty() {
 		return e.Message

vm/vm.go

@@ -61,6 +61,9 @@ func (vm *VM) Run(program *Program, env interface{}) (_ interface{}, err error)
 				Location: program.Locations[vm.ip-1],
 				Message:  fmt.Sprintf("%v", r),
 			}
+			if err, ok := r.(error); ok {
+				f.Wrap(err)
+			}
 			err = f.Bind(program.Source)
 		}
 	}()

vm/vm_test.go

@@ -274,6 +274,10 @@ func TestRun_MethodWithError(t *testing.T) {
 	out, err := vm.Run(program, env)
 	require.EqualError(t, err, "error (1:1)\n | WillError(\"yes\")\n | ^")
 	require.Equal(t, nil, out)
+
+	selfErr := errors.Unwrap(err)
+	require.NotNil(t, err)
+	require.Equal(t, "error", selfErr.Error())
 }
 
 func TestRun_FastMethods(t *testing.T) {