Fix getting default values out of maps

Author: antonmedv

expr_test.go

@@ -828,15 +828,20 @@ func TestExpr_fetch_from_func(t *testing.T) {
 	assert.Contains(t, err.Error(), "cannot fetch Value from func()")
 }
 
-func TestExpr_reference_options(t *testing.T) {
-	options := []expr.Option{expr.Env(map[string]interface{}{})}
+func TestExpr_map_default_values(t *testing.T) {
+	env := map[string]interface{}{
+		"foo": map[string]string{},
+		"bar": map[string]*string{},
+	}
 
-	e, err := expr.Compile("'hello world'", options...)
-	assert.NoError(t, err)
+	input := `foo['missing'] == '' && bar['missing'] == nil`
+
+	program, err := expr.Compile(input, expr.Env(env))
+	require.NoError(t, err)
 
-	output, err := expr.Run(e, "'hello world'")
-	assert.NoError(t, err)
-	assert.Equal(t, "hello world", output)
+	output, err := expr.Run(program, env)
+	require.NoError(t, err)
+	require.Equal(t, true, output)
 }
 
 //

vm/runtime.go

@@ -36,8 +36,13 @@ func fetch(from interface{}, i interface{}) interface{} {
 
 	case reflect.Map:
 		value := v.MapIndex(reflect.ValueOf(i))
-		if value.IsValid() && value.CanInterface() {
-			return value.Interface()
+		if value.IsValid() {
+			if value.CanInterface() {
+				return value.Interface()
+			}
+		} else {
+			elem := reflect.TypeOf(from).Elem()
+			return reflect.Zero(elem).Interface()
 		}
 
 	case reflect.Struct: