Define TypedValue comparators; Make parser init more flexible

pkg/expr/values.go

@@ -1,6 +1,8 @@
 package expr
 
-import "reflect"
+import (
+	"reflect"
+)
 
 type (
 	Type interface {
@@ -14,6 +16,10 @@ type (
 		Get() interface{}
 	}
 
+	Comparable interface {
+		Compare(TypedValue) (int, error)
+	}
+
 	typedValueWrap struct {
 		Value interface{} `json:"@value"`
 		Type  string      `json:"@type"`