Extractors

Quick and easy extraction of structured data from unstructured data

An Example

scala> import com.twitter.extractors.json.JsonObjectExtractor

scala> case class Person(firstName: String, lastName: String, mi: Option[String], age: Int)

scala> val personFromJson = JsonObjectExtractor(Person, "first", "last", "middle_initial", "age")

scala> personFromJson("""{ "first": "Alice", "last": "Smith", "middle_initial": "B", "age": 35 }""")
res0: Person = Person(Alice,Smith,Some(B),35)

Quick-and-dirty Usage

Currently, there are extractor factories for JSON, using Jackson, JDBC ResultSets, and unstructured Map[String,Any].

To create an extractor function, you give your object factory function as the first argument, followed by the keys for each factory parameter, (generally strings):

case class Person(firstName: String, lastName: String, mi: Option[String], age: Int)
val personFromJson = JsonObjectExtractor(Person, "first", "last", "middle_initial", "age")

Based on the type signature of the factory function, converter functions for each parameter are determined at compile time via Scala's implicit parameter functionality. You will get a compile time error if the type of a constructor parameter is unsupported by the specific extractor.

Nested Extractors

Additionally, extractors can be nested. In order for this to work, the extractor should be an implicit value on the companion object of your class:

case class Child(name: String)

object Child extends (String => Child) {
  implicit val fromJson = JsonObjectExtractor(apply, "name")
}

case class Parent(name: String, children: List[Child])

object Parent extends ((String, List[Child]) => Parent) {
  implicit val fromJson = JsonObjectExtractor(apply, "name", "children")
}


scala> Parent.fromJson("""{ "name": "Dad", "children": [{"name": "Son"}] }""")
res1: Parent(Dad,List(Child(Son)))

Recursive Extractors

Extractors can created for recursive data types, however, it requires adding an explicit type annotation, as the resulting extractor is a recursive definition. (Not sure how to get around this.)

case class IntCons(item: Int, next: Option[IntCons])

object IntCons extends ((Int, Option[IntCons]) => IntCons) {
  implicit val fromJson: JsonObjectExtractor.Extractor[IntCons] =
    JsonObjectExtractor(IntCons, "item", "next")
}


scala> IntCons.fromJson("""{"item": 1, "next": {"item": 2, "next": { "item": 3 } } }""")
res1: IntCons(1,Some(IntCons(2,Some(IntCons(3, None)))))