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scala> val archer = new Dog("archer") with Athlete with Runner with Male
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archer: Dog with Athlete with Runner with Male = $anon$1@18bbc98
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CHAPTER 7 TRAITS AND TYPES AND GNARLY STUFF FOR ARCHITECTS
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scala> val dpp = new Person("David") with Athlete with Biker with Male
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dpp: Person with Athlete with Biker with Male = $anon$1@7b5617
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scala> val john = new Person("John") with Athlete with Runner with Male
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scala> val annette = new Person("Annette") with Athlete with Runner with Female
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We ve got a bunch of Animals. Let s see what we can do with them:
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scala> def goBiking(b: Biker) = println(b.name+" is biking") goBiking: (Biker)Unit scala> goBiking(dpp)
David is biking
What happens if we try to send Annette on a bike ride
scala> goBiking(annette)
<console>:7: error: type mismatch; found : Person with Athlete with Runner with Female required: Biker goBiking(annette)
CHAPTER 7 TRAITS AND TYPES AND GNARLY STUFF FOR ARCHITECTS
This makes sense. The method requires a Biker, and Annette is not a Biker. However, just as we can compose a class out of traits, we can require that a class implement more than one trait in order to be the parameter to a method:
scala> def charityRun(r: Person with Runner) = r.run()
charityRun: (Person with Runner)Unit
The charityRun method can only be called with a parameter that is a subclass of Person and also implements the Runner trait.
scala> charityRun(annette)
I'm running
What if we try to call the method with a Runner that is not a Person
scala> charityRun(archer)
<console>:7: error: type mismatch; found : Dog with Athlete with Runner with Male required: Person with Runner charityRun(archer)
We can define the parameter in terms of traits. The womensRun method may only be called with a parameter that s both a Runner and a Female:
scala> def womensRun(r: Runner with Female) = r.run()
womensRun: (Runner with Female)Unit
CHAPTER 7 TRAITS AND TYPES AND GNARLY STUFF FOR ARCHITECTS
scala> womensRun(annette)
I'm running
scala> val madeline = new Cat("Madeline") with Athlete with Runner with Female
madeline: Cat with Athlete with Runner with Female = $anon$1@11dde0c
scala> womensRun(madeline)
I'm running
In this way, we ve modeled complex relationships. We ve modeled things in a way that you cannot model with Java. Scala s compositional rules are very powerful tools for defining very complex class hierarchies and for specifying the rules for composing classes as well as the rules for passing parameters into methods. In this way, we can make sure that the charityRun method can only be called with valid parameters rather than testing for parameter correctness at runtime and throwing an exception if the parameter is not correct. This increased modeling flexibility combined with enhanced type safety gives the architect another tool to help developers write correct code.
Types It s Beyond Generic
We ve seen how Scala allows complex modeling and compile-time type checking that allows us to build complex class hierarchies. What about things that make a developer s life easier What about things that make a developer, a library consumer, think that Scala is as easy and flexible as a scripting language In this section, we re going to write a database abstraction layer that allows a developer to define a table mapping simply as follows:
class val val val val MyTable extends Table[MyTable] { table = "mytable" id = IntColumn("id") name = StringColumn("name") birthday = DateColumn("birthday")
CHAPTER 7 TRAITS AND TYPES AND GNARLY STUFF FOR ARCHITECTS
type ColumnTypes = (Int, String, Date) def columns = id ~ name ~ birthday }
The class will allow type-safe queries such as this:
MyTable.findAll(By(MyTable.id, 33))
And the compiler will flag an error on a query such as this:
MyTable.findAll(By(MyTable.id, "33"))
Thus, the library consumer gets the benefits of type safety along with very lightweight syntax for defining the mapping to the table and for defining queries. Scala s type system and type inferencer make this possible.
Table Code
Let s look at the entire listing for the Query Builder (Listing 7-3). After that, we ll slice and dice it to understand how it works from the library-producer standpoint and see how it makes a library consumer s life easier.
Listing 7-3. Query Builder import java.util.Date import java.sql._ import scala.collection.mutable.ListBuffer trait def def def def } BasicColumn[TableType <: Table[TableType], T] { default: T name: String getField(rs: ResultSet): T set(st: PreparedStatement, offset: Int, value: T)
trait Table[MyType <: Table[MyType]] extends SuperTuple { this: MyType => def table: String type ColumnTypes <: Product
def columns: ColumnTypes with FieldProduct[MyType]
CHAPTER 7 TRAITS AND TYPES AND GNARLY STUFF FOR ARCHITECTS
trait MyColumn[T] extends BasicColumn[MyType, T] { def ~[OT](p: MyColumn[OT]): MyTuple2[MyType, T, OT] = { val col = this new MyTuple2[MyType, T, OT](col.default, p.default) { def fields = List(col, p) def fieldProduct: (MyColumn[T], MyColumn[OT]) = (col, p) } } } case class IntColumn(name: String) extends MyColumn[Int] { def default = 0 def set(st: PreparedStatement, offset: Int, value: Int) { st.setInt(offset, value) } def getField(rs: ResultSet): Int = rs.getInt(name) } case class LongColumn(name: String) extends MyColumn[Long] { def default = 0 def set(st: PreparedStatement, offset: Int, value: Long) { st.setLong(offset, value) } def getField(rs: ResultSet): Long = rs.getLong(name) } case class StringColumn(name: String) extends MyColumn[String] { def default = "" def set(st: PreparedStatement, offset: Int, value: String) { st.setString(offset, value) } def getField(rs: ResultSet): String = rs.getString(name) } case class DateColumn(name: String) extends MyColumn[Date] { def default = new Date(0) def set(st: PreparedStatement, offset: Int, value: Date) { st.setDate(offset, new java.sql.Date(value.getTime)) } def getField(rs: ResultSet): Date = rs.getDate(name) }
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