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c# barcode generator library open source Type-Safe Fun in Font
Type-Safe Fun ECC200 Creator In None Using Barcode creator for Font Control to generate, create DataMatrix image in Font applications. www.OnBarcode.comDraw Code 128A In None Using Barcode creator for Font Control to generate, create Code 128 image in Font applications. www.OnBarcode.comBut, oh how we can have some type-safe fun. We can find the id and name columns matching everyone whose name is David and whose id is 44. The type inferencer does the right thing and knows that the return type is a Tuple2[(Int, String)]. Creating Barcode In None Using Barcode generation for Font Control to generate, create Barcode image in Font applications. www.OnBarcode.comBarcode Creator In None Using Barcode drawer for Font Control to generate, create Barcode image in Font applications. www.OnBarcode.comMyTable.find(MyTable.id ~ MyTable.name, By(MyTable.name, "David"), By(MyTable.id, 44), OrderBy(MyTable.name, Ascending)) match { case (_, name) :: _ => name.length // name is a String case _ => } Draw UCC.EAN - 128 In None Using Barcode generator for Font Control to generate, create UCC - 12 image in Font applications. www.OnBarcode.comCode 3 Of 9 Generator In None Using Barcode maker for Font Control to generate, create Code 3/9 image in Font applications. www.OnBarcode.comAnd we can get all the columns from our table where the id is 33.
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Generate Code-128 In None Using Barcode generation for Word Control to generate, create Code-128 image in Microsoft Word applications. www.OnBarcode.comEncode GS1-128 In Objective-C Using Barcode drawer for iPhone Control to generate, create UCC.EAN - 128 image in iPhone applications. www.OnBarcode.comCHAPTER 7 TRAITS AND TYPES AND GNARLY STUFF FOR ARCHITECTS
Linear Barcode Printer In Visual C#.NET Using Barcode maker for Visual Studio .NET Control to generate, create Linear image in .NET applications. www.OnBarcode.comPaint EAN / UCC - 13 In Java Using Barcode printer for Java Control to generate, create EAN128 image in Java applications. www.OnBarcode.comWe ve got MT2, which looks just like MyTable. Will the compiler let us build a query partially from MyTable and MT2 No. MyTable.find(MyTable.name ~ MyTable.id, OrderBy(MT2.name, Ascending)) Query.scala:256: error: type mismatch; found : MT2.StringColumn required: BasicColumn[MyTable, _] MyTable.find(MyTable.name ~ MyTable.id, OrderBy(MT2.name, Ascending)) ^ Next, let s see what happens if we try to pass an Int where we expect a String.
MyTable.find(MyTable.name ~ MyTable.id, By(MyTable.name, 33)) Query.scala:258: error: no implicit argument matching parameter type (Int) => String was found. MyTable.find(MyTable.name ~ MyTable.id, By(MyTable.name, 33)) ^ In this section, we ve seen how Scala s type system provides us a very powerful mechanism for defining the rules for passing parameters. We were able to go through a lot of work to create a library that was complex underneath but simple to use. Let s go on to see a little more about how types and class hierarchies work in Scala. Variance
Variance is an important and challenging concept. It defines the rules by which parameterized types can be passed as parameters. In the beginning of the chapter, we showed how passing a String[] (Java notation) to a method expecting an Object[] can cause problems. Java allows you to pass an array of something to a method expecting an array of something s superclass. This is called covariance. On the surface, this makes a lot of sense. If you can pass a String to a method expecting an Object, why can t you pass an Array[String] (Scala notation) to a method expecting an Array[Object] Because Array is mutable: it can be written to in addition to being read from, so a method that takes an Array[Object] may modify the Array by inserting something that cannot be inserted into an Array[String]. CHAPTER 7 TRAITS AND TYPES AND GNARLY STUFF FOR ARCHITECTS
Defining the type variance for type parameters allows you to control how parameterized types can be passed to methods. Variance comes in three flavors: invariant, covariant, and contravariant. Type parameters can be individually marked as covariant or contravariant and are by default invariant. Invariant Parameter Types
In Scala, Array[T] is invariant. This means that you can only pass an Array[String] to foo(a: Array[String]) and that you can only pass an Array[Object] to bar(a: Array[Object]). This ensures that what is read from or written to the array is something of the correct type. So, for anything that s mutable, the type parameter should be invariant. You do this by doing nothing with the type parameter. So, let s define an invariant class: class Holder[T](var data: T) The class holds data of type T. Let s write a method: scala> def add(in: Holder[Int]) {in.data = in.data + 1} add: (Holder[Int])Unit
scala> val h = new Holder(0) h: Holder[Int] = Holder@bc0eba
scala> add(h) scala> h.data
res2: Int = 1
Because the add method expects an Int to come out of Holder and puts an Int back into the Holder, the type of the Holder must be invariant. That does not mean that invariant containers lose their ability to hold subclasses of their declared type. A Holder[Number] can
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