c# barcode generator library open source Type-Safe Fun in Font

Painting DataMatrix in Font Type-Safe Fun

Type-Safe Fun
ECC200 Creator In None
Using Barcode creator for Font Control to generate, create DataMatrix image in Font applications.
www.OnBarcode.com
Draw Code 128A In None
Using Barcode creator for Font Control to generate, create Code 128 image in Font applications.
www.OnBarcode.com
But, 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.com
Barcode Creator In None
Using Barcode drawer for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
MyTable.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.com
Code 3 Of 9 Generator In None
Using Barcode maker for Font Control to generate, create Code 3/9 image in Font applications.
www.OnBarcode.com
And we can get all the columns from our table where the id is 33.
EAN 13 Generation In None
Using Barcode maker for Font Control to generate, create GTIN - 13 image in Font applications.
www.OnBarcode.com
Code11 Maker In None
Using Barcode printer for Font Control to generate, create Code11 image in Font applications.
www.OnBarcode.com
MyTable.find(MyTable.columns, By(MyTable.id, 33)) match { case (_, name, date) :: _ => name.length; date.getTime case _ => }
Data Matrix 2d Barcode Generation In Objective-C
Using Barcode creator for iPhone Control to generate, create Data Matrix ECC200 image in iPhone applications.
www.OnBarcode.com
Recognize Data Matrix 2d Barcode In None
Using Barcode scanner for Software Control to read, scan read, scan image in Software applications.
www.OnBarcode.com
Pretty neat, huh We ve got the type safety of Scala with very readable library consumer code. But how type-safe is it Let s define a second table and see what happens when we try to mix things up.
QR Code 2d Barcode Generation In VS .NET
Using Barcode generator for Visual Studio .NET Control to generate, create Quick Response Code image in Visual Studio .NET applications.
www.OnBarcode.com
Create European Article Number 13 In .NET
Using Barcode creation for .NET framework Control to generate, create European Article Number 13 image in VS .NET applications.
www.OnBarcode.com
class val val val val MyTable2 extends Table[MyTable2] with ConnectionSupplier { table = "mytable" id = IntColumn("id") name = StringColumn("name") birthday = DateColumn("birthday")
Printing Code 39 In Java
Using Barcode creation for Java Control to generate, create USS Code 39 image in Java applications.
www.OnBarcode.com
Drawing DataMatrix In None
Using Barcode encoder for Microsoft Word Control to generate, create ECC200 image in Office Word applications.
www.OnBarcode.com
type ColumnTypes = (Int, String, Date) def columns = id ~ name ~ birthday } object MT2 extends MyTable2
Print Code39 In Visual Basic .NET
Using Barcode creation for .NET framework Control to generate, create Code 39 Full ASCII image in .NET applications.
www.OnBarcode.com
Barcode Encoder In Java
Using Barcode generation for BIRT Control to generate, create Barcode image in Eclipse BIRT applications.
www.OnBarcode.com
10. See http://www.nabble.com/Re:--scala--Cyclic-reference--p22024739.html.
Generate Code-128 In None
Using Barcode generation for Word Control to generate, create Code-128 image in Microsoft Word applications.
www.OnBarcode.com
Encode GS1-128 In Objective-C
Using Barcode drawer for iPhone Control to generate, create UCC.EAN - 128 image in iPhone applications.
www.OnBarcode.com
CHAPTER 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.com
Paint EAN / UCC - 13 In Java
Using Barcode printer for Java Control to generate, create EAN128 image in Java applications.
www.OnBarcode.com
We 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
Copyright © OnBarcode.com . All rights reserved.