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crystal reports qr code generator INTRODUCING LANGUAGE-ORIENTED PROGRAMMING in VB.NET
CHAPTER 9 INTRODUCING LANGUAGE-ORIENTED PROGRAMMING Data Matrix ECC200 Printer In VB.NET Using Barcode drawer for .NET Control to generate, create Data Matrix 2d barcode image in .NET applications. www.OnBarcode.comData Matrix ECC200 Reader In Visual Basic .NET Using Barcode recognizer for .NET Control to read, scan read, scan image in Visual Studio .NET applications. www.OnBarcode.comIntroducing Active Patterns
Code 128 Code Set C Creator In Visual Basic .NET Using Barcode encoder for .NET Control to generate, create Code 128C image in .NET applications. www.OnBarcode.comLinear Encoder In VB.NET Using Barcode creation for .NET Control to generate, create Linear Barcode image in VS .NET applications. www.OnBarcode.comPattern matching is one of the key techniques provided in F# for decomposing abstract syntax trees and other abstract representations of languages. So far in this book, all the examples of pattern matching have been directly over the core representations of data structures: for example, directly matching on the structure of lists, options, records, and discriminated unions. But pattern matching in F# is also extensible that is, you can define new ways of matching over existing types. You do this through a mechanism called active patterns. This book covers only the basics of active patterns. However, they can be indispensable, because they can let you continue to use pattern matching with your types even after you hide their representations. Active patterns also let you use pattern matching with .NET object types. The following section covers active patterns and how they work. Draw UCC - 12 In Visual Basic .NET Using Barcode generation for .NET Control to generate, create UCC.EAN - 128 image in VS .NET applications. www.OnBarcode.comMake PDF 417 In VB.NET Using Barcode encoder for .NET Control to generate, create PDF-417 2d barcode image in .NET applications. www.OnBarcode.comConverting the Same Data to Many Views
Draw Barcode In Visual Basic .NET Using Barcode encoder for .NET framework Control to generate, create Barcode image in VS .NET applications. www.OnBarcode.comRM4SCC Printer In Visual Basic .NET Using Barcode maker for VS .NET Control to generate, create Royal Mail Barcode image in Visual Studio .NET applications. www.OnBarcode.comIn high-school math courses, you were probably taught that you can view complex numbers in two ways: as rectangular coordinates x + yi or as polar coordinates of a phase r and magnitude . In most computer systems, complex numbers are stored in the first format, although often the second format is more useful. Wouldn t it be nice if you could look at complex numbers through either lens You could do this by explicitly converting from one form to another when needed, but it would be better to have your programming language look after the transformations needed to do this for you. Active patterns let you do exactly that. First, here is a standard definition of complex numbers: [<Struct>] type Complex(r: float, i: float) = static member Polar(mag,phase) = Complex(mag * cos phase, mag * sin phase) member x.Magnitude = sqrt(r*r + i*i) member x.Phase = atan2 i r member x.RealPart = r member x.ImaginaryPart = i Here is a pattern that lets you view complex numbers as rectangular coordinates: let (|Rect|) (x:Complex) = (x.RealPart, x.ImaginaryPart) And here is an active pattern to help you view complex numbers in polar coordinates: let (|Polar|) (x:Complex) = (x.Magnitude, x.Phase) The key thing to note is that these definitions let you use Rect and Polar as tags in pattern matching. For example, you can now write the following to define addition and multiplication over complex numbers: let addViaRect a b = match a, b with | Rect (ar, ai), Rect (br, bi) -> Complex (ar+br, ai+bi) let mulViaRect a b = match a, b with | Rect (ar, ai), Rect (br, bi) -> Complex (ar*br - ai*bi, ai*br + bi*ar) Generating Data Matrix 2d Barcode In .NET Framework Using Barcode creator for Reporting Service Control to generate, create DataMatrix image in Reporting Service applications. www.OnBarcode.comMaking ECC200 In None Using Barcode drawer for Font Control to generate, create Data Matrix image in Font applications. www.OnBarcode.comCHAPTER 9 INTRODUCING LANGUAGE-ORIENTED PROGRAMMING
Encoding UCC - 12 In None Using Barcode generation for Software Control to generate, create UPC-A Supplement 5 image in Software applications. www.OnBarcode.comEncode GS1 DataBar In Visual Studio .NET Using Barcode creator for .NET framework Control to generate, create GS1 DataBar-14 image in .NET framework applications. www.OnBarcode.comAs it happens, multiplication on complex numbers is easier to express using polar coordinates, implemented as follows: let mulViaPolar a b = match a, b with | Polar (m, p), Polar (n, q) -> Complex.Polar (m*n, p+q) Here is an example of using the (|Rect|) and (|Polar|) active patterns directly on some complex numbers via the pattern tags Rect and Polar. You first make the complex number 3+4i: > let c = Complex (3.0, 4.0);; val c : complex > c;; val it : complex = 3.0r+4.0i > match c with | Rect (x, y) -> printfn "x = %g, y = %g" x y;; x = 3, y = 4 val it : unit = () match c with | Polar (x, y) -> printfn "x = %g, y = %g" x y;; x = 5.0, y = 0.927295 val it : unit = () > addViaRect c c;; val it : complex = 6.0r+8.0i > mulViaRect c c;; val it : complex = -7.0r+24.0i > mulViaPolar c c;; val it : complex = -7.0r+24.0i As you may expect, you get the same results if you multiply via rectangular or polar coordinates. However, the execution paths are quite different. Let s look closely at the definition of mulViaRect. The important lines are in bold here: let mulViaRect a b = match a, b with | Rect (ar, ai), Rect (br, bi) -> Complex (ar*br - ai*bi, ai*br + bi*ar) When F# needs to match the values a and b against the patterns Rect (ar, ai) and Rect (br, bi), it doesn t look at the contents of a and b directly. Instead, it runs a function as part of pattern matching (which is why they re called active patterns). In this case, the function executed is (|Rect|), which produces a pair as its result. The elements of the pair are then bound to the variables ar and ai. Likewise, in the definition of mulViaPolar, the matching is performed partly by running the function (|Polar|). The functions (|Rect|) and (|Polar|) are allowed to do anything, as long as each ultimately produces a pair of results. Here are the types of (|Rect|) and (|Polar|): > Decode GS1 - 13 In Java Using Barcode recognizer for Java Control to read, scan read, scan image in Java applications. www.OnBarcode.comRecognize Data Matrix 2d Barcode In C#.NET Using Barcode scanner for VS .NET Control to read, scan read, scan image in Visual Studio .NET applications. www.OnBarcode.comBarcode Maker In None Using Barcode printer for Excel Control to generate, create Barcode image in Excel applications. www.OnBarcode.comBarcode Printer In .NET Using Barcode generator for ASP.NET Control to generate, create Barcode image in ASP.NET applications. www.OnBarcode.comGenerate Barcode In Java Using Barcode encoder for BIRT reports Control to generate, create Barcode image in BIRT applications. www.OnBarcode.comGenerating Barcode In None Using Barcode maker for Software Control to generate, create Barcode image in Software applications. www.OnBarcode.comBarcode Scanner In None Using Barcode reader for Software Control to read, scan read, scan image in Software applications. www.OnBarcode.comCode 3/9 Creation In Objective-C Using Barcode generator for iPad Control to generate, create Code 3 of 9 image in iPad applications. www.OnBarcode.com |
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