.net data matrix barcode generator Getting Started with Pattern Matching in VB.NET

Creating Data Matrix ECC200 in VB.NET Getting Started with Pattern Matching

Getting Started with Pattern Matching
Data Matrix Creation In Visual Basic .NET
Using Barcode creator for Visual Studio .NET Control to generate, create DataMatrix image in Visual Studio .NET applications.
www.OnBarcode.com
Recognize DataMatrix In VB.NET
Using Barcode recognizer for .NET Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
One important tool in F# programming is pattern matching, a general construct that combines decomposition and control. In the previous sections, you got a taste of how you can use pattern matching with tuple, list, and option values. However, you can also use pattern matching in many other situations. You see many other examples of pattern matching in this book, but let s start with some simple pattern matching over strings and integers. As you ve already seen, pattern matches on explicit values are introduced using the match ... with ... construct: let urlFilter url agent = match (url,agent) with | "http://www.control.org", 99 -> true | "http://www.kaos.org" , _ -> false | _, 86 -> true | _ -> false The inferred type of the function is as follows: val urlFilter : string -> int -> bool The expression (url,agent) after the keyword match is a tuple of type (string*int). Each rule of the match is introduced with a | followed by a pattern, then ->, and then a result expression. When executed, the patterns of the rules are used one by one, and the first successful pattern match determines which result expression is used. In the previous example, the first pattern matches if url and agent are "http://www.control.org" and 99, respectively. The next matches if url is "http://www.kaos.org". The third pattern matches if agent is 86. The last three rules all use wildcard patterns represented by the underscore character; these match all inputs.
Generate Data Matrix ECC200 In VB.NET
Using Barcode encoder for .NET Control to generate, create ECC200 image in .NET framework applications.
www.OnBarcode.com
Generating Code 3/9 In VB.NET
Using Barcode printer for VS .NET Control to generate, create Code 3/9 image in Visual Studio .NET applications.
www.OnBarcode.com
CHAPTER 3 CREATING YOUR FIRST F# PROGRAM
Draw Barcode In VB.NET
Using Barcode creator for Visual Studio .NET Control to generate, create Barcode image in .NET applications.
www.OnBarcode.com
PDF-417 2d Barcode Encoder In Visual Basic .NET
Using Barcode generator for VS .NET Control to generate, create PDF417 image in .NET applications.
www.OnBarcode.com
The overall conditions under which urlFilter returns true can be determined by reading through the pattern match: agent 99 can access "http://www.control.org", no one can access "http://www.kaos.org", and, excluding the latter, agent 86 can access anything. Patterns are a rich and powerful technique for simultaneous data analysis and decomposition. Table 3-12 summarizes all the ways to form patterns in F#; many of these involve building up patterns from other patterns. The sections that follow look at some of these constructs, and 9 covers active patterns. Table 3-12. Different Ways to Form Patterns
Printing 1D In VB.NET
Using Barcode drawer for Visual Studio .NET Control to generate, create Linear image in VS .NET applications.
www.OnBarcode.com
Identcode Generation In Visual Basic .NET
Using Barcode creator for .NET Control to generate, create Identcode image in .NET framework applications.
www.OnBarcode.com
General Form
ECC200 Generator In None
Using Barcode encoder for Microsoft Word Control to generate, create DataMatrix image in Word applications.
www.OnBarcode.com
Data Matrix 2d Barcode Encoder In .NET
Using Barcode drawer for ASP.NET Control to generate, create Data Matrix ECC200 image in ASP.NET applications.
www.OnBarcode.com
(pat, ... ,pat) [pat; ... ;pat] [| pat; ...; pat |] { id=pat; ...; id=pat } Tag(pat, ... ,pat) pat | pat pat & pat pat as id id _ Any literal : type null
Creating Barcode In None
Using Barcode encoder for Software Control to generate, create Barcode image in Software applications.
www.OnBarcode.com
Recognizing Data Matrix In .NET Framework
Using Barcode reader for Visual Studio .NET Control to read, scan read, scan image in Visual Studio .NET applications.
www.OnBarcode.com
Kind
EAN 13 Drawer In Java
Using Barcode generator for Java Control to generate, create EAN13 image in Java applications.
www.OnBarcode.com
Code 3/9 Decoder In .NET
Using Barcode scanner for .NET framework Control to read, scan read, scan image in VS .NET applications.
www.OnBarcode.com
Tuple pattern List pattern Array pattern Record pattern Tagged union or active pattern Or pattern And pattern Named pattern Variable pattern Wildcard pattern Constant pattern Type test pattern Null test pattern
Code 128A Creator In Java
Using Barcode creation for BIRT reports Control to generate, create USS Code 128 image in Eclipse BIRT applications.
www.OnBarcode.com
EAN / UCC - 13 Drawer In .NET Framework
Using Barcode encoder for ASP.NET Control to generate, create EAN128 image in ASP.NET applications.
www.OnBarcode.com
Example
GS1 128 Maker In None
Using Barcode drawer for Excel Control to generate, create EAN / UCC - 14 image in Excel applications.
www.OnBarcode.com
Creating Barcode In None
Using Barcode generator for Office Word Control to generate, create Barcode image in Word applications.
www.OnBarcode.com
(1,2,("3",x)) [x;y;z] [| "cmd"; arg1; arg2 |] { X=1; Y=2 } Point(x,y) [x] | ["X";x] [p] & [Point(x,y)] [x] as inp x _ 36, "36", 27L,System.DayOfWeek.Monday : string null
EAN-13 Supplement 5 Creator In None
Using Barcode generator for Font Control to generate, create EAN-13 Supplement 5 image in Font applications.
www.OnBarcode.com
Create PDF-417 2d Barcode In Java
Using Barcode generator for Android Control to generate, create PDF-417 2d barcode image in Android applications.
www.OnBarcode.com
Matching on Structured Values
Pattern matching can be used to decompose structured values. Here is an example where you match nested tuple values: > let highLow a b = match (a,b) with | ("lo", lo), ("hi", hi) -> (lo,hi) | ("hi", hi), ("lo", lo) -> (lo,hi) | _ -> failwith "expected a both a high and low value";;
CHAPTER 3 CREATING YOUR FIRST F# PROGRAM
The match examines two pairs and looks at the strings in the first element of each, returning the associated values: > highLow ("hi",300) ("lo",100);; val it : int * int = (100,300) The first rule matches if the first parts of the input pairs are the strings "lo" and "hi", respectively. It then returns a pair made from the respective second parts of the tuples. The second rule is the mirror of this in case the values appeared in reverse order. The final cases of both of the previous examples use wildcard patterns to cover remaining cases. This makes the patterns exhaustive. Frequently, no wildcard is needed to ensure this, because for many input types F# is able to determine whether the given set of rules is sufficient to cover all possibilities for the given shape of data. However, if a match isn t exhaustive, a warning is given: > let urlFilter3 url agent = match url,agent with | "http://www.control.org", 86 -> true | "http://www.kaos.org", _ -> false;; match url,agent with ^^^^^^^^^^^^^^^^^^^^ warning: Incomplete pattern matches on this expression. ... In these cases, it may be necessary to add an extra exception-throwing clause to indicate to the F# compiler that the given inputs aren t expected: let urlFilter4 url agent = match url,agent with | "http://www.control.org", 86 -> true | "http://www.kaos.org", _ -> false | _ -> failwith "unexpected input" Nonexhaustive matches are automatically augmented by a default case where a MatchFailureException is thrown. 4 discusses exceptions. F# is frequently able to determine whether pattern-matching rules are redundant, such as if a rule can never be selected because previous rules subsume all such cases. In this case, a warning is given. For example: > let urlFilter2 url agent = match url,agent with | "http://www.control.org", _ -> true | "http://www.control.org", 86 -> true | _ -> false;; | "http://www.control.org", 86 -> true ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ warning: This rule will never be matched.
Copyright © OnBarcode.com . All rights reserved.