.NET/Java PDF, Tiff, Barcode SDK Library

If, as is commonly the case, you just want to raise an exception with a text description of what went wrong, you can use failwith to raise a generic exception that contains the text passed to the function let primes = [ 2; 3; 5; 7; 11; 13; 17; 19; 23; 29; 31; 37; 41; 43; 47; 53; 59 ] let testSecond() = try let currentSecond = SystemDateTimeNowSecond in if Listexists (fun x -> x = currentSecond) primes then failwith "A prime second" else raise (WrongSecond currentSecond) with WrongSecond x -> printf "The current was %i, which is not prime" x testSecond() As shown in testSecond, the try and with keywords handle exceptions The expressions that are subject to error handling go between the try and with keywords, and one or more pattern matching rules must follow the with keyword.

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The last two items in Figure 7-4 are informational warnings. These are instances in which something is technically not correct but from which Firefox can still recover. The first informational message describes a style sheet whose MIME type was sent as text/plain instead of text/ css as it should be. You can correct this error by ensuring that the server supplying the page

Figure 13-16. Keypad schematic The eight connections to the keypad matrix are arranged as pins in a row that can be soldered directly into the holes of the Mindsensors prototype board I/O connector. The connector is labeled D0 to D7, but they are really just pins P0 to P7 of the PCF8574. Make sure that you use the middle eight holes in lower row, as shown in Figure 13-17. You don t need the outer holes that are for VCC and ground. It is a good idea to physically space the keypad as high above the top of the circuit board as possible and still have the pins reach all the way through to the bottom of the board.

When trying to match an F# exception, the syntax follows that of trying to match an F# constructor from a union type The first half of the rule consists of the exception name, followed by identifiers or wildcards to match values that the exception contains The second half of the rule is an expression that states how the exception should be handled One major difference between this and the regular pattern matching constructs is that no warning or error is issued if pattern matching is incomplete This is because any exceptions that are unhandled will propagate until they reach the top level and.

correctly sets the MIME type for CSS files to text/plain. The message shown in Figure 7-4 hints to the developer that the W3C standard of referencing an object should be used instead of a proprietary method.

stop execution. The example handles exception wrongSecond, while leaving the exception raised by failwith to propagate. F# also supports a finally keyword, which is used with the try keyword. You can use the finally keyword in conjunction with the with keyword as well as some rules for error handling or no rules. Always place the finally expression after the with block, if it exists; the finally expression will be executed whether an exception is thrown. The next example shows a finally block being used to ensure a file is closed and disposed of after it is written to: #light let writeToFile() = let file = System.IO.File.CreateText("test.txt") in try file.WriteLine("Hello F# users") finally file.Dispose() writeToFile()

Figure 13-17. Soldered connections on bottom of circuit board A complete listing of an NXC keypad data entry program can be found in Appendix C. The program builds up a number one digit at a time; when you are done, it saves the number in a file with the name A, B, C, or D.txt, depending on which letter key you press. Pressing the asterisk (*) button clears the number, and pressing the pound sign (#) button changes the sign of the number. A program that actually uses the number simply opens the appropriate file and reads the value out of it. The keypad can be temporarily connected or more permanently attached to the side with beams, as shown in Figure 13-18.