.NET/Java PDF, Tiff, Barcode SDK Library

Enums allow you to define a type made up of a finite set of identifiers, with each identifier mapping to an integer. This defines a type that can then take the value associated with any one of the defined identifiers. To define an enum, give the names of the identifiers followed by equals signs and then the values of the constants associated with the identifiers. The identifiers that are members of the enum are separated by vertical bars. To demonstrate this, an enum Scale is defined in the following example: #light type Scale = | C = 1 | D = 2 | E = 3 | F = 4 | G = 5 | A = 6 | B = 7 It s quite common to define enums that are intended to be combined logically. To do this, choose constants so that each number is represented by a single bit, that is, the numbers 0, 1, 2, 4, 8, and so on. F# s binary literals are a great help here, since you can easily see how the constants can combine.

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The PCF8591 prototype board kit is shown in Figure A-23. It has many of the same convenient features of the PCF8574 kit. It includes a 2.5V reference for the analog signals, which means that the range of the analog inputs is 0 for 0V and 255 for 2.5V. The analog output is limited to the 2.5V range.

#light [<System.Flags>] type ChordScale = | C = 0b0000000000000001 | D = 0b0000000000000010 | E = 0b0000000000000100 | F = 0b0000000000001000 | G = 0b0000000000010000 | A = 0b0000000000100000 | B = 0b0000000001000000 The module Enum provides functionality for dealing with enums in F#; I discuss it in 7.

Listing 8-8. weatherForecast.jsp <taconite-root> <taconite-replace-children contextNodeID="forecastLocation" parseInBrowser="false"> <![CDATA[ var element0 = document.createDocumentFragment(); element0.appendChild(document.createTextNode ("Weather for NEW ULM, MN")); while (document.getElementById("forecastLocation") .childNodes.length > 0) { document.getElementById("forecastLocation").removeChild (document.getElementById("forecastLocation").childNodes[0]); } document.getElementById("forecastLocation").appendChild(element0); ]]> </taconite-replace-children> <taconite-replace-children contextNodeID="weatherContent" parseInBrowser="false"> <![CDATA[ var element0 = document.createDocumentFragment(); var element1 = document.createElement("table"); element0.appendChild(element1); var element2 = document.createElement("tbody"); element1.appendChild(element2); var element3 = document.createElement("tr"); element2.appendChild(element3); var element4 = document.createElement("td"); element3.appendChild(element4); element4.appendChild(document.createTextNode("Sun. 08/28")); var element5 = document.createElement("br"); element4.appendChild(element5); element4.appendChild(document.createTextNode("High: 78")); element5 = document.createElement("br"); element4.appendChild(element5); element4.appendChild(document.createTextNode("Low: 54")); element5 = document.createElement("br"); element4.appendChild(element5); element5 = document.createElement("img"); element5.setAttribute("src", "http://www.nws.noaa.gov/weather/images/fcicons/sct.jpg"); element5.setAttribute("alt", "forecast image"); element4.appendChild(element5); <!-- Other cells omitted for brevity -->

Figure A-23. PCF8591 prototype kit The connector area for the PCF8591 PCB is shown in Figure A-24. All four analog inputs are available, along with the 2.5V reference and the analog output. In 13, we use this prototype board to build the Light Compass project. In that project, the four CdS LDRs were pulled up to this 2.5V reference with 10K resistors. Just like the PCF8574 PCB, there is also a connector labeled 5V, but it is really the 4.3V supply.

Summary

while (document.getElementById("weatherContent").childNodes .length > 0) { document.getElementById("weatherContent").removeChild (document.getElementById("weatherContent").childNodes[0]); } document.getElementById("weatherContent").appendChild(element0); ]]> </taconite-replace-children> </taconite-root> Note how the XML begins with a taconite-root tag that has two taconite-replace-children tags, which is similar to the structure of the tags in Listing 8-7. Each taconite-replace-children tag has a CDATA section that contains the embedded JavaScript to appropriately update the page s content. If you carefully read the embedded JavaScript in Listing 8-8, you ll easily be able to see exactly what the JavaScript is doing. In fact, it s likely identical to the JavaScript you would write to update the page! Upon receiving this XML response from the server, the Taconite browser-side library automatically extracts and executes the embedded JavaScript using the eval function.

You ve now seen how to use the three major programming paradigms in F# and how flexible F# is for coding in any mix of styles. In the next chapter, you ll look at how code is organized in F# and how to annotate and quote it.

This appendix provides links to go beyond the information presented in the book. The links are categorized by chapter, with an additional General Interest category. Some of the links are to the web sites of manufactures of electronic parts, and others are to companies that sell them. The book web site at http://www.apress.com will maintain an updated list of links.

n important part of any programming language is the ability to organize code into logical chunks. It s also important to be able to annotate code with notes about what it does, for future maintainers and even yourself. It has also become common to use attributes and data structures to annotate assemblies and the types and values within them. Other libraries or the CLR can then interpret these attributes. I cover this technique of marking functions and values with attributes in the section Attributes. The technique of compiling code into data structures is known as quoting, and I cover it in the section Quoted Code toward the end of the chapter.