.NET/ASP.NET/C#/VB.NET PDF Document SDK

Any discussion of browser-based technologies wouldn t be complete without mentioning security. The XMLHttpRequest object is subjected to the browser s security sandbox. Any resources requested by the XMLHttpRequest object must reside within the same domain from which the calling script originated. This security restriction prevents the XMLHttpRequest object from requesting resources outside the domain from which the script was originally served. The strength of this security restriction varies by browser (see Figure 2-5). Internet Explorer shows an alert stating that a potential security risk exists but gives the user a choice of whether to continue with the request. Firefox simply stops the request and shows an error message in the JavaScript console.

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nonempty !matsuoBasho) do print_string (Listhd !matsuoBasho); matsuoBasho := Listtl !matsuoBasho You enumerate over a list, and the Boolean expression to terminate the loop is based on whether the list is empty Within the body of the loop, you print the head of the list and then remove it, shortening the list on each iteration The results of this example, when compiled and executed, are as follows: An old pond! A frog jumps in- The sound of water.

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You can use loops using for to enumerate collections, performing an imperative action, one that returns unit, on each element. This is similar to the foreach loop available in many programming languages. The syntax for using a comprehension to enumerate a collection is the for keyword followed by the identifier that will be bound to each item in the collection, then the collection, and then the keyword do. The code for processing each item in the collection comes next indented to show it belongs to the for loop. The following example demonstrates this, enumerating an array of strings and printing each one: #light let words = [| "Red"; "Lorry"; "Yellow"; "Lorry" |] for word in words do print_endline word The results are as follows: Red Lorry Yellow Lorry As you ll see later in this chapter, and in many examples throughout the book, this can be a convenient way to work with collections returned by .NET BCL methods.

Firefox does provide some JavaScript tricks to allow XMLHttpRequest to request resources from an outside URL. However, since these techniques are specific to the browser, it s best to not use them and to avoid attempting to access outside URLs using XMLHttpRequest.

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One extremely useful feature of imperative programming in F# is being able to use just about any library written in a .NET programming language, including the many methods and classes available as part of the BCL itself. I consider this to be imperative programming, because libraries written in other languages make no guarantees about how state works inside them, so you can t know whether a method you call has side effects. A distinction should be made between calling libraries written in F# and libraries written in any other language. This is because libraries written in F# have metadata that describes extra details about the library, such as whether a method takes a tuple or whether its parameters can be curried. This metadata is specific to F# and in a binary form understood by the F# compiler. This is largely why the Microsoft.FSharp.Reflection API is provided to bridge the gap between F# and .NET metadata. The basic syntax when calling static or instance properties or methods is the same. Method calls to a non-F# library must have their arguments separated by commas and surrounded by parentheses. (Remember, F# function calls usually use whitespace to separate arguments, and parentheses are needed only to impose precedence.) Method calls to a non-F# library cannot be curried, and the methods themselves are not equivalent to values, so they cannot be passed as arguments. Despite this difference, calling a method from a non-F# library is pretty straightforward. You ll start off by using static properties and methods:

So far, none of the solutions we ve discussed is a standard Though XMLHttpRequest enjoys widespread support, you ve already seen how the process of creating it differs from browser to browser Many people mistakenly believe Ajax has W3C backing; however, it does not The W3C has addressed this and other shortcomings in a new standard with a rather lengthy name: the DOM Level 3 Load and Save Specification This specification is designed to be a platformand language-independent way to modify the content of a DOM document with XML content The 10 version was recommended in April 2004, but currently no browser has implemented it When will Load and Save replace Ajax Your bet is as good as ours.