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Another common mistake is to write your own code for what one of the supplied packages or an existing SQL function already does for you. It is well worth the effort to become familiar with the PL/SQL-supplied packages that Oracle provides. Otherwise, you ll end up writing, debugging, and testing code that is already provided by Oracle for you.

Record types can also support members (for example, properties and methods) and give implicit implementations of interfaces, discussed in 6. Practically speaking, this means you can use them as one way to implement object-oriented abstractions.

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For the remainder of this chapter, we ll look at some useful techniques to improve the performance and maintainability of your PL/SQL code.

Once you ve determined the hash code algorithm you wish to use, you can create an instance of the algorithm using the static HashAlgorithm.Create method. Simply pass in a string name of the algorithm you require (MD5, SHA1, SHA256, SHA384, or SHA512). Assume you wish to generate a hash code for a file on your local machine: static void Main(string[] args) { // Open a local file on the C drive. FileStream fs = new FileStream(@"C:\MyData.txt", FileMode.Open); // Now generate a hash code for this file using MD5. HashAlgorithm alg = HashAlgorithm.Create("MD5"); byte[] fileHashValue = alg.ComputeHash(fs);

Packaging Matters!

One particularly useful mutable record is the general-purpose type of mutable reference cells, or ref cells for short. These often play much the same role as pointers in other imperative programming languages. You can see how to use mutable reference cells in the following example: > let cell1 = ref 1;; val cell1 : int ref > cell1;; val it : int ref = { contents = 1 } > !cell1;; val it : int = 1 > cell1 := 3;; val it : unit = () > cell1;; val it : int ref = { contents = 3 } > !cell1;; val it : int = 3 The key type is 'a ref, and its main operators are ref, !, and :=. The types of these operators are as follows: val ref : 'a -> 'a ref val (:=) : 'a ref -> 'a -> unit val (!) : 'a ref -> 'a These allocate a reference cell, read the cell, and mutate the cell, respectively. The operation cell1 := 3 is the key one; after this operation, the value returned by evaluating the expression !cell1 is changed. You can also use either the contents field or the Value property to access the value of a reference cell. Both the 'a ref type and its operations are defined in the F# library as a simple record data structure with a single mutable field: type 'a ref = { mutable contents: 'a } let (!) r = r.contents let (:=) r v = r.contents <- v let ref v = { contents = v }

You can write your PL/SQL methods as stand-alone functions and procedures, or you can make them part of modular PL/SQL packages. There are several advantages to using PL/SQL packages: Packages break the dependency chain. This is perhaps the biggest advantage of using packages. In fact, we will go through this benefit in detail in the next subsection. Packages expand the namespace for your procedures and functions. If you use packages, a method signature has to be unique only within the package. Thus, you have to worry much less about signature clashes with other methods in your application. Packages allow you to hide information and implementation details. You can declare private methods and variables in a package body. This hides the implementation details from the users of the package and also improves the maintainability of the code tremendously. This is similar to how you declare private variables and define private methods in your Java classes, and declare public interfaces in your Java interfaces. Packages improve the readability and manageability of your code. Imagine what would happen if all the methods in Oracle s supplied PL/SQL packages were stand-alone. There won t be any decent way to go about looking for a method you need. Fortunately, Oracle supplies its common utility methods in various packages, each catering to a distinct functionality requirement. In this respect, organizing methods in packages is very similar to organizing your files in different directories.

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