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Mutable data is often hidden behind an encapsulation boundary. We ll look at encapsulation in more detail in 7, but one easy way to do this is to make data private to a function. For example, the following shows how to hide a mutable reference within the inner closure of values referenced by a function value: let generateStamp = let count = ref 0 (fun () -> count := !count + 1; !count)

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Bulk binding allows you to improve performance of inserts, updates, and deletes in a loop With bulk binding, you insert, delete, or update tables using values from an initialized collection (such as a varray, a nested table, or associative arrays) Assuming that l_x_coll is a collection variable, without bulk binding a typical for loop processing that involves an insert may look like for i in 1.xcount loop insert into t1(x) values( l_x_coll(i) ); end loop; Of course, if you can replace the preceding code with a single SQL statement, you should In many cases, though, you may have some complicated processing on the data before inserting it Bulk binding can be applied in such cases to improve performance significantly When using bulk binding, the preceding syntax changes slightly to forall i in 1.x.

Figure 5-6. Configuration inheritance under ASP .NET Notice that this site has a subdirectory named SiteAdmin that contains its own unique configuration file, while the site as a whole is configured using the settings found within the root level web.config file. The idea (as you may suspect) is that access to SiteAdmin is more restrictive than the than of the root directory s content, and is, therefore, constrained using custom configuration settings.

The line let count = ref 0 is executed once, when the generateStamp function is defined. Here is an example of the use of this function: > generateStamp();; val it : int = 1 > generateStamp();; val it : int = 2 This is a powerful technique for hiding and encapsulating mutable state without resorting to writing new type and class definitions. It is good programming practice in polished code to ensure that all related items of mutable state are collected under some named data structure or other entity such as a function.

count insert into t1(x) values( l_x_coll(i) ); Thus, instead of the keyword for, we use the keyword forall, and we lose the keywords loop and end loop The reason forall works faster in most cases is that it avoids switching the context back and forth between PL/SQL and SQL To elaborate on this concept further, during execution of PL/SQL code, the PL/SQL engine hands over execution of an embedded SQL statement to the SQL engine When the SQL statement execution is over, the control passes back to PL/SQL engine, and so on The overhead of this context switch adds up in a loop such as the one without bulk binding shown previously In the first for loop, with each iteration the loop counter i needs to be incremented (using the PL/SQL context), and then the insert statement needs to be executed (using the SQL context).

F# encourages the use of objects whose logical identity (if any) is based purely on the characteristics (for example, fields and properties) of the object. For example, the identity of a pair of integers (1,2) is determined by the two integers themselves; two tuple values both containing these two integers are for practical purposes identical. This is because tuples are immutable and support structural equality, hashing, and comparison, discussed further in 5 and 8. Mutable reference cells are different; they can reveal their identities through aliasing and mutation. However, not all mutable values necessarily reveal their identity through mutation. For example, sometimes mutation is used just to bootstrap a value into its initial configuration, such as when connecting the nodes of a graph. These are relatively benign uses of mutation. Ultimately you can detect whether two mutable values are the same object by using the function System. Object.ReferenceEquals. You can also use this function on immutable values to detect whether two values are represented by the physically same value. However, in this circumstance, the results returned by the function may change according to the optimization settings you apply to your F# code.