Microsoft Word OpenIssuesInOO. doc

4. Abstraction 

mechanisms 

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In the following examples, BETA is used as a reference language. One of the unique 

characteristics of BETA is that abstraction mechanisms such as class, procedure, function, 

control abstractions, process type, exception type, etc, have been unified into one single 

abstraction mechanisms called pattern. By abstraction mechanism we mean any language 

construct that describe the structure of a set of entities and may be used to instantiate such 

entities. A class describe the structure of objects and may be used to instantiate objects; a 

procedure describe the structure of procedure activations and may be used to instantiate 

procedure activations, etc.  The following is a simple example of BETA patterns: 

Account: 

   (# balance: @integer; 

      deposit: 

         (# amount: @integer 

         enter amount 

         do balance + amount -> balance 

         exit balance 

         #); 

       withdraw: (# ... #) 

   #) 

The pattern Account describes objects that have 3 attributes: balance,  deposit and 

withdraw.  Balance is a simple integer variable. Deposit and Withdraw are 

patterns. In this example Account corresponds to a class and is called a class pattern, 

whereas  deposit and withdraw corresponds to procedures and are called procedure 

patterns. The enter-part (enter amount) describes the input parameter, and the exit-part 

(exit balance) describes the output parameter of deposit. If myAccount refers to 

an instance of Account, then the following imperative makes a deposit of 500 and returns 

the new balance in currentBalance 

500 -> myAccount.deposit -> currentBalance