Take-home Final Exam (Fall 2005)

Denotational Semantics

1. [15%] Let's start with the denotational definition given in your class notes called "Example: language with combined expressions and commands".

   I wish to add to this language 3 new features:

   (1) normal For loop
       Syntax: for I = 1 to N do E
       Informal semantics: well known, I have no surprises in mind.
         If N is < 1, then go through the loop zero times. 
         If N is = 1, then go through the loop one time.
   
   (2) funky loop
       Syntax: loop E1, B, E2 end
       Informal semantics: B is an expression that is meant to evaluate to true or false; 
         the loop body is entered, and E1 is evaluated; then B is evaluated, and if
         true E2 is also evaluated and the loop goes back for another iteration; 
         however, if B evaluates to false, then looping ends and E2 is not evaluated
         on that last time thorugh.  This loop can implement both traditional while loops
         (E1 a null experssion), repeat loops (E2 a null expression), and hybrids.
   
   (3) 2-way multi-assignment
       Syntax: L1, L2 := E1, E2 
       Semantics: right-side expressions are all evaluated first,
       then assignments to left-side L-values are done in the 
       order written.  
   
       This is allowed: x, x := 2, 7 and would result in x having 
       the value 7 when done.  
   

   Modify the denotational definition by adding formal semantics for these three language features. You will need to add the syntactic BNF, the semantic functions, and the semantic equations that work off the BNF you create.

2. [5%] Continuing in the spirit of the previous question, consider this n-way multi-assignment

   
       Syntax: L1, L2, ... Ln := E1, E2, ... En
       Semantics: right-side expressions are all evaluated first,
       then assignments to left-side L-values are done in some order;
       shown here in left to right order.
   

   Show an extenstion to the denotational definition from the previous question that defines the meaning of this n-way multiassignment. You can pick how you would want the order of assignment to go... you might, for example, find it easier to write a definition if assignment were "inside out", like this

       Syntax: L1, L2, ... Ln := En ...,  E2, E1
   

   It's your choice... pick your semantics, and formally define them. Make sure to tell me informally what you are trying to define.

   Design Patterns

3. [80%] Suppose you have been tasked with developing a system for inventory control in a mail-order catalog retailing company. Your system must keep the catalog on-line (Web mania, you know, eCommerce and all that), keep track of how many of each item you have in stock, order more items from suppliers when low, take customer orders, maintain a shopping cart, make shipping orders, and arrange billing and collections. It should also keep statistics on what parts of the catalog are being visited by customers.

   In designing this system you, of course, want to use the best software engineering techniques to ensure that the final product is nicely extensible and easily maintainable. Your client is a growing company, so while they sell mostly books on-line right now, they expect to expand their product lines into CDs, videos, electronics, software... whatever the market seems to support. Your system must be designed to allow this expansion without business interruption.

   Sketch out a basic design for this system. Feel free to add your own details and enhancements to the (admittedly) very loose requirements given above.

   In your design description explain what object-oriented design patterns you have used, and why. To give a complete answer, you should not simply name the patterns, but rather for each pattern you choose to use, explain briefly what its structure is and give a rationale for using it in this design. Explain how your use of particular patterns will help reach your goals of an extensible and maintainable system.