Comp 372 Study Questions  Dordal  Dec 13, 1999

I apologize for being so late to my office hours Tuesday;
I got stuck in traffic. If it will help, I'll be in my office
by 7:15 am on Wednesday, the day of the exam, for last-minute
questions etc.

The following material may be covered on the exam:
Polymorphism
functional v oo styles
ML types and functions
Lisp functions (basics of ch5, ch6, 8.2, 10.2, 10.3)
Polymorphism in Java
CLOS (11.5, 11.6)
double-dispatch (ie CLOS generic functions with two parameters)
game program

The exam will *not* be open book. However, I would not
expect you to be able to answer all the *study guide* questions 
without consulting a reference; if these were used on the
exam then some supplemental notes or hints would be supplied on 
the exam.


1. Consider the following DO loop in lisp:
	(defun badfact (n)
	    (do
	        ((i 1 (1+ i)) 
	         (prod 1 (* i prod)))
		  ((equal i n) prod)))		;; exit condition

It doesn't quite work.

(a). Explain what (badfact 4) returns.


(b). Explain why DO* would fix this. (You may need to look up DO* 
in the textbook).


(c). Fix it while leaving it as a DO (not DO*) loop.

NOTE: recall that DO evaluates its variable initializations
and updates in *parallel*; DO* evaluates them strictly sequentially.

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2. Suppose class Deriv is derived from class Base in C++.
(a). Suppose we have the following declarations:
	Base b;
	Deriv d;
What is the problem with the assignment
	b=d;

(b). Suppose we have the declarations
	Base ** pp; 
	Deriv ** qq;
Explain the hole in the type system that would be introduced
if we allowed the assignment pp = qq;

HINT: Suppose we have 
	Base * p;
	Deriv * q;

	p = & b;
	q = & d;
	pp = & p;
	qq = & q;
// the four preceding assignments are all legit 
	And then:
	pp = qq;	// the dangerous one
	*pp = p;	// fundamentally legit

To what does p now point? To what does q now point?

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3. What are the ML types of the following functions?

(a)  fun rev(nil, L) = L | rev(h::t, L) = rev(t, h::L);

(b)  fun foo(nil,    nil)    = nil
      |  foo(h1::t1, h2::t2) = h1(h2) :: foo(t1, t2);  

(c)  fun c x = fn y => y*x;

(d) The function c in (c) above is Curried. 
Give the type of c(x), and also give an uncurried 
equivalent uc(x,y).

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4. In class I gave the following CLOS example. I assumed that 
classes Room and Word were defined, and that damControlRoom
was a particular Room.

(defmethod response((r Room) (v Verb)) ...)	; definition 1

Then, to define response in damControlRoom, I defined

(defmethod response ((r (eql damControlRoom)) (v (eql TURN))) ...)
;; definition 2: interprets TURN VALVE in damControlRoom

Finally I defined
(defmethod response ((r room) (v (eql TURN))) ...) 
; definition 3: default interpretation of TURN outside the damcontrolroom.

(a). What function is applied if r is damControlRoom but v isn't TURN?

(b). As things stand now, what function is applied if r isn't damControlRoom but v *is* TURN?

(c). Suppose I define swampRoom, and
(defmethod response ((r (eql swampRoom)) (v Verb))...) 
Suppose swampRoom has no special interpretation for TURN.
What would happen if I tried to TURN something in swampRoom?

(d). Explain why the problem of (c) might not be expected
to arise in practice.


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5. Suppose I write a function my_if(a, b, c); a is boolean,
and if it evaluates to true then we return b otherwise c.
However, I'm not too clever, and so my_if always evaluates
all three parameters in full before proceding (that is, 
a, b, and c are ordinary value parameters). 

(a). What can go wrong with 
	my_if (x==0, 0, 1/x);


(b). what can go wrong with
	int fact(int n) {
	    return my_if(n==1, 1, n*fact(n-1));
	}

You should also know about value-result and reference parameters.

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6. Lisp lets you attach essentially *anything* to the property
list of an atom, with
	(get x 'key)
	(setf (get x 'key) 'value)
Attempting to retrieve the value for an undefined key 
simply results in nil.

Write a function (calldynamic x y keyname default) that invokes 
the attached function (on the property list of x with key "keyname") 
if present, with parameter y, and otherwise calls (default_interpret y)

Hint:
	(funcall FN ARG)  calls function FN with argument ARG

==============================================================


Other topics:

Lisp macros

CLOS routine examples

Polymorphism: ML v Lisp v C++ templates

Polymorphism in Java

Parameters