CIS 352 Quiz 3a Practice Questions
Question 1. Answer 3 out of the following 6 questions.
(a) Briefly explain what a thunk is and how thunks are
used in implementing programming languages.
(b) Give a brief definition and an example of variable
aliasing.
(c) Briefly explain the difference between how let and
letrec are implemented. Hint: What goes wrong if
we try to evaluate the following?
let fact = proc(n) if zero?(n) then 1
else *(n,(fact (sub1(n))))
in (fact 6)
(d) Explain the difference between expressed values
and denoted values and sketch a situation where
they differ.
(e) Briefly explain the difference between abstract syn-
tax and concrete syntax and the rˆ
ole abstract syn-
tax in programming language implementation.
(f) In the continuation-passing interpreter, eval-expres-
sion takes three arguments: an expression, an en-
vironment, and a continuation. Briefly explain the
purpose of the environment and the continutation.
Question 2. This question assumes that call-by-value parameter passing is used.
(a) (8 points) Consider the following code.
letx=2 y=4 z=6
in let f = proc(x) +(x,y)
g = proc(t,y) *(t,y)
y=8
in
(f (g z 10))
(i) What is the value of this expression under lexical scoping?
(ii) What is the value of this expression under dynamic scoping?
(b) (8 points) Consider the following code.
letm=0 q=1 r=10
in let m = 4
p = proc(x,t) begin
set r = +(m,200);
*(r,t)
end
in let q = 5
k = proc(a,m) begin
set m = 2;
(p add1(m) *(a,q))
end
in
(k m r)
(i) What is the value of this expression under lexical scoping?
(ii) What is the value of this expression under dynamic scoping?
Page 1 of 3 29 April 2008
