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CIS425 - Midterm Exam
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1
(10 points) Consider the grammar G = (T,NT,R, P ) where T = {e, f, g}, NT = {S, T, U}, R = S, and P is given
as follows:
S ::= e S|T
T ::= f T |U
U ::= g U |
1. What is L(G)?
2. Is the grammar regular, context free or context sensitive? Briefly explain your answer
3. Give a parse tree of a string belonging to the language
2
(10 points) Consider the following grammar G:
E ::= E + E | E ∗ E | (E) | a | b
1. Show that the grammar is ambiguous
2. Rewrite the grammar into a non ambiguous grammar reflecting the fact that + and ∗ are right
associative and ∗ has lower precedence than +
3. Briefly explain with an example the difference between a parse tree and an abstract syntax tree
3
(4 points) Draw the abstract syntax tree of the lambda expression λx.(y z w)
(6 points) Lambda-calculus expressions are represented by the following datatype
datatype M = Var of string | App of M*M | lam of string*M
Represent the lambda-expression λx.(y z w) as a value of type M .
4
(9 points) Give all the free and bound variables in the following lambda-expressions. If a variable is bound draw
a line between the variable and its definition.
1. λx.x z λy.x y
2. (λx.x z) λy.w λw.w y z x
3. λx.x y λx.y x
(6 points) Define the function FV which returns the set of free variables occurring in a lambda-expression. You
do not need to use ML code, define it as we did in class.
FV (x) =
FV (λx.M) =
FV (M N) =
(5 points) Explain using an example what is the variable capture problem
5
(6 points) Apply β-reduction to the following expressions as much as possible :
1. (λy.y)(λx.λz.z)w
2. (λx.x λy.y x) y
3. λx.(λy.y y) w z
(2 points) Can I α-convert the expression (λx.λy.x y) y to (λx.λy.x y) z? Explain briefly why or why not.
(2 points) In the expression (λx.λy.x y) y, can I rename variable x to y ? Explain briefly why or why not.
6
(10 points) Consider the following simple language E of arithmetic:
E ::= num | E + E | E * E
where num is any integer. We represent strings of E using a corresponding datatype:
datatype E = NUM of int | PLUS of E * E | TIMES of E * E
1. Write (5 ∗ 3) + 9 as a value of type E.
2. Write a function interp that accepts as input a program of type E, interprets it, and returns the
integer result. For example:
- interp (PLUS (NUM 1, NUM 2));
val it = 3 : int
fun interp
| interp
| interp
7
(10 points) Write a function foldl (also called reduce) with type:
foldl: (’a * ’b -> ’b) -> ’b -> ’a list -> ’b
The type (’a * ’b -> ’a) is the type of the input function (say f), ’b is the type of an initial value,
and ’a list is the type of the input list.
foldl(f,init,[x1,...xn])
returns
f(xn,...,f(x2, f(x1,init))...)
For example, we have:
- fun plus (x,y) = x + y + 0 ;
- foldl (plus, 0, [1,2,3,4]) ;
val it = 10 : int
fun foldl (f: ’a*’b->’b) (init: ’b) (l: ’a list): ’b =
case l of
8
(5 points) Briefly explain what the following function is doing:
fun foo l1 x = foldl (fn (hd, acc) => if hd = x then acc else hd::acc) [] l1
foo : ’a list -> ’a -> ’a list
For example, what is the result of foo ["a","b","c"] "b"?
(5 points) Briefly explain what the following function is doing:
fun foo l1 l2 = foldl (fn (hd, acc) => if (exists l2 hd) then acc else hd::acc) [] l1
foo: ’a list -> ’a list -> ’a list
assuming that function exists is defined as
fun exists [] n = false
| exists (x :: xs) n = if n = x then true else exists xs n
For example, what is the result of foo ["a","b","c"] ["b","c"]?

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