CS 3721: Programming Languages Lab
Lab #09 Solutions: Imperative Programming in ML
Due date: April 1, 3:30pm. At the beginning of the next recitation.
Goals of this lab:
ā¢Be able to read/write short imperative programs in ML using ref cells, if, while do.
1. Reference Cells. Areference cell is the assignable region of memory in ML. A reference is itself
a value; if xhas type Ļthen a reference to xis written ref x and has type Ļref. Operations on
reference cells are:
re f v => c rea te s a re f er enc e ce ll co nta in ig va lue v
!r => d ere fe r en cin g : re tur ns t he va lu e c on tai ne d i n reference cell r
r := v = > pl ace s val ue v i n reference cell r
Reference cells correspond to the variables of C, Pascal and similar languages where as the value
declarations that we have seen so far correspond to the constants of languages like C. Here are
some examples:
-val r = re f 5;
va l r = ref 5 : int ref
The identiļ¬er ris bound to a new reference cell with contents 5. The output says that the value
of ris this reference cell having type int ref.
- r := !r + 1;
va l it = () : un it
The value in the reference cell, r, is incremented.
- !r ;
va l it = 6 : i nt
!r returns the contents of r, which is the integer 6.
(a) Create three real reference cells x,yand zwhich have the initial values 3.5, 4.2 and 5.1,
respectively. Then, assign the mathematical expression, x+yāxāy
zto the variable x(in this
case, the reference cell x.). Finally, return the contents of x. (Use let.)
Solution:
le t
val x = re f 3 .5 ;
val y = re f 4 .2 ;
val z = re f 5 .1
in
x := ( ! x + !y * ! x - ! y) / ! z ;
!x
en d ;
=> val it = 2.74509803922 : real
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