CS321 Prog Lang & Compilers                       Assignment #10
Assigned: Feb. 28, 2007            Due: Wednesday. March 7, 2007

1) The goal of this assignment is to extend the type checker
discussed in lecture 13 to handle refereces and assinments.
To do this you will need to

A) Add a new constructed type to MLtype to represent reference types.
Do this by adding a new type constructor to MLtype called "Ref".

B) Add two new type constructors to "Exp" called "Init" and
"Assign". They should coorespond to the underlined ML
expressions in the examples below:

val x = ref 5
       -------

val y = ( x := 6  ; print x)
         --------

C) Extend the function "typeeq" to handle the new "Ref" type.

D) Extend the function "TCExp" to handle the two new kinds of
expressions. The cases should coorespond to the type rules
below.

S |- x : t  ref     S |- y : t
---------------------------------
  S |-  x := y  : unit


  S |- x : t
------------------------
  S |-  ref x  : t ref


You may copy and paste the code from the lecture to get started.
Test your code, and highlight all the additions to the lecture
code you made in what you hand in.