CSC 533: Programming Languages
Spring 2023

HW4: Implementing Subroutines

This assignment is optional. If you choose not to submit anything, it will not affect your grade.
If you do submit, it will only count if it improves your homework average.

The SILLY interpreter you wrote for HW3 extends the original version to allow for local variables and nested scopes. For this final SILLY assignment, you are to extend the language further by implementing simple subroutines (without return values).

In order to implement simple subroutines, you will need to define two new classes, a subDecl class for declaring a subroutine and a subCall class for calling a subroutine. The grammar rules for these new statements are:

<subdecl> --> 'sub' <id> '(' [ <paramseq> ] ')' <compound> <exprseq> --> <expr> { ',' <expr> } <subcall> --> 'call' <id> '(' [ <exprseq> ] ')' <paramseq> --> <type> <id> { ',' <type> <id> }

A subroutine declaration specifies the name of the subroutine, an arbitrary number of parameters in parentheses (separated by commas), and a compound statement specifying the body of the subroutine. Executing a subroutine declaration when there already exists a variable or subroutine with that same name should result in a run-time error. Note: when a subroutine is declared, it is assumed to be in the global scope, regardless of where that declaration occurs.

A subroutine call specifies the subroutine name and the sequence of expressions (in parentheses and separated by commas) that correspond to the parameters. Executing a subroutine call should result in a run-time error if there is no declared subroutine with that name, if the number of expressions does not match the number of parameters, or if there are any type mismatches. Note: when a subroutine is called, it defines a new, independent scope. Parameters are treated as local variables in that scope, initialized based on the expressions provided in the subroutine call. Because a subroutine's scope is independent, statements within a subroutine cannot access variables outside that subroutine.

SAMPLE CODE (output in red)
>>> sub foo( ) {
        output "foo"
    }
>>> call foo( )
foo
>>> sub add(int val1, int val2) {
        int sum = (val1 + val2)
        output {val1, "+", val2, "=", sum}
    }
>>> call add(2, 4)
2 + 4 = 6 
>>> call add("foo", "bar")
foo + bar = foobar   
>>> sub sumTo(int num) {
        int sum = 0
        while (num > 0) do {
            sum = (sum + num)
            num = (num - 1)
        }
        output sum
    }
>>> call sumTo(10)
55 
>>> call sumTo((10*10))
5050  
>>> sub stamp(str word, int times) {
        str final = "" 
        while (times > 0) do {
            final = (final + word)
            times = (times-1)
        }
        output final
    }
>>> call stamp("foo", 3)
foofoofoo 
>>> call stamp(("X"+"Y"), (4*2))
XYXYXYXYXYXYXYXY 

>>> sub hailstone(int n) {
        if (n == 1) then {
            output "1"
            output "STUCK"
        } else {
            output n
            if ((n % 2) == 0) then {
                n = (n / 2)
            } else {
                int triple = (n * 3)
                n = (triple + 1)
            }
            call hailstone(n) 
        }
    }
>>> call hailstone(5)
5
16
8
4
2
1
STUCK 
>>> call hailstone(52)
52 
26 
13 
40 
20 
10 
5 
16 
8 
4 
2 
1 
STUCK

It is important to recognize that executing a subroutine declaration does NOT result in its code being executed. Declaring a subroutine simply stores the code associated with that subroutine in memory so that it can be called later. You will need to add an additional field to the MemorySpace class corresponding to the code segment. When a subroutine declaration is executed, the parameters and code for that subroutine should be stored in the code segment. Subsequently, when a subroutine call is executed, the corresponding parameters and statements must be accessed from the code segment so that they can be executed.