Language features

We will modify our compiler, to add the following features.

1 An arbitrary number of `print` statements

The programmer will be able to print an arbitrary number of string constants

2 Support for input

The hmc programmer will be able to write

input x;

The program will prompt the end-user to input a number, then store that number in the variable x.

3 Conditionals

The hmc programmer will be able to write something like:

if (<expr>) {
    <true-statements>
} else {
    <false-statements>
}
<next-statements>

The compiled program will evaluate <expr>. If the result is anything other than 0, the program will execute <true-statements>. If the result is 0, the program will execute <false-statements>. After the if statement, the program will continue on to <next-statements>.

4 Loops

The hmc programmer will be able to write something like:

while (<expr>) {
    <loop-body-statements>
}
<next-statements>

The compiled program will evaluate <expr>. If the result is anything other than 0, the program will execute <loop-body-statements>. Then the program will evaluate <expr> again, to determine whether to reenter the loop. When <expr> evaluates to 0, the program will continue on to <next-statements>.

Example

With this new grammar, we could write a factorial program!

input n;
x := 1;
while (n) {
  x := x * n;
  n := n - 1;
}
printString "Factorial is: ";
printNumber x;

Implementation

The updated grammar appears on the next page. I recommend adding features one-at-a-time, rather than all-at-once.

1 An arbitrary number of print statements