Adding references

References are a simple way to add mutable data to functional programming languages. A reference is a pointer to a single cell whose value can change as the program executes.

We'll add support for the following operations:

• (ref e) creates a reference with the initial value e.
• (deref r) gets the value from a reference.
• (set-ref r e) sets the value of the reference r to e. It should evaluate to e.

References should be displayed by the interpreter and runtime as (ref e), where e is the representation of the currently stored value.

Starting point

We'll build off the stencil for homework 4.

Adding references to the interpreter

In the interpreter, implement references using OCaml's built-in ref. Add a Ref constructor to the value type, then add support for the reference operations described above.

Adding references to the compiler

The compiler should put references on the heap and use 0b001 as the tag for runtime reference values. Don't forget to update the runtime!

Testing

Testing mutable data

To make testing mutable data easier, we've implemented a common lisp construct in the interpreter and compiler: do blocks. do blocks take in a sequence of expressions (at least 1), execute each of them in order, and evaluate to whatever the last expression evaluates to. For instance,

(let ((r (ref 1)))
  (do (set-ref r 2)
      (deref r)))

should evaluate to 2.

CSV

In addition to the usual testing scheme based on *.lisp, *.out and *.err files, you can now include a CSV file of anonymous tests in examples/examples.csv. The format of each line is either <PROGRAM>,<EXPECTED> or <PROGRAM>, depending on whether or not you want to specify an expected output. The CSV does not support tests that are expected to error--for those, and for more complicated tests for programs that span multiple lines, use file-based tests. Leading and trailing whitespace is stripped from each of the CSV's cells.