Homework 4: Error Handling and the Heap
Note: This is a 1-week assignment (due 1 week after release). Introduction
In this homework, you’ll implement lists in two ways: first using pairs, then using arrays. You’ll get practice handling runtime errors and dealing with data on the heap.
At the end of this homework assignment, your interpreter and compiler should support the following grammar (we’ve highlighted what you’ll be adding):
1 2 3 4 5 6 7 8 9
| (
| (
| (
| (if
| (let ((
| (do
+ | vector?
+ | vector-length
+ | vector
+ | vector-get
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37 38 39 40
Tests will not be graded
We will NOT be grading your tests for this homework.
However, it will still be the case that when you submit your implementation to Gradescope (to the assignment hw4 ), your suite of examples in the examples/ directory will be run against the reference interpreter and compiler. If the reference implementation fails on any of your examples, Gradescope will show you how its output differed from the expected output of your example if you provided one.
You can do this as many times as you want. We encourage you to use this option to develop a good set of examples before you start working on your interpreter and compiler!
Running tests
You can use dune runtest -f to run all your tests in the examples/ directory. As before, the testing framework supports both .lisp / .out files and the examples/examples.csv file.
Running the compiler and the interpreter manually
In addition to using dune runtest -f , you can manually run the interpreter and compiler on input files. To run the interpreter on a file, execute the following command:
1 duneexecbin/interp.exe–
To run the compiler on a file, execute the following command: 1 duneexecbin/compile.exe–
The resulting .s file (containing assembly code) and .exe file (an executable) will be in the output/ directory. You can then run the executable with:
1 ./output/
You can also tell the compiler to run the executable immediately after compiling by adding -r , e.g.:
1 duneexecbin/compile.exe–
Inspecting and debugging assembly
If you run the compiler on a .lisp file (as described above), you can take a look at the emitted assembly by looking in the .s file in the output directory. This is a really useful way to understand how your compiler is working as a whole and to debug any issues you may be facing.
You can also take a look at _build/default/test/test_output/*.s to take a look at the assembly for your test cases, including those in examples/examples.csv .
For a more interactive way to debug assembly, check out Section 4 on the course website!
Manually assembling .s files
If it ever happens that the assembly emitted by your compiler is invalid and you’re not sure why, you can try manually compiling a test file (see above), then manually running nasm on the .s file in the output directory:
1 nasm
This may give you more helpful error messages than the automated testing infrastructure.
1. Lists (5 subtasks)
As we saw in class, lists can be implemented using pairs. In fact, this is how lists are generally implemented in Scheme and other Lisp-like languages. A list is either:
The empty list, for which we used false in class. A pair where the second element is a list.
Instead of using false to signal the empty list, Scheme includes a special value () (pronounced “nil”). Task 1.1: Add support for () to the interpreter.
Task 1.2: Add support for () to the compiler. Use 0b11111111 as the runtime representation of () . Hint: You’ll need to modify the runtime to properly display nil values.
Now we can build lists using pair and () . Here are a few such lists:
(pair 1 ())
(pair 1 (pair 2 ()))
(pair (pair 1 2) ())
We can now define a primitive (list? e) that evaluates to true when its argument is a list and false otherwise.
Task 1.3 (ungraded): Write tests for the list? primitive in the examples/ directory. Task 1.4: Add support for list? to the interpreter.
Hint: You can use a recursive helper function to implement list? in the interpreter.
Programming Help
Task 1.5: Add support for list? to the compiler.
Hint: You can generate a loop to execute this primitive; to do so, you’ll need to define a label and repeatedly
jump back to that label.
2. Vectors (3 subtasks)
We can also implement lists using arrays. In Scheme, array-backed lists are called vectors. In this task, you’ll implement the following five vector primitives:
(vector n e) , which creates a vector of length n where all of the elements are e . Precondition: n evaluates to a positive integer.
(vector? e) returns true if e is a vector and false otherwise. Precondition: none.
(vector-length v) returns the length of the vector v . Precondition: v is a vector.
(vector-get v n) returns the element of the vector v at (0-based) index n .
Precondition: v is a vector and n evaluatase to a non-negative integer less than the length of v .
(vector-set v n e) sets the element at index n of the vector v to e and evaluates to the vector. Precondition: v is a vector and n evaluates to a non-negative integer less than the length of v .
An important note about error handling
For this assignment, unlike previous assignments, you are expected to implement error-handling in the compiler.
An expression that does not meet its corresponding precondition must result in a runtime error. (This is sometimes what is meant by a language being “safe.”) Here’s what your code should do when evaluating an expression that does not meet its precondition:
In the interpreter, you must throw an exception. Any exception will do; we will not check the exception type nor its contents.
In the compiler, you must jump to the lisp_error symbol, defined in lib/runtime/runtime.c . Take a look at ensure_num and ensure_pair for some examples of the error handling we’ve implemented so far.
Hint: C functions assume their first argument is stored in the register Rdi , and the dq directive lets you embed bytes into the location in memory where the instruction is stored.
Displaying vector outputs
Vectors should be displayed as space-delimited lists of their values enclosed in square brackets. For instance, here are some simple vectors of numbers:
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Note that this syntax is not supported in our grammar, so you cannot use [1 1] as shorthand for (vector 2 1) , for example.
Task 2.1 (ungraded): Write tests for the five vector primitives in the examples/ directory. Implementing vectors
Task 2.2: Add support for vectors and the five vector primitives to the interpreter. In the interpreter, you should implement vectors using OCaml’s built-in array type and the functions in the Array module.
Task 2.3: Add support for vectors and the five vector primitives to the compiler. In the compiler, you should implement vectors on the heap: A vector of length n should occupy n+1 8-byte cells, where the first cell should be used to store its length. Use the three-bit tag 0b101 for vector values.
Hint: In compile.ml , look at where we call compile_binary_primitive to see that the second argument is held in Rax , and the first is on the stack. See where we call compile_trinary_primitive to see where we store arguments for primitives that accept three arguments.
Hint: You may need to make use of an additional register in order to implement some of the vector primitives. If you do, you can use R9 in addition to the usual Rax and R8 .
Hint: You’ll need to modify the runtime to properly display vector values. In order to do so, it will likely be helpful to cast a vector’s pointer to the heap into a C pointer to a uint64_t and then use pointer arithmetic to access each of the values in the vector. Recall how we handled pairs in class.
We will not test on vectors over size 100, and we will not test display of cyclic vectors.