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cs330_f2016:labpaint

Objective:

This lab is designed to help you go deeper into program transformations, and how we can analyze the AST of a program and perform different transformations on it, either for performance or for end-user ease-of-use.

Pre-requisite:

For this lab, you should have Julia 0.4+ installed, the same as for the rudimentary interpreter. You will need both the Lexer and Error modules, and you will need your code from the Extended Interpreter.

Deliverable:

For this lab, you will expand the interpreter that you have already built, and add in a new “analysis” function that performs three different flavors of syntactic de-sugaring.

The primary deliverable for this lab is a new Julia module. Your module should export parse, calc and analyze functions, and two return types, NumVal and ClosureVal.

Your module should be able to do everything that your simple interpreter could do and everything that your extended interpreter can do. However, you will use program analysis to both eliminate With nodes, and to add two new simple features.

The CI7.jl module is available on LearningSuite, under “Content → Julia → Program analysis”.

Description:

For this lab, you will implement an analyze function similar to what we covered in class. The analyze function accepts as input an AST, rewrites it, and returns a new AST.

There are three rewrites you must implement:

1. You must rewrite multi-argument with nodes to be multi-argument lambda nodes.
2. You must create a new multi-argument + operation that is implemented with only binary + operations.
3. You must create a new multi-argument and operation that is implemented with only if0 operations.

Each is discussed in more detail in the following sections.

The grammar for our new language is the following:

<OWL> ::= number
| (+ <OWL> <OWL> <OWL>*)   # at least two parameters, possibly infinity.  note extra star!
| (- <OWL> <OWL>)
| (* <OWL> <OWL>)
| (/ <OWL> <OWL>)
| (mod <OWL> <OWL>)
| (collatz <OWL>)
| (- <OWL>)
| id
| (if0 <OWL> <OWL> <OWL>)
| (with ( (id <OWL>)* ) <OWL>)
| (lambda (id*) <OWL>)
| (and <OWL> <OWL> <OWL>*)  # new operation 'and'
| (<OWL> <OWL>*)         

Expanded functionality:

Here we discuss the three new features you must support in our language.

Part 1: Multi-site with → multi-site lambda

In class we showed how to implement with expressions using lambda expressions, but only for with and lambda expressions that involve one parameter. For the first part of the lab, you must rewrite the AST of a program to eliminate all with nodes, even with multiple parameters, and replace them with equivalent function creation / function calls.

Remember from class that

(with (x 5) (+ x 1))

becomes

((lambda x (+ x 1)) 5)

Your code should make a similar transformation for multiple-identifier with expressions to applying multiple-parameter lambda expressions.

No changes to the grammar are necessary to support this change, and therefore, no changes to your parser from the Extended Interpreter are necessary to implement this under-the-hood syntactic rewrite. The transformation should happen in the analyze function.

Part 2: Addition operation with arbitrary number of parameters

For the second feature, you must implement a more flexible + operator.

Specifically, instead of only two operands, the new + operator should support an arbitrary number of operands, as long as there are more than two.

However, this will be syntactic sugar – you should implement this using your existing binary operations.

So, we would essentially rewrite this:

(+ a b c d)

into this:

(+ a (+ b (+ c d)))

Note that this involves a change to the grammar, and therefore you will need to change your parser! You may need to change the definition of the plus node – it can no longer by implemented as a BinOp. The analyze function should eventually convert it to a sequence of BinOp operations only, and your calc function should handle this without change.

Part 3: A simple and operator

For the final part of this lab you will implement a simple short-circuited and operation in terms of if0 calls. The semantics of the and node is that it should return 1 iff all subexpressions evaluate to nonzero, and return 0 otherwise. Or in other words, it should return 0 if any subexpression evaluates to zero.

As an example, we will transform this:

(and (+ 4 0) (- 3 3) (* 4 3))

into this:

(if0 (+ 4 0)
0
(if0 (- 3 3)
0
(if0 (* 4 3)
0
1)))

Note that this is a new construct in our grammar, so you will need to create a new AST node type (call it And) and parse it appropriately. However, because it is being implemented in terms of if0 calls, you will not need to modify your calc function to support it. The transformation from the And node to the if0 nodes should happen in the analyze function.