CS170

Project Assignment 3

TARGET DATE : Thursday 4/16.

Interpreting Simple Functions

Add an eval() function that can evaluate a list, given its internal representation. For now, just implement the basic functions quote, car, cdr, cons and symbol?. All these functions are one to three lines long with the exception of cons, which is perhaps 6 or 7 lines long. Each of these functions (eval, car, cdr, quote) should have a List parameter (cons has two) and return a List. When this phase is complete, a sample run might look like this.

A few tips on evaluation:

• You will probably have to keep quote, car, cdr, symbol? and cons in the same file as the parser, since they have to look into the implementation of the cons-cell struct. In fact, you could also put eval() in that same module. But even if you don't get to it, think of some way of separating the evaluation from the parsing as much as possible, by putting most of the evaluation functions (including eval) in a separate module.
• Remember that when a Scheme function is called, the parameters are evaluated first. That means that when eval is called for a particular function (say you're evaluating "(car (cdr (quote (a b c))))"), it should first evaluate the parameters (so in the cited example, we first have to evaluate "(cdr (quote (a b c)))"). Thus eval() should be recursive. It will bottom out when it gets to the level of a symbol or an empty list.
• If a function is undefined (internally to your C program), just have your evaluator return the list, rather than have the program crash.
• The most important tip of all: Think functionally! Build on what you have! Use the functions you have written to write new ones. For example, you can use your C implementation of car and cdr to get at the parameters in a function call.

Submission location is here.

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