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An "explicit control" Lisp interpreter written in assembly-like C

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# lispinc
An "explicit-control" Lisp interpreter written in assembly-like C

INSTRUCTIONS

clone, then make. call with lispinc.

This will bring up the REPL. Besides code, a few user commands can be entered:
* .help for help
* .quit to quit
* .stats to toggle stats mode
* .info to toggle info mode
* .step to toggle step mode (pauses between each step of the evaluator; useful in conjunction with info mode)
* .debug to toggle debug mode
* .tail to toggle tail recursion mode (turning this off is really only of any interest in conjunction with stats mode)
* .demo turns on info, stats, and step modes
* .repl turns off info, stats, and step modes

The interpreter starts in info and stats modes.

TODO

* Update documentation to include compiler! (https://github.com/nickdrozd/Lisp-C-Compyler)

* <del>Variable-arity functions</del> Done!

* Reader macros (' for quotation and ` for quasiquotation, for instance)

* Memory management is poor. The main culprits are the parsing functions and the environment functions. Figure out how to free that memory, ie figure out some kind of garbage collection. (Does anything in mem.c work?)

* <del>There is rudimentary syntax checking, but any other error unceremoniously crashes the entire interpreter. Figure out an error-checking system (although as SICP says, "this is lots of work" and "this is a major project").</del> Actually, because of the way the object system was set up, this turned out to be a pretty simple addition!

* Add documentation to individual functions.

* Add links to acknowledgements section of readme.

* Figure out how to format this readme.

BACKGROUND

In the textbook Structure and Interpretation of Computer Programs (SICP), Abelson and Sussman present two styles of Lisp (Scheme) interpreters. In the first style, the semantics of the target language are specified (e.g. an "if" statement has its test condition evaluated first, and then either its consequent or alternative are evaluated according as whether the test condition returned true or false), but nothing else is. In particular, the memory requirements of the target language cannot be specified, since the evaluation of the target language is left up to the language of the interpreter. (They call this interpreter a *metacircular evaluator*, since it is written in Lisp and interprets Lisp, but this is an inessential detail.)

The second style of interpreter, which they call an *explicit-control evaluator*, remedies this problem by showing how to interpret the language at a more primitive level. To implement this interpreter, they develop a toy assembly language complete with goto statements and a stack capable of handling recursion of arbitrary depth. Being written in directly in assembly, the explicit-control evaluator makes no (nontrivial) function calls, and so leaves (almost) nothing up to the underlying language. Every part of the computation is completely visible at all times, and it is possible to control exactly how interpretation is to be undertaken. In particular, it's possible to specify that the interpreter be (or not be) tail recursive, allowing one to implement iterative processes using function calls without incurring more than constant memory use.

The book's penultimate exercise, Exercise 5.51, reads "Develop a rudimentary implementation of Scheme in C (or some other low-level language of your choice) by translating the explicit-control evaluator...into C. In order to run this code you will need to also provide appropriate storage-allocation routines and other run-time support".

lispinc is just such a translation. In fact, the program's main function (found in ec_main.c) is basically a literal translation of SICP's toy assembly code. Here is a comparison of the section of the interpreter that handles lambda expressions:

SICP (toy assembly):

ev-lambda
	(assign unev (op lambda-parameters) (reg exp))
	(assign exp (op lambda-body) (reg exp))
	(assign val (op make-procedure)
				(reg unev) (reg exp) (reg env))
	(goto (reg continue))

lispinc (C):

LAMBDA:
	unev = lambdaParams(expr);
	expr = lambdaBody(expr);
	val = makeFunc(unev, expr, env);
	goto CONTINUE;

Aside from minor terminological differences, it should be clear that these two pieces of code are basically the same. There is one nontrivial difference: SICP's toy assembly code allows for goto labels to be passed around as values of variables (in other words, labels are first-class objects), while C does not. Instead of passing around goto labels, lispinc passes enum labels and then adds an extra goto label CONTINUE that dispatches on the enums.

The part of lispinc that actually does the interpretation -- a seriously clever tangle of gotos and stack pushes and pops -- was copied more or less straight out of SICP. The rest of it -- input, parsing, environment manipulation, and printing -- was written from scratch. However, anyone familiar with the programming style advocated by SICP would immediately recognize certains parts of this program (especially the part dealing with environments) as striving to emulate it.

FAQ

* Why did you write this in C? Why not something easier, like Python?

One way of "translating" SICP's explicit-control evaluator would be to write it in actual assembly for a real chip or whatever. I don't know assembly, and I didn't feel like learning. I did, however, know enough C to get off the ground, and since C is still respectably low-level, I figured it would be good enough as a substitute. (C is also mentioned explicitly in the SICP problem statement.) Certainly the interpreter could be adapted to a higher-level language, but it would be harder to ensure that the underlying language wasn't "doing the work".

* Why did you use gotos? Don't you know that those are bad?

Yes, I know they're bad and antiquated. The code in lispinc, while written in C, is intended to resemble assembly code. Assembly uses gotos, so gotos are used here. The goto arrangement of the main function were basically copied from SICP, while the gotos of the tokenizing function in read.c are of my own devising.

* Why is there so much C++?

There's no C++, but GitHub seems to recognize C header files as C++. Code generated by my compiler is included in headers and there's a lot of that, so that makes it look like a C++ repo.

ACKNOWLEDGEMENTS

Many people helped me with this project in small ways, but a few people helped in large ways:

* Jeff Fowler's own C Lisp interpreter gave me some general inspiration, and his blog posts were incredibly helpful in the early stages of the project.

* Andrew Kelley showed me the difference between tokenizing and parsing and why it's a good idea to keep the two processes separate. Without his advice, I almost certainly wouldn't have gotten past parsing and the whole project would have died on the vine.

* Michael Cordover discovered the source of an annoying little bug in the tokenizer.

* Phil Jost and Max Bittker were tirelessly enthusiastic about the project and spent quite a bit of time discussing various aspects of it with me.

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