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drewvid edited this page Dec 30, 2015 · 14 revisions

LISP has a long distinguished history and is the second oldest high level programming language in use today. My interest in this language led me to create a version for the Parellella board. The starting point was a blog post on the parallella forum discussing how much LISP could fit on the board. In trying to answer this question for myself, I came across numerous references to John McCarthy’s paper, "A Micro-Manual for Lisp - not the whole Truth" and decided to start there. The result is a small implementation of lisp which is void of any garbage collection but does run on the Parallella 16 board.

So how many primitives does one need to implement a turing complete version of LISP. Well, according to John McCathy’s original paper just 10:

  1. (quote expr) - expr
  2. (car l) - the head of the list - (car (quote (1 2 3))) = 1
  3. (cdr l) - the tail of the list - (cdr (quote (1 2 3))) = (2 3)
  4. (cons expr1 expr2) - cons constructs lists and is the inverse of car and cdr - (cons (quote a) (quote (b c))) = (a b c)
  5. (equal symbol1 symbol2) - T if symbol1 = symbol2 - (equal (car (quote (a b))) (quote a)) = true (t)
  6. (atom expr) - T if expr is a symbol (atom)
  7. (cond (predicate_1 expr_1) ... (predicate_n expr_n)) - the value of expr_i when predicate_i is true - (cond ((atom (quote a)) (quote b)) ((quote t) (quote c))) = b.
  8. (label x 1) - 1 is assigned to x
  9. (label ff (lambda (x y) (cons (car x) y))) - defines the function ff - (ff '(a b) (cdr '(c d))) = (a d)
  10. (eval expr env) - Evaluate an expression within the current environment

The aditoinal primitives added by me are:

  1. nilp
  2. append
  3. concat
  4. loop
  5. block
  6. progn
  7. if
  8. define
  9. ldefine
  10. print
  11. terpri
  12. <
  13. >
  14. +
  15. -
  16. /
  17. *
  18. =

The extra functions are for printing, looping, block processing, list processing and integer arithmetic.

The code isn’t documented because I intend to do that with a series of blog posts over the next few months or so. This being the first.

Because I was concentrating on writing the code I didn't really checked that everything works as expected. I'll be doing this over the next few months as I add error checking. The code and examples I've released work OK so only minor changes are probably needed, which is good because I'm running out of heap space!

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