A small implementation of LISP for the Parallella-16 board with basic, mark and sweep, garbage collection
LISP has a long distinguished history and is the second oldest high level programming language in use today. My interest in the language led me to create a version for the Parallella 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 started there. The result is a small implementation of lisp which runs on the Parallella-16 board.
In the two latest versions of Parallella-lisp, released in early Feb 2016, the printing of input and evaluated input has been moved to the host. Also the freelist containing unallocated nodes is constructed on the host and written to the device. This makes the LISP interpreter smaller and frees up space for more functionality. The version of Parallella-lisp with Garbage Collection now contains more primities. Garbage collection is done at the end of each iteration of the REPL. Variables can be defined with ldefine (local) or define (global). Loop variables can be either local or global thus the following will work and x and y should be 100 when printed.
(defun foldr (f zero lst)
(if (null lst) zero
(funcall f (car lst) (foldr f zero (cdr lst)))
)
)
(defun append (a b) (foldr cons b a))
(define x 2)
(define y 2)
(define result nil)
(define func
(lambda (n1 n2)
(progn
(ldefine y n2)
(loop (> y -1)
(ldefine x n1)
(loop (> x -1)
(define result (append result ((x y))))
(ldefine x (- x 1))
)
(ldefine y (- y 1))
)
nil
)
)
)
(func x y)
result
(block x y)
More functionality is slowly being added as space permits.
The version with gc is in the directory plisp-gc. The lastest version without garbage collection is in the directory plisp. The original version is in the directory plisp-initial. All versions were compiled and tested with esdk.2015.1. To build LISP for the parallella in either of the above mentioned directories type:
make build
To run LISP type:
make run
If you would like to build lisp to run on your local machine then type:
make
Two versions of lisp will be built which can be run by typing:
./fl
./onefile
Both onefile and fl can now be called with a filename. If no filename is given testfuncs.lisp will be interpreted. For instance, to interpret the code that will run on core 3 type in:
./fl code/p2.lisp
Adding more tests to testfuncs.lisp in the folder plisp-gc is not a good idea because there is a chance you will run out of memory. The shell script plisp is also provided to execute the same lisp code on every core, try:
./plisp prog.lisp
Parallella-lisp includes the 10 primitives suggested by John McCarthy which are:
atom
equal
car
cdr
cons
quote
cond
lambda
label
eval
In addition the following primitives are included on top of the original 10.
nilp
append
concat
loop
block
progn
if
define
ldefine
terpri
<
>
+
-
/
*
=
<=
>=
defun
funcall
null
consp
times
zerop
greaterp
lessp
sub1
add1
numberp
eq
and
or
not
setflag
The tests directory now contains the code for the derivation of the y-combinator. For things to work I added one c function:
node *bind_variables(node *expr, node *env)
This function is called by a modified evlambda. The Y-Combinator implemented in plisp is very similar to the emacs LISP version and you can read about how this function is derived in this blog post Y-Combinator in emacs LISP
The plisp version along with an example looks like this:
(setyc)
(defun YCombinator (f)
(funcall (lambda (x)
(funcall f (lambda (y)
(funcall (funcall x x) y)
)
)
)
(lambda (x)
(funcall f (lambda (y)
(funcall (funcall x x) y)
)
)
)
)
)
(funcall
(YCombinator
(lambda (f)
(lambda (n)
(if (eq n 0)
1
(* n (funcall f (1- n)))
)
)
)
)
6
)
(unsetyc)
The file, ycomb.lisp, cotains the the above code along with a modified version of the code listed in the blog post on how to derive the function to work with emacs LISP. To see everything working, do this in the plisp or plisp-gc directory:
make
./fl ../tests/ycomb.lisp
or
make build
./plisp ../tests/ycomb.lisp
./fl runs the local version and ./plisp runs the code on the parallella 16.
The LISP code that runs on each core is generated anew by gencode.py when you build or run LISP. The files generated are named p0.lisp to p15.lisp. If you want to change the filenames then you will have to edit fl-device.c and recompile. Python is also used to generate a single file which can be ported easily to another machine for testing and development. The code isn’t documented because I intend to do so with a series of blog posts towards the end of February of this year (2016).
To see the output after evaluating one of the files in the tests directory, click on the appropriate link below. The files are evaluated by a compiled version of plisp-gc hosted by pythonanywhere.