Awesome

SedLisp

Lisp implementation in (GNU) sed

How to Use

$ sed -f sedlisp.sed  # '>' won't be shown.
> (car (quote (a b c)))
a
> (cdr (quote (a b c)))
(b c)
> (cons 1 (cons 2 (cons 3 ())))
(1 2 3)
> (defun fact (n) (if (eq n 0) 1 (* n (fact (- n 1)))))
(lambda (n) (if (eq n 0) 1 (* n (fact (- n 1)))))
> (fact 10)
3628800
> (defun fib (n) (if (eq n 1) 1 (if (eq n 0) 1 (+ (fib (- n 1)) (fib (- n 2))))))
(lambda (n) (if (eq n 1) 1 (if (eq n 0) 1 (+ (fib (- n 1)) (fib (- n 2))))))
> (fib 12)
233
> (defun gen (n) ((lambda (x y) y) (define G n) (lambda (m) (define G (+ G m)))))
(lambda (n) ((lambda (x y) y) (define G n) (lambda (m) (define G (+ G m)))))
> (define x (gen 100))
(lambda (m) (define G (+ G m)))
> (x 10)
110
> (x 90)
200
> (x 300)
500

Builtin Functions

• car
• cdr
• cons
• eq
• atom
• +, -, *, /, mod
• neg?
• print

Special Forms

• quote
• if
• lambda
• defun
• define

More Complicated Examples

You can test a few more examples.

FizzBuzz:

$ cat fizzbuzz.l | sed -f sedlisp.sed
(lambda (n) (if (eq n 101) nil (if (print (if (eq (mod n 15) 0) FizzBuzz (if (eq (mod n 5) 0) Buzz (if (eq (mod n 3) 0) Fizz n)))) (fizzbuzz (+ n 1)) nil)))
PRINT: 1
PRINT: 2
PRINT: Fizz
...
PRINT: 98
PRINT: Fizz
PRINT: Buzz
nil

Sort:

$ cat sort.l /dev/stdin | sed -f sedlisp.sed
...
(sort (quote (4 2)))
(2 4)
(sort (quote (4 2 99 12 -4 -7)))
(-7 -4 2 4 12 99)

Though this Lisp implementation does not support eval function, we can implement eval on top of this interpreter - eval.l is the implementation:

$ cat eval.l /dev/stdin | sed -f sedlisp.sed
(eval (quote (+ 4 38)))
42
(eval (quote (defun fact (n) (if (eq n 0) 1 (* n (fact (- n 1)))))))
(fact (lambda (n) (if (eq n 0) 1 (* n (fact (- n 1))))))
(eval (quote (fact 4)))  ; Takes 10 seconds or so.
24

This essentially means we have a Lisp interpreter in Lisp. evalify.rb is a helper script to convert a normal Lisp program into the Lisp in Lisp. You can run the FizzBuzz program like:

$ ./evalify.rb fizzbuzz.l | sed -f sedlisp.sed
...
PRINT: 1
PRINT: 2
PRINT: Fizz

This takes very long time. For me, it took 45 minutes.

Though sedlisp.sed does not support defmacro, eval.l also defines defmacro:

$ ./evalify.rb | sed -f sedlisp.sed
(defmacro let (l e) (cons (cons lambda (cons (cons (car l) nil) (cons e nil))) (cons (car (cdr l)) nil)))
(let (x 42) (+ x 7))  ; Hit ^d after this.
...
49
$ ./evalify.rb | sed -f sedlisp.sed
(defun list0 (a) (cons a nil))
(defun cadr (a) (car (cdr a)))
(defmacro cond (l) (if l (cons if (cons (car (car l)) (cons (cadr (car l)) (cons (cons (quote cond) (list0 (cdr l))))))) nil))
(defun fb (n) (cond (((eq (mod n 5) 0) "Buzz") ((eq (mod n 3) 0) "Fizz") (t n))))
(fb 18)  ; Hit ^d after this. This will take about one minute.
...
Fizz

You can apply ./evalify.rb multiple times. However, sedlisp seems to be too slow to run the generated program. purelisp.rb, which is a reference implementation of sedlisp, can run it:

$ ./evalify.rb fizzbuzz.l | ./evalify.rb | ruby purelisp.rb
...
PRINT: 1
PRINT: 2
PRINT: Fizz
PRINT: 4
PRINT: Buzz
PRINT: Fizz
PRINT: 7
PRINT: 8

test.l is the test program I was using during the development. test.rb runs it with sedlisp.sed and purelisp.rb and compare their results. You can run the test with evalify.rb by passing -e:

$ ./test.rb -e

Limitations

There should be a lot of limitations. sedlisp behaves very strangely when you pass a broken Lisp code.

I don't know how many GNU extensions I used, so it would not be easy to port this to other sed implementations.