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A lisp interpreter and compiler written in Scala, inspired by Peter Norvig

Download

You can either clone this repo and use sbt or you can download the precompiled jar and execute that.

To be able to compile the resulting scala code, you should also download compiled_builtins.scala and place it in the same directory as the compiled .scala file.

Interpeter

It's a bit incomplete and probably buggy, but it works good enough to for most applications:

(define fact (lambda (n) (if (< n 2) 1 (* n (fact (- n 1))))))
(fact 10) 

produces 3628800

Also, Merge sort can be implemented without a problem:

; note that you can also use comments
; and split functions over multiple lines for readability
(define msort 
  (lambda (list) 
    (if (<= (length list) 1) 
      list 
      (begin 
        (define split (/ (length list) 2)) 
        (merge 
          (msort (subseq list 0 split)) 
          (msort (subseq list split)) 
        ) 
      ) 
    ) 
  )
)

; ordering is not important, as functions are evaluated lazily
(define merge
  (lambda (a b)
    (if (< (length a) 1)
      b
      (if (< (length b) 1)
        a
        (if (< (car a) (car b))
          (cons (car a) (merge (cdr a) b))
          (cons (car b) (merge a (cdr b)))
        )
      )
    )
  )
)

Usage is just like you'd expect

(msort '(5 7 2 1 3 4 6))

And results in '(1 2 3 4 5 6 7)

It's also possible to execute files, simply do

scalisp filename.l

or, from the sbt-console

run filename.l

Compiler

Instead of taking the traditional route and compiling to byte code (which Clojure already does), I decided to compile to Scala instead. Just use scalisp like you would for interpetation, but add the -c switch.

For example, the compiled merge sort looks like this:

def merge(a: Any, b: Any): Any = {
  if(length(a) < 1l) {
    b
  } else {
    if(length(b) < 1l) {
      a
    } else {
      if(car(a) < car(b)) {
        car(a) :: merge(cdr(a), b)
      } else {
        car(b) :: merge(a, cdr(b))
      }
    }
  }
}

def msort(list: Any): Any = {
  if(length(list) <= 1l) {
    list
  } else {
    {
      var split = (length(list) / 2l)
      merge(msort(subseq(list, 0l, split)), msort(subseq(list, split)))
    }
  }
}

If you specify a whole file, you'll get a complete .scala file back, ready to be compiled. If you just execute scalisp -c, you get a REPL, which simply compiles snippets of code.