skim

This is skim, a toy Lisp written in Clojure.

Usage

You can run a repl with:

./skim

or evaluate a program with:

./skim example.skm

Syntax

Pretty straightforward Lisp syntax, our special forms are quote, if, func and bind.

Clojure is our definitional language so comments are preceded with any number of semicolons ;.

(- 4 1)
;; 3

(quote (foo bar baz))
;; (foo bar baz)

(if (= 3 4)
  "same"
  "not same")
;; "not same"

(bind (a 1
       b 13)
      (- b a))
;; 12

(bind (minus-5 (func (n)
                     (- n 5)))
      (minus-5 20))
;; 15

Initial environment

We have an intentionally minimal initial environment:

• For arithmetic we start with just the - function, as it can be used to create +, *, / and %.

• For boolean logic we start with just the nand function, as it can be used to emulate the other primitive boolean functions not, and and or.

• For numerical comparision we just have < and =, creating other comparison operators is achieved by combining these with the boolean functions.

• For sequences we have cons, first and rest, with these we can implement map, filter, reduce etc.

• Finally we have coll? and symbol? which are included specifically to allow you to implement your own Lisp in skim.

See the examples directory for implementation ideas.

Recursion

Inside the lambda form func is an implicitly available binding recur.

;; need to define `+` first
(bind (+ (func (a b)
               (- a (- b)))

       ;; finding Fibonnaci numbers with multiple recursive calls
       fib (func (n)
                 (if (< n 2)
                   n
                   (+ (recur (- n 1))
                      (recur (- n 2))))))

      (fib 10))
;; 55

TODO:

• can we create an emacs mode with correct indentation rules?