Explanation:
(defun my-assoc (A L)
(cond
((null L) nil)
((eq A (car (car L))) (car L))
(t (my-assoc A (cdr L)))
)
)
;; This one is done
(defun my-eval (e alist)
(cond ((atom e) (my-eval-atom e alist))
(t (my-apply (car e) (cdr e) alist))
)
)
;; You need to write this one.
(defun my-eval-atom (e alist)
;; how do you evaluate an atom???
;; Remember there are special cases: T, NIL, ASYMBOL, 10, "Hello"
(cond
((eq e T) T)
((eq e nil) nil)
((symbolp e) (cdr (my-assoc e alist)))
((numberp e) e)
((stringp e) e)
(t nil))
)
;; This one is done, but you must write the functions it calls
(defun my-apply (fn args alist)
(cond ((atom fn) (my-apply-atom fn args alist))
( t (my-apply-lambda fn args alist)))
)
;; You need to write this one.
;; Utility function for eval-cond and apply-lambda. Evaluates each expression
;; in l and returns the value of the last expression
(defun my-eval-list (l alist)
nil
)
;; You need to write this one.
(defun my-apply-lambda (fn args alist)
;; bind the formals to the evaluated actuals then evaluate the body in that
;; new scoping context (i.e., that becomes the new alist for recursive
;; evaluation of the function body. Return the value of the last
;; expression in the body (using eval-list).
)
;; You need to write this one.
(defun my-bind-formals (formals actuals alist)
;; This takes a list of formals and unevaluated actuals. It should evaluate
;; each actual and bind it to its corresponding formal placing them all on
;; the front of the alist. It should return the alist with the new bindings
;; on the front. This will be used to evaluate calls to functions defined
;; via defun.
;; e.g., (my-bind-formals '(a) '((add 1 b)) '((b . 10)))
;; will return ((a . 11) (b . 10))
;; Note there will be one actual parameter for each formal parameter.
)
;; You need to write this one. Handle the primitives as special cases, then
;; handle user defined functions (defined via defun) in the default case.
;; These are the only functions we handle: eq, car, cdr, cons, quote, cond,
;; defun, eval, setq, and user defined functions (defined via defun) that
;; we have evaluated. You can add more built-ins (like plus, times, atom,
;; listp) as you like for testing.
(defun my-apply-atom (fn args alist)
(cond ((eq fn 'eq)
(eq (my-eval (car args) alist) (my-eval (cadr args) alist)))
;; I wrote the first one, eq, for you, you write the rest
((eq fn 'car)
(car(car(my-eval (car args) alist
)
;((eq fn 'cdr)
;)
;((eq fn 'cons)
;)
;((eq fn 'quote)
;)
;((eq fn 'setq))
;; these are (nearly) done, but you must write the sub-functions
;((eq fn 'cond) (my-eval-cond args alist))
;((eq fn 'defun) (my-eval-defun args alist))
;((eq fn 'eval) (my-eval (my-eval (car args) alist) alist))
;(T (my-apply ;; get the lambda from the alist, args alist))
;)
)
;; You need to write this one.
(defun my-eval-setq (var val)
;; just push a new association of the var and its evaluated val onto the
;; global alist
)
;; You need to write this one. You should know how cond works at this point.
(defun my-eval-cond (clauses alist)
)
;; You need to write this one.
(defun my-eval-defun (body alist)
;; just push the function body onto the global alist. It is already an
;; association, e.g., (equal (L1 L2) (cond (...))) and (assoc 'equal in
;; the global alist will return this. You can then take the cdr and you
;; have a list containing the formal parameters and the expressions in
;; the function body.
)
;; This one is done, it just initializes the global alist where global
;; settings, like those defined via setq and defun, go.
(setq global-alist nil)
;; to push a new value, (setq global-alist (cons (cons 'newvar 'newval) global-alist))
;; This one is done, it will become the new top-level for LISP. After you
;; load this file, call (my-top) and then you can type in expressions and
;; define and call functions to test your my-eval.
(defun my-top ()
(prog ()
top (print (my-eval (read) global-alist))
(terpri) ;; prints a newline
(go top) ;; loops forever
)
)
