function - LISP, cond returning nil everytime -

why return nil when adding lst???

please help!

thank you!

cl-user 1 : 1 > (defun constants-aux (form lst)                   (cond ((null form) lst)                         ((eq (car form) 'implies) (constants-aux (cdr form) lst))                         ((eq (car form) 'not) (constants-aux (cdr form) lst))                         ((eq (car form) 'and) (constants-aux (cdr form) lst))                         ((eq (car form) 'or) (constants-aux (cdr form) lst))                         ((atom (car form)) (print (car form)) (cons (car form) (list lst)) (delete-duplicates lst) (constants-aux (cdr form) lst))                         (t (constants-aux (car form) lst) (constants-aux (cdr form) lst)))) constants-aux  cl-user 2 : 1 > (defun constants (form)                    (constants-aux form nil)) constants  cl-user 3 : 1 > constants '(implies (not q) (implies q p))  q  q  p  nil 

you doing wrong in many ways.

1. - why create -aux function, when can use optional arguments?

(defun constants (form &optional lst)   (cond     ((null form) lst) ... 

2. - don't need many similar branches, can write:

((find (car form) '(implies not , or))  (constants (cdr form) lst)) 

3. - delete-duplicates may modify list, don't think have to, , if will, modification makes not wanted. should use result of it. think got style-warning, in sbcl looks code:

; caught style-warning: ;   return value of delete-duplicates should not discarded. 

read warnings, helps.

i don't understand result expecting, can't modify function work properly, i'll try show problem(s) is. code last 2 cond branches:

((atom (car form))          (print (car form))          (cons (car form) (list lst)) ;; result ignored          (delete-duplicates lst)      ;; result ignored          (constants-aux (cdr form) lst)) 

you should write:

(constant-aux (cdr form) (delete-duplicates lst))          (t            (constants-aux (car form) lst) ;; result ignored            (constants-aux (cdr form) lst)))) 

may (depends on want get) should write:

(cons    (constants-aux (car form) lst)    (constants-aux (cdr form) lst)) 

4 i'm not shure, looks using print debug, use trace instead. code it'll give pretty information going on list during execution:

  0: (constants-aux (implies (not q) (implies q p)) nil)     1: (constants-aux ((not q) (implies q p)) nil)       2: (constants-aux (not q) nil)         3: (constants-aux (q) nil)  q           4: (constants-aux nil nil)           4: constants-aux returned nil         3: constants-aux returned nil       2: constants-aux returned nil       2: (constants-aux ((implies q p)) nil)         3: (constants-aux (implies q p) nil)           4: (constants-aux (q p) nil)  q             5: (constants-aux (p) nil)  p               6: (constants-aux nil nil)               6: constants-aux returned nil             5: constants-aux returned nil           4: constants-aux returned nil         3: constants-aux returned nil         3: (constants-aux nil nil)         3: constants-aux returned nil       2: constants-aux returned nil     1: constants-aux returned nil   0: constants-aux returned nil 

5. it'll easyer answer if explain transformation expecting code.

good luck.