functional programming - Function that creates a list of all indexes where an element appears in a list -

i attempting write function (positions n l) returns list of each index appears in l , n number given first element of l.

for example,

(positions 0 'a '(a b c d e a)) => (0 3 6)

(positions 1 'a '(a b c d e a)) => (1 4 7)

so far, have come (which isn't working correctly):

(define (positions n l)   (cond     ((null? l)      '())      ((= (car l) a)       (cons n (positions (+ n 1) (cdr l))))      (#t       (positions (+ n 1) (cdr l))))) 

try this:

(define (positions n l)   (cond     ((null? l)          '())     ((equal? (car l) a) (cons n (positions (+ n 1) (cdr l))))     (else               (positions (+ n 1) (cdr l))))) 

the problem = defined numbers. if you're that list contain symbols, use eq?. otherwise use equal?, general equality comparison , works many data types (numbers, symbols, booleans, etc.) also, use else last condition, using #t common lisp convention doesn't apply in scheme.