Scheme - sum of list

问题

I'm trying to implement a function which calc sum of list , its name is sum -

(define (sum elemList)
  (if
   (null? elemList)
   (+ (car elemList) (sum (cdr elemList)))
   0
  )
 )

The above implementation gives wrong result , for example -

> (sum (list 1 2 3 4 ))
0

What I did wrong here ?

回答1:

I think you swapped the then and the else part of the if:

(define (sum elemList)
  (if
    (null? elemList)
    0
    (+ (car elemList) (sum (cdr elemList)))
  )
)

In the original function, for every non-empty list, 0 is returned.

来源：https://stackoverflow.com/questions/15019498/scheme-sum-of-list