Question

I’m writing a function that takes a list and returns the sum of the squares of all items in the list. Called on (1 2 3) it should return 14: (1² + 2² + 3²).

I have this sqsum function:

(define (sqsum lis)
    (if (null? lis)
        0
        (+ (* (car lis) (car lis)) (sqsum (cdr lis)))
    )
)

Is this tail recursive? I think it is recursive but not tail recursive.
This is part of my homework and I’m not looking for solution: I just want to know if this is tail recursive or not. If not then I need to read more and find new solutions.

Answer 1

To be tail recursive the recursion has to happen at the very last function, i.e. you cannot do anything with the results from the recursiv call. Here you pass it to +, which makes it non-tail-recursive. The compiler could optimize this away though, and it’s pretty easy to do yourself too.