(define-syntax returning
  (syntax-rules (=>)
    ((_ expr => binding body ...)
     (let ((binding expr))
       body ...
       binding))
    ((_ expr body ...)
     (let ((tmp expr))
       body ...
       tmp))))


(returning (vector 1 2 3) => vec
  (vector-set! vec 0 100)
  (print "I set the vector"))

(returning (list 1 2 3)
  (print "I return a list"))