Understanding Common Lisp do macro syntax

Question

(do ((n 0 (1+ n))
     (cur 0 next)
     (next 1 (+ cur next)))
    ((= 10 n) cur)))

This is an example from Lisp textbook about keyword "do"

the "do" basic template is:

(do (variable-definitions*)
    (end-test-form result-form*)
 statement*)

But, for this example, it's not clear to me which part is which. And also, what do te middle 2 lines do?

Thank you!

Answer 1

Your good indentation clearly shows which part is which:

(do ((n 0 (1+ n))
    ^(cur 0 next)
    |(next 1 (+ cur next)))
    |
    +-- first argument of do

    ((= 10 n) cur)))
    ^
    |
    +-- start of second argument of do

Look, they line up nicely, and the inner material is indented:

   ((n 0 (1+ n))
    (cur 0 next)
    (next 1 (+ cur next)))
    ^
    |
    +- inner material of argument: three forms which are
       indented by 1 character and aligned together.

Your do doesn't have a third argument there: there is no body of statements (empty loop).

Answer 2

Sometimes it can help to 1. annotate the forms with comments and 2. print the current values in the body like so:

(do
 ;; varlist
 ((n 0 (1+ n))
  (cur 0 next)
  (next 1 (+ cur next)))
 ;; endlist
 ((= 10 n) cur)
  ;; body
  (print (list n cur next)))

This prints

(0 0 1) 
(1 1 1) 
(2 1 2) 
(3 2 3) 
(4 3 5) 
(5 5 8) 
(6 8 13) 
(7 13 21) 
(8 21 34) 
(9 34 55) 
55

which should clear matters up. @_@