'((foo) (bar) (baz))
 '(((((((42)))))))

we often are interested in traversing just the spine of a list. That is, we often want to inspect just the elements at the top level of the list -- which may themselves be lists -- without traversing those elements. We have already seen how to take the length of a list. This handout looks at some other typical uses of ``flat recursion'' on lists.

There are some Scheme forms we'll need, so let's cover these first.

A cond is a multi-way conditional (if only allows two branches). The syntax is:

 (cond
  [q1 a1]
  [q2 a2]
   ...
  [else an])

where the qs are questions, and the as are answers. The Scheme evaluator checks the questions from top to bottom. If any question is non-#f, the value of the corresponding answer is the result of the cond. The else question is always true. A cond does not need to have an else: in that case, the result of the cond is unspecified if no question returns non-#f.

The following forms are short-circuiting.

 (and exp ...)

evaluates each of its arguments, left to right. As soon as an argument returns #f, the and returns #f. If all the expressions return non-#f, the and returns the value of the last argument. Observe: if each exp is the application of a predicate, the and returns either #t or #f.

 (or exp ...)

evaluates its arguments left to right. If any returns non-#f, that's the value of the or. Otherwise, if all the arguments return #f, the or returns #f.