I was in the #lisp chat room at Freenode when someone posed a question about performing a map function over a nested lists. This was an interesting problem, so I thought that I would cover it here.
Suppose there was a complex tree of numbers:
((1 2) (3 4))
And you want to map each to a function (let’s say a squaring function) and maintain its structure. This is where a good utility function would be in order. The function must be able to handle trees of an unknown structure. So, the only solution that I know would be to use recursion:
(defun nested-map (fn item)
(cond ((null item) nil)
((atom item) (funcall fn item))
((consp item) (cons (nested-map fn (car item))
(nested-map fn (cdr item))))))
Even though it looks complex, the function seems correct:
CL-USER> (nested-map #'(lambda (x) (* x x))
'((1 2) (3 4)))
((1 4) (9 16))
CL-USER> (nested-map #'(lambda (x) (* x x))
'(1 2 (3 4 (5 6 7))))
(1 4 (9 16 (25 36 49)))
I think I’ll keep this in my utility toolkit. I am concerned about the performance, but I won’t worry too much about it until it becomes a problem.
Working in mostly M$ shops in the past 10 years or so, coupled with my home projects in the functional languages, I don’t get to do much with Java or the JVM anymore. But today, it was treated to my first experience in programming in 
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