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Here are some source code for an example from the amazing book "Land of Lisp" :

(defun random-node ()
  (1+ (random *node-num*)))

(defun edge-pair (a b)
  (unless (eql a b)
     (list (cons a b) (cons b a))))

(defun make-edge-list ()
    (apply #'append (loop repeat *edge-num*
                          collect (edge-pair (random-node) (random-node)))))

Since I don't have the Lisp instinct , I find it is useful to break a method into multiple lines (as an imperative style) and then trying to morph it into the functional style.

Would you please help me to break make-edge-list function into multiple lines?

Joshua Taylor
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Chiron
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  • It could be helpful to at least replace components of the expression with let expressions. I'm not sure about the exact dialect you're using here, but you should be able to break it down into at least a more descriptive form, e.g. `(defun make-edge-list () (let ((pair (edge-pair (random-node) (random-node))) (...`, etc. Look up `let` binding for more info if you're unfamiliar. – Asherah Nov 18 '10 at 02:17
  • Am I missing something -- is there a reason for doing it in two steps in the first place? Is this different from `(loop repeat *edge-num* append (edge-pair (random-node) (random-nome)))`? – Ken Nov 18 '10 at 02:29

1 Answers1

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Lines are meaningless in Lisp. Lisp notation is based on s-expressions and the textual line is not seen by Lisp during evaluation. You can break up an expression in any way on whitespace.

(defun make-edge-list ()
   (apply #'append
          (loop repeat *edge-num*
                collect (edge-pair (random-node)
                                    (random-node)))))

You have to read the code like this:

  • each edge pair is a list of conses. Each cons stores a position.
  • the loop returns a list of edge pairs.
  • Then append is applied to this list and returns a list of conses. The list of conses is really a list of positions.

Note that the function has several slight problems:

  • APPLY works not for lists of arbitrary lengths. So let's hope that the list is not too long.

  • More important, the apply of the function APPEND is not really needed if we change the LOOP slightly. LOOP can not only COLLECT, but also APPEND.

Rainer Joswig
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  • I want to break the method into the imperative style, just like Arlen suggested earlier. – Chiron Nov 18 '10 at 13:42
  • If you want to write your code in imperative style, you might want to use a different programming language. In above function it makes little sense... – Rainer Joswig Nov 18 '10 at 14:58
  • Please read my original post: "Since I don't have the Lisp instinct , I find it is useful to break a method into multiple lines (as an imperative style) and then trying to morph it into the functional style." – Chiron Nov 21 '10 at 08:37