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I'm new to Common Lisp, and found myself taking advantage of way functions returns values. The following are to two trivial examples:

(defun safe-avg (a b)
    (and (numberp a) (numberp b) (/ (+ a b) 2)))

(defun safe-div (a b)
    (and (numberp a) (numberp b) (> b 0) (/ a b)))

But I could've written it like this (arguably clearer):

(defun safe-avg (a b)
    (if (and (numberp a) (numberp b))
        (/ (+ a b) 2)))

(defun safe-div (a b)
    (if (and (numberp a) (numberp b) (> b 0))
        (/ a b)))

I wanted to know what is the preferred method of doing something like this and the reasoning behind it, before I start abusing this habit.

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  • I would use the 2nd form. Why? Because it's "less tricky" and shows the intent better. I value code readability -- both now and in 6 months from now. Commented Sep 24, 2011 at 8:16
  • I was used to scheme, are you sure the function are equivalent? To me not, as the first couple of function return false if the constraint are not meet the second one seems not specify a return value Commented Sep 24, 2011 at 8:55

2 Answers 2

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7

Since you do not use the "else" branch, you could use when:

(defun safe-div (a b)
  (when (and (numberp a) (numberp b) (> b 0))
    (/ a b)))

which is the same as:

(defun safe-div (a b)
  (if (and (numberp a) (numberp b) (> b 0))
      (/ a b)
      nil))

which is the same as your version, but more explicit.

Anyway, these are all functionally equivalent. I would rather think about how these functions are to be used. If you do it like this, you will have to do null checks each time you call these functions, which is tedious.

It would be better to use conditions, either through type declarations, through asserts, or through explicit when … signal forms. You can then define handlers and restarts for these conditions for entire parts of your program. Further reading: Practical Common Lisp, ch. 19.

In this case, I would not handle this here at all:

(defun safe-div (a b)
  (/ a b))

(or rather, just use /).

If / gets the wrong arguments, it will signal an error which you can then handle outside, where you know what that could mean.

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Comments

3

The first form is idiomatically acceptable. It isn't the best example of using AND's return value effectively, since the second form is a little clearer without being any lengthier. But you shouldn't be afraid to use LISP as intended!

For instance, going in an (inadvisable) direction...someone might argue that the implicit "nil return" of if statements could be confusing and try and parallel if/else structure to be more clear:

(defun safe-avg (a b)
  (cond ((and (numberp a) (numberp b))
         (/ (+ a b) 2))
        (t
         nil)))

That's bad. And you don't want to go down that road. So go ahead and use the expressions and trim the amount of code with nice evaluations, and use comments to pick up the slack to remind others (and yourself) of how it works if there's anything non-obvious about it.

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12 Comments

Ah, that's funny. I was coming from the functional programming mindset, where undefined results are bad. :-)
@Svante - Garbled how? It copies and pastes into the interpreter I got with sudo apt-get install clisp and seems to work as expected. What doesn't work? Also, due to you picking on a body of code trying to demonstrate "don't do this" you are missing the point.
@HostileFork: Typically, you wrap the tests inside the and and put the return value outside the condition. However, if COND is not given a return value, it'll return the value of the test, so this (maybe accidentally) works.
@ArtTaylor: These days, COND is usually a macro on top of IF.
@HostileFork You put the (/ (+ a b) 2) inside the and clause. That's what Svante was talking about. I've fixed it.
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