I have been using Common Lisp a bit lately and have been working on a web-server/framework called “Bog” to server up this website as a way of digging deeper into some practical CL applications. I was wanting to conditionally display some markup content when a URI had a file that matched the path, and if there was no file, then don’t add any additional content, just display the dynamic data. My first iteration did this:

(let ((val (function-returning-content-or-nil)))
  (when val val))

I thought to myself that this might be a good function to bottle into a macro, so I came up with:

(defmacro when-val (f)
  `(let ((val ,f))
    (when val val)))

I was talking about this with a friend and he mentioned you can just call `and`, and sure enough you can!

(and (function-returning-content-or-nil))

This works great and with a couple less calls, but it’s not very readable. Luckily we can just write a quick macro to enhance the readability without adding any overhead to the call:

(defmacro when-yield (f)
  `(and ,f))

And then I realized that you can just call the function, (function-returning-content-or-nil), so this was mostly useless…