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#+Ignore is fine, #+(or) is bliss
Thursday, June 29, 2006

Although Common Lisp has a multiline-comment facility (that even handles nesting), many Lispers tend to use the #+ / #- reader macros to temporarily remove chunks of code from the view of the evaluator (note that these do not remove the code from the view of the reader, an issue which often leads to confusion the first time you can't load code that you think is 'commented' out — which is why it's important to remember that #+/#+ is not a commenting facility even though you may be using it like one...). The reason for this in my case is simple: laziness. I'd rather do something in one place than in two and it's easier to place (and remove) #+ignore in front of a form rather than wrapping the form in #| and |#. Frankly, I think that this sort of laziness is part of the human condition. If we really want most people to do the right thing most of the time, then we need to make it easy and convenient to do the right thing (e.g., recycle, carpool, keeping documentation up to date, and so on).

Using the reader macro to hide code isn't a bad practice as practices go (it's certainly healthier than coding in C or C++ <smile>) but lurking in the back of every coder's mind is that horrible question: "What if," they say to themselves in the still hours of the night, "What if someone pushes :ignore onto the *features* list?" (or :no, :later, :old, :new, :nil, :never, :not-yet, ...)? Well, I was looking at some of Peter Seibel's code recently and found a very nice way to have my cake even after eating it. There is form that is guaranteed to return nil regardless of what is on *features*? That form is #+(or) and it's a very nice thing indeed.


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