Lisp golf

Posted by Holger Schauer in Lisp
Friday, November 9. 2007

Some time ago, I was looking at splitting some text with Elisp, Perl, Ruby and Common Lisp. Yesterday, when I again had to do quite the same thing, it occurred to me that the Common Lisp solution was unnecessary complex/long. I'm not a Perl guru, but I believe the following is probably hard to beat even with Perl:


CL-USER> (format t "~{<li>~A</li>~%~}" (cl-ppcre:split "\\|" "Kim Wilde|Transvision Vamp|Ideal|Siouxsie and the Banshees|Nena|Iggy Pop"))

<li>Kim Wilde</li>
<li>Transvision Vamp</li>
<li>Ideal</li>
<li>Siouxsie and the Banshees</li>
<li>Nena</li>
<li>Iggy Pop</li>
NIL

For the uninitiated, it's not the cl-ppcre library which is interesting here but the built-in iteration facilities of format. See the Hyperspec on the control-flow features of format for details. Now, I usually tend to avoid the mini languages that come with Common Lisp like the one of format or loop when writing real programs, but when using Lisp as a glorified shell they come in very handy.

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In other languages you'd do the iteration with list comprehension, I guess.
E.g. in Python you could do

print "\n".join(["%s" % x for x in "foo|bar|baz".split("|")])

which is arguably more readable -- although I like lisp I find the loop constructs of format quite hard to grok.

#1 H Durer on 2007-11-09 11:58 (Reply)

Of course my version did have the anglebracket - "li" - angle bracket and anglebracket - "/li" - angle bracket text in it. Your blog software swallows that up for some reason.

I also agree on the near-impossibility to pass the CAPTCHA -- had to reload the page a few time to get an image that I could decipher.

#1.1 H Durer on 2007-11-09 12:22 (Reply)

Wrt. to the captchas: I experienced a lot of spam for some time, so I had strong captchas enabled. I've now reverted to the normal ones, I hope these are easier for humans to decipher.

#1.1.1 Holger Schauer on 2007-11-09 13:10 (Reply)

If my Perl isn't too rusty, it't turns out something like this:
print "[li]$_[/li]\n" for split '|', "Whatever"
The iteration is external to format's equivalent, but it's readable enough for me.

PS: your spam prevention is too hard for my human eyes.

#2 Alphonso Nonymous on 2007-11-09 12:02 (Reply)

This is a very trivial thing for anyone who's used to scripting languages. Here's a natural Perl solution:

map "$_\n", split /|/, "foo|bar|baz"

#3 funktio (Homepage) on 2007-11-09 12:37 (Reply)

I hope it displays properly this time:

map "[li]$_[/li]\n", split /\|/, "foo|bar|baz"

#3.1 funktio (Homepage) on 2007-11-09 12:39 (Reply)

Thanks for reminding me of the existence of map in Perl. For some reason I've almost never used it -- Perl seems to be located in the imperative half of my brain.

Wrt. to disappearing elements: Serendipity probably tries to interpret the braces/brackets as HTML elements or as smileys.

#3.1.1 Holger Schauer on 2007-11-09 13:08 (Reply)

Perl meens regexp.

s/([^|]+)|?/[li]${1}[\/li]\n/g

#4 pdw on 2007-11-09 13:14 (Reply)

This one doesn't count as it modifies the input. A similar approach is doable in CL with the same RE library (cl-ppcre) as in the blog post.

#4.1 Holger Schauer on 2007-11-11 14:57 (Reply)

We've been persecuted long enough. I'm going to start a club for Lisp Users Who Actually Like Format And Loop Damnit!

#5 ken on 2007-11-09 21:43 (Reply)

Lisp Users Who Actually Like Format And Loop Damnit -- LUWALFALD. I like that. And if you add the C in front for Club, you actually get the usual CL- prefix.

BTW: It's not like I dislike format or loop. It's more a chicken and egg-problem: I don't use them regularly, so I sometimes find solutions that use them harder to understand than others involving, say, dolist, which I do use regularly. So my comment in the blog post says a lot more about me than about format/loop.

#5.1 Holger Schauer on 2007-11-11 14:55 (Reply)

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