Lisp mentality vs. Python mentality

Carl Banks pavlovevidence at gmail.com
Sat Apr 25 08:06:30 CEST 2009


In answering the recent question by Mark Tarver, I think I finally hit
on why Lisp programmers are the way they are (in particular, why they
are often so hostile to the "There should only be one obvious way to
do it" Zen).

Say you put this task to a Lisp and a Python programmer: Come up with
a good, generic, reusable way to compare two lists.  What are their
respective trains of thought?


Lisp programmer:

Well, there is a standard function called mismatch that does it, but I
can't recommend it.  First of all, you don't know where that
function's been.  Anyone and their mother could have worked on it, did
they have good, sound programming practice in mind when they wrote
it?  Of course not.  Let's be real here, we have to implement this by
hand.

(defun lists-are-equal (a b)
   (or (and (not a) (not b))
       (and (= (car a) (car b)) (lists-are-equal (cdr a) (cdr b))))

There, much better than the standard function, and better yet, it's in
the *absolute minimal form possible*.  There is no way to express list
comparison in a more reduced form.  It's almost erotic how awesome it
is.  I'm---whoa, ok, I'm getting a little excited now, settle down.
Well, come to think of it, that's really not that good.  First of all
it's a function.  I mean, it just sits there and does nothing till you
call it.  How boring is that?  It can't react to the current
situation.  Plus it compares all lists the same way, and that is
really inefficient.  Every list compare is a new problem.  Different
lists need different comparative strategies.  This function simply
won't do.  I need a macro that can intelligently compile the right
list compare methodology in.  For instance, if we want to compare two
lists that are known at compile time, we don't want to waste time
comparing them at runtime.  No, the macro must detect constant
arguments and special case them.  Good start.  Now, we have to
consider the conditions this comparison is being done under.  If the
user is passing the result of a sort to this macro, it's almost
certain that they are trying to see whether the lists have the same
elements.  We can do that a lot more efficiently with a countset.  So
let's have the macro check to see if the forms passed to it are all
sort calls.  Better yet, let's check for my own powerful sort macro.
Hmm.  Wait... I think my 4600-line sort macro already checks its
calling context to see if its results are being fed to a list
comparison.  I'll have to refactor that together with this macro.  Ok,
good, now I am sure other users will eventually want to customize list
comparison for their own use, after all every list comparison is
different and I can't possibly anticipate all of them.  A user needs
to be able to adapt to the situation, so it's vitally important to
create a plug-in infrastructure to give them that flexibility.  Now,
what about exceptions, there's a millions ways to deal with that...

...and so on until eyelids can no longer stay open....



Python programmer:

a == b.  Next question.



Carl Banks, who might be exaggerating

...a little.



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