The Surprisingly Tense History of the Schwartzian Transform

Alpine Perl Workshop 2016 • brian d foy
The Surprisingly Tense
History of the
Schwartzian Transform

ThePerlReview•www.theperlreview.com
TheSurprisinglyTenseHistoryoftheSchwartzianTransform
http://bit.ly/2bHNNx4

TheSurprisinglyTenseHistoryoftheSchwartzianTransform Randal Schwartz
Just another
hacker.
•troff
•emacs
•Lisp
•Perl
•Smalltalk
•…

Tom Christiansen
•Languages geek
•Csh god
•Perl evangelist

The Technique
Its Expression
The Arguments
Its Name
The Reckoning

decorate
sort
undecorate

compute
associate
sort
map

Richard Stallman
•Editor editor
•GNU
•Free Software

TECO - Text Editor and Corrector
Emacs - Editing MACroS
EINE - E. Is Not Emacs
ZWEI - Z. Was EINE Initially
GNU Emacs

Lisp
(defun schwartzian-transform (list costly-function predicate)
(mapcar #'cdr
(stable-sort (mapcar (lambda (x)
(cons (funcall costly-function x) x))
list)
predicate
:key #'car)))
(require :sb-posix)
(schwartzian-transform
(directory "/etc/*")
(lambda (x) (sb-posix:stat-mtime (sb-posix:stat x)))
#'<=)
http://codereview.stackexchange.com/a/138436/13050

December 16, 1994
#!/usr/bin/perl
require 5; # new features, new bugs!
print
map { $_->[0] }
sort { $a->[1] cmp $b->[1] }
map { [$_, /(S+)$/] }
<>;
Randal Schwartz in comp.unix.shells

Unfamiliar
Complex
Undocumented
Hard to learn
Too clever
New
Different

OMFG
Awesome
New features
Lists of lists
Power tools

Not documented

Easy to learn
Power tools

FMTEYEWTK
I'm not ragging on Randal, merely teasing a bit. He's
just trying to be clever, and that's what he does. I'm
just submitting a sample chapter for his perusal for
inclusion the mythical Alpaca Book :-)
Tom Christiansen
Far More Than Everything You've Ever Wanted to Know About Sorting,
http://www.perl.com/doc/FMTEYEWTK/sort.html

August 30, 1995
Or for possibly more efﬁciency, ensure that
the calls only happen once per record, rather than
approximately NlogN times, with the Schwartz
transformation:-)
@keys =
map { $_->[0] }
sort { $a->[1] <=> $b->[1] or $a cmp $b }
map { [ $_, datexform($foo{$_}) ] } keys
%foo;
Bennett Todd in comp.lang.perl.misc
https://groups.google.com/forum/?hl=en#!topic/comp.lang.perl.misc/fLo0RNV8oW8

January 1996
http://www.stonehenge.com/merlyn/UnixReview/col06.html
Randal explains map-sort-map in Unix Review
I'm going to talk about some sorts of sorts, and
hopefully you'll sort everything out by the time
you're ﬁnished reading. (And no, despite the
similarity to my name, I will not talk about
``random sorts''.)

April 1996
Tom labels a benchmark "Schwartzian XFrom" in
comp.lang.perl.misc for “Read directory in
timestamp order?”
https://groups.google.com/d/msg/comp.lang.perl.misc/pw-Hl4byLnc/yzejRnku3RoJ

July 1996
https://groups.google.com/d/msg/comp.lang.perl.misc/6NEeX4XJx54/nmpMmReMIbcJ
Colin Howarth in comp.lang.perl.misc
"Schwartzian transform of %$self ... help?"

October 1996
Tom posts a draft of perllol
with "Schwartzian Transform"

December 1996
https://groups.google.com/d/msg/comp.lang.perl.misc/fPx42DB2jd8/cC_6osV70mMJ
Tom in comp.lang.perl.misc
I'm still pissed at Randal for having posted it,
especially the way he did.

I hereby propose that the aforementioned
Lispism, yclept “Schwartzian Transform",
be recast in a less Teutonic albeit darker
light; to wit, "Black Transform", as this
spelling more accurately connotes its
sinister subtleties. :-)
Tom later in the same thread

December 1996
I wonder if this chunk of code will
haunt us forever.
Eric Arnold in comp.lang.perl.misc
https://groups.google.com/d/msg/comp.lang.perl.misc/fPx42DB2jd8/CTRmyWyJW6MJ

1999

Orcish Manuever
@sorted = sort {
( $times{$a} ||= -M $a ) <=>
( $times{$b} ||= -M $b )
} @old_array;
Perl 4 compatible version

2000

Python 2.4, 3
sorted = sorted(string_list, key=…)
string_list.sort(key=lambda …)
map(lambda x: x[1],
sorted(
map(lambda x: (f(x), x),
old_list)))
new_list = zip(*
sorted(
zip(map(f, old_list),
old_list)))[1]

Python 2
tmp2 = [ (int(s[10:15]), s) for s in L ]
tmp2.sort()
Isorted = [ x[1] for x in tmp2 ]
sorted([5, 2, 4, 1, 3], cmp=…)

Ruby
[1,1.1,1.2,10].sort_by {|n| n.to_s}

•Innsbruck - CC BY 2.0 by Reisender1701 (Flickr)
https://goo.gl/PuILnA
•Randal Schwartz - CC BY-SA 3.0 by Chris Marquardt
https://commons.wikimedia.org/wiki/File:Randal_Schwartz_headshot_by_Chris_Marquardt.png
•Tom Christiansen - CC BY-SA 3.0 by Tim Bray
https://en.wikipedia.org/wiki/File:Tom_Christiansen_in_2008.jpg
•Richard Stallman - CC BY-SA 3.0 by Thesupermat
https://commons.wikimedia.org/wiki/File:Richard_Stallman_-_Fête_de_l%27Humanité_2014_-_010.jpg

The Surprisingly Tense History of the Schwartzian Transform

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The Surprisingly Tense History of the Schwartzian Transform