The Universe of Disco

Easy exhaustive search with the list monad

(Haskell people may want to skip this article about Haskell, because the technique is well-known in the Haskell community.)

Suppose you would like to perform an exhaustive search. Let's say for concreteness that we would like to solve this cryptarithm puzzle:

    S E N D
+   M O R E
-----------
  M O N E Y

This means that we want to map the letters S, E, N, D, M, O, R, Y to distinct digits 0 through 9 to produce a five-digit and two four-digit numerals which, when added in the indicated way, produce the indicated sum.

(This is not an especially difficult example; my 10-year-old daughter Katara was able to solve it, with some assistance, in about 30 minutes.)

If I were doing this in Perl, I would write up either a recursive descent search or a solution based on a stack or queue of partial solutions which the program would progressively try to expand to a full solution, as per the techniques of chapter 5 of Higher-Order Perl. In Haskell, we can use the list monad to hide all the searching machinery under the surface. First a few utility functions:

    import Control.Monad (guard)

    digits = [0..9]

    to_number = foldl (\a -> \b -> a*10 + b) 0
    remove rs ls = foldl remove' ls rs
      where remove' ls x = filter (/= x) ls

to_number takes a list of digits like [1,4,3] and produces the number they represent, 143. remove takes two lists and returns all the things in the second list that are not in the first list. There is probably a standard library function for this but I don't remember what it is. This version is !!O(n^2)!!, but who cares.

Now the solution to the problem is:

    --     S E N D
    --   + M O R E
    --   ---------
    --   M O N E Y

    solutions = do
      s <- remove [0] digits
      e <- remove [s] digits
      n <- remove [s,e] digits
      d <- remove [s,e,n] digits
      let send = to_number [s,e,n,d]
      m <- remove [0,s,e,n,d] digits
      o <- remove [s,e,n,d,m] digits
      r <- remove [s,e,n,d,m,o] digits
      let more = to_number [m,o,r,e]
      y <- remove [s,e,n,d,m,o,r] digits
      let money = to_number [m,o,n,e,y]
      guard $ send + more == money
      return (send, more, money)

Let's look at just the first line of this:

    solutions = do
      s <- remove [0] digits
      …

The do notation is syntactic sugar for

    (remove [0] digits) >>= \s -> …

where “…” is the rest of the block. To expand this further, we need to look at the overloading for >>= which is implemented differently for every type. The mote on the left of >>= is a list value, and the definition of >>= for lists is:

    concat $ map (\s -> …) (remove [0] digits)

where “…” is the rest of the block.

So the variable s is bound to each of 1,2,3,4,5,6,7,8,9 in turn, the rest of the block is evaluated for each of these nine possible bindings of s, and the nine returned lists of solutions are combined (by concat) into a single list.

The next line is the same:

      e <- remove [s] digits

for each of the nine possible values for s, we loop over nine value for e (this time including 0 but not including whatever we chose for s) and evaluate the rest of the block. The nine resulting lists of solutions are concatenated into a single list and returned to the previous map call.

      n <- remove [s,e] digits
      d <- remove [s,e,n] digits

This is two more nested loops.

      let send = to_number [s,e,n,d]

At this point the value of send is determined, so we compute and save it so that we don't have to repeatedly compute it each time through the following 300 loop executions.

      m <- remove [0,s,e,n,d] digits
      o <- remove [s,e,n,d,m] digits
      r <- remove [s,e,n,d,m,o] digits
      let more = to_number [m,o,r,e]

Three more nested loops and another computation.

      y <- remove [s,e,n,d,m,o,r] digits
      let money = to_number [m,o,n,e,y]

Yet another nested loop and a final computation.

      guard $ send + more == money
      return (send, more, money)

This is the business end. I find guard a little tricky so let's look at it slowly. There is no binding (<-) in the first line, so these two lines are composed with >> instead of >>=:

      (guard $ send + more == money) >> (return (send, more, money))

which is equivalent to:

      (guard $ send + more == money) >>= (\_ -> return (send, more, money))

which means that the values in the list returned by guard will be discarded before the return is evaluated.

If send + more == money is true, the guard expression yields [()], a list of one useless item, and then the following >>= loops over this one useless item, discards it, and returns yields a list containing the tuple (send, more, money) instead.

But if send + more == money is false, the guard expression yields [], a list of zero useless items, and then the following >>= loops over these zero useless items, never runs return at all, and yields an empty list.

The result is that if we have found a solution at this point, a list containing it is returned, to be concatenated into the list of all solutions that is being constructed by the nested concats. But if the sum adds up wrong, an empty list is returned and concated instead.

After a few seconds, Haskell generates and tests 1.36 million choices for the eight bindings, and produces the unique solution:

    [(9567,1085,10652)]

That is:

    S E N D            9 5 6 7 
+   M O R E        +   1 0 8 5
-----------        -----------
  M O N E Y          1 0 6 5 2

It would be an interesting and pleasant exercise to try to implement the same underlying machinery in another language. I tried this in Perl once, and I found that although it worked perfectly well, between the lack of the do-notation's syntactic sugar and Perl's clumsy notation for lambda functions (sub { my ($s) = @_; … } instead of \s -> …) the result was completely unreadable and therefore unusable. However, I suspect it would be even worse in Python because of semantic limitations of that language. I would be interested to hear about this if anyone tries it.

[ Addendum: Thanks to Tony Finch for pointing out the η-reduction I missed while writing this at 3 AM. ]

[ Addendum: Several people so far have misunderstood the question about Python in the last paragraph. The question was not to implement an exhaustive search in Python; I had no doubt that it could be done in a simple and clean way, as it can in Perl. The question was to implement the same underlying machinery, including the list monad and its bind operator, and to find the solution using the list monad.

[ Peter De Wachter has written in with a Python solution that clearly demonstrates that the problems I was worried about will not arise, at least for this task. I hope to post his solution in the next few days. ]

[ Addendum 20150803: De Wachter's solution and one in Perl ]

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Another use for strace (isatty)

(This is a followup to an earlier article describing an interesting use of strace.)

A while back I was writing a talk about Unix internals and I wanted to discuss how the ls command does a different display when talking to a terminal than otherwise:

ls to a terminal

ls not to a terminal

How does ls know when it is talking to a terminal? I expect that is uses the standard POSIX function isatty. But how does isatty find out?

I had written down my guess. Had I been programming in C, without isatty, I would have written something like this:

    @statinfo = stat STDOUT;
    if (    $statinfo[2] & 0060000 == 0020000
        && ($statinfo[6] & 0xff) == 5) { say "Terminal" }
    else { say "Not a terminal" }

(This is Perl, written as if it were C.) It uses fstat (exposed in Perl as stat) to get the mode bits ($statinfo[2]) of the inode attached to STDOUT, and then it masks out the bits the determine if the inode is a character device file. If so, $statinfo[6] is the major and minor device numbers; if the major number (low byte) is equal to the magic number 5, the device is a terminal device. On my current computers the magic number is actually 136. Obviously this magic number is nonportable. You may hear people claim that those bit operations are also nonportable. I believe that claim is mistaken.

The analogous code using isatty is:

    use POSIX 'isatty';
    if (isatty(STDOUT)) { say "Terminal" } 
    else { say "Not a terminal" }

Is isatty doing what I wrote above? Or something else?

Let's use strace to find out. Here's our test script:

    % perl -MPOSIX=isatty -le 'print STDERR isatty(STDOUT) ? "terminal" : "nonterminal"'
    terminal
    % perl -MPOSIX=isatty -le 'print STDERR isatty(STDOUT) ? "terminal" : "nonterminal"' > /dev/null
    nonterminal

Now we use strace:

    % strace -o /tmp/isatty perl -MPOSIX=isatty -le 'print STDERR isatty(STDOUT) ? "terminal" : "nonterminal"' > /dev/null
    nonterminal
    % less /tmp/isatty

We expect to see a long startup as Perl gets loaded and initialized, then whatever isatty is doing, the write of nonterminal, and then a short teardown, so we start searching at the end and quickly discover, a couple of screens up:

    ioctl(1, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7ffea6840a58) = -1 ENOTTY (Inappropriate ioctl for device)
    write(2, "nonterminal", 11)             = 11
    write(2, "\n", 1)                       = 1

My guess about fstat was totally wrong! The actual method is that isatty makes an ioctl call; this is a device-driver-specific command. The TCGETS parameter says what command is, in this case “get the terminal configuration”. If you do this on a non-device, or a non-terminal device, the call fails with the error ENOTTY. When the ioctl call fails, you know you don't have a terminal. If you do have a terminal, the TCGETS command has no effects, because it is a passive read of the terminal state. Here's the successful call:

    ioctl(1, SNDCTL_TMR_TIMEBASE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
    write(2, "terminal", 8)                 = 8
    write(2, "\n", 1)                       = 1

The B38400 opost… stuff is the terminal configuration; 38400 is the baud rate.

(In the past the explanatory text for ENOTTY was the mystifying “Not a typewriter”, even more mystifying because it tended to pop up when you didn't expect it. Apparently Linux has revised the message to the possibly less mystifying “Inappropriate ioctl for device”.)

(SNDCTL_TMR_TIMEBASE is mentioned because apparently someone decided to give their SNDCTL_TMR_TIMEBASE operation, whatever that is, the same numeric code as TCGETS, and strace isn't sure which one is being requested. It's possible that if we figured out which device was expecting SNDCTL_TMR_TIMEBASE, and redirected standard output to that device, that isatty would erroneously claim that it was a terminal.)

[ Addendum 20150415: Paul Bolle has found that the SNDCTL_TMR_TIMEBASE pertains to the old and possibly deprecated OSS (Open Sound System) It is conceivable that isatty would yield the wrong answer when pointed at the OSS /dev/dsp or /dev/audio device or similar. If anyone is running OSS and willing to give it a try, please contact me at mjd@plover.com. ]

[ Addendum 20191201: Thanks to Hacker News user jwilk for pointing out that strace is now able to distinguish TCGETS from SNDCTL_TMR_TIMEBASE. ]

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Another use for strace (groff)

The marvelous Julia Evans is always looking for ways to express her love of strace and now has written a zine about it. I don't use strace that often (not as often as I should, perhaps) but every once in a while a problem comes up for which it's not only just the right thing to use but the only thing to use. This was one of those times.

I sometimes use the ancient Unix drawing language pic. Pic has many good features, but is unfortunately coupled too closely to the Roff family of formatters (troff, nroff, and the GNU project version, groff). It only produces Roff output, and not anything more generally useful like SVG or even a bitmap. I need raw images to inline into my HTML pages. In the past I have produced these with a jury-rigged pipeline of groff, to produce PostScript, and then GNU Ghostscript (gs) to translate the PostScript to a PPM bitmap, some PPM utilities to crop and scale the result, and finally ppmtogif or whatever. This has some drawbacks. For example, gs requires that I set a paper size, and its largest paper size is A0. This means that large drawings go off the edge of the “paper” and gs discards the out-of-bounds portions. So yesterday I looked into eliminating gs. Specifically I wanted to see if I could get groff to produce the bitmap directly.

GNU groff has a -Tdevice option that specifies the "output" device; some choices are -Tps for postscript output and -Tpdf for PDF output. So I thought perhaps there would be a -Tppm or something like that. A search of the manual did not suggest anything so useful, but did mention -TX100, which had something to do with 100-DPI X window system graphics. But when I tried this groff only said:

    groff: can't find `DESC' file
    groff:fatal error: invalid device `X100`

The groff -h command said only -Tdev use device dev. So what devices are actually available?

strace to the rescue! I did:

    % strace -o /tmp/gr groff -Tfpuzhpx

and then a search for fpuzhpx in the output file tells me exactly where groff is searching for device definitions:

    % grep fpuzhpx /tmp/gr
    execve("/usr/bin/groff", ["groff", "-Tfpuzhpx"], [/* 80 vars */]) = 0
    open("/usr/share/groff/site-font/devfpuzhpx/DESC", O_RDONLY) = -1 ENOENT (No such file or directory)
    open("/usr/share/groff/1.22.2/font/devfpuzhpx/DESC", O_RDONLY) = -1 ENOENT (No such file or directory)
    open("/usr/lib/font/devfpuzhpx/DESC", O_RDONLY) = -1 ENOENT (No such file or directory)

I could then examine those three directories to see if they existed, and if so find out what was in them.

Without strace here, I would be reduced to groveling over the source, which in this case is likely to mean trawling through the autoconf output, and that is something that nobody wants to do.

Addendum 20150421: another article about strace. ]

[ Addendum 20150424: I did figure out how to prevent gs from cropping my output. You can use the flag -p-P48i,48i to groff to set the page size to 48 inches (48i) by 48 inches. The flag is passed to grops, and then resulting PostScript file contains

   %%DocumentMedia: Default 3456 3456 0 () ()

which instructs gs to pretend the paper size is that big. If it's not big enough, increase 48i to 120i or whatever. ]

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I'm old

This week I introduced myself to Recurse Center, where I will be in residence later this month, and mentioned:

I have worked as a professional programmer for a long time so I sometimes know strange historical stuff because I lived through it.

Ms. Nikki Bee said she wanted to hear more. Once I got started I had trouble stopping.

I got interested in programming from watching my mom do it. I first programmed before video terminals were common. I still remember the smell of the greasy paper and the terminal's lubricating oil. When you typed control-G, the ASCII BEL character, a little metal hammer hit an actual metal bell that went "ding!".

I remember when there was a dedicated computer just for word processing; that's all it did. I remember when hard disks were the size of washing machines. I remember when you could buy magnetic cores on Canal Street, not far from where Recurse Center is now. Computer memory is still sometimes called “core”, and on Unix your program still dumps a core file if it segfaults. I've worked with programmers who were debugging core dumps printed on greenbar paper, although I've never had to do it myself.

I frequented dialup BBSes before there was an Internet. I remember when the domain name system was rolled out. Until then email addresses looked like yuri@kremvax, with no dots; you didn't need dots because each mail host had a unique name. I read the GNU Manifesto in its original publication in Dr. Dobb's. I remember the day the Morris Worm hit.

I complained to Laurence Canter after he and his wife perpetrated the first large scale commercial spamming of the Internet. He replied:

People in your group are interested. Why do you wish to deprive them of what they consider to be important information??

which is the same excuse used by every spammer since.

I know the secret history of the Java compiler, why Java 5.0 had generics even though Sun didn't want them, and why they couldn't get rid of them. I remember when the inventors of LiveScript changed its name to JavaScript in a craven attempt to borrow some of Java's buzz.

I once worked with Ted Nelson.

I remember when Sun decided they would start charging extra to ship C compilers with their hardware, and how the whole Internet got together to fund an improved version of the GNU C compiler that would be be free and much better than the old Sun compiler ever was.

I remember when NCSA had a web page, updated daily, called “What's New on the World Wide Web”. I think I was the first person to have a guest book page on the Web. I remember the great land rush of 1996 when every company woke up at the same time and realized it needed a web site.

I remember when if you were going to speak at a conference, you would mail a paper copy of your slides to the conference people a month before so they could print it into books to hand out to the attendees. Then you would photocopy the slides onto plastic sheets so you could display them on the projector when you got there. God help you if you spilled the stack of plastic right before the talk.

tl;dr i've been around a while.

However, I have never programmed in COBOL.

[ Addendum 20150609: I'm so old, I once attended a meeting at which Adobe was pitching their new portable document format. ]

(I'm not actually very old, but I got started very young.)

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Your kids will love a cookie-decorating party

We had a party last week for Toph's 7th birthday, at an indoor rock-climbing gym, same as last year. Last year at least two of the guests showed up and didn't want to climb, so Lorrie asked me to help think of something for them to do if the same thing happened this year. After thinking about it, I decided we should have cookie decorating.

This is easy to set up and kids love it. I baked some plain sugar cookies, bought chocolate, vanilla, and strawberry frosting, several tubes of edible gel, and I mixed up five kinds of colored sugar. We had some colored sprinkles and little gold dragées and things like that. I laid the ingredients out on the table in the gym's side room with some plastic knives and paintbrushes, and the kids who didn't want to climb, or who wanted a break from climbing, decorated cookies. It was a great success. Toph's older sister Katara had hurt her leg, and couldn't climb, so she helped the littler kids with cookies. Even the tiny two-year-old sister of one of the guests was able to participate, and enjoyed playing with the dragées.

(It's easy to vary the project depending on how much trouble you want to take. I made the cookies from scratch, which is pretty easy, but realized later I could have bought prefabricated cookie batter, which would have been even easier. The store sold colored sugar for $3.29 for four ounces, which is offensive, so I went home and made my own. You put one drop of food coloring per two ounces of sugar in a sealed container and shake it up for a minute, for a total cost of close to zero; Toph helped with this. I bought my frosting, but when my grandmother used to do it she'd make a simple white frosting from confectioners' sugar and then color it with food coloring.)

I was really pleased with the outcome, and not just because the guests liked it, but also because it is a violation of gender norms for a man to plan a cookie-decorating activity and then bake the cookies and prepare the pastel-colored sugar and so forth. (And of course I decorated some cookies myself.) These gender norms are insidious and pervasive, and to my mind no opportunity to interfere with them should be wasted. Messing with the gender norms is setting a good example for the kids and a good example for other dads and for the rest of the world.

I am bisexual, and sometimes I feel that it doesn't affect my life very much. The sexual part is mostly irrelevant now; I fell in love with a woman twenty years ago and married her and now we have kids. I probably won't ever have sex with another man. Whatever! In life you make choices. My life could have swung another way, but it didn't.

But there's one part of being bisexual that has never stopped paying dividends for me, and that is that when I came out as queer, it suddenly became apparent to me that I had abandoned the entire gigantic structure of how men are supposed to behave. And good riddance! This structure belongs in the trash; it completely sucks. So many straight men spend a huge amount of time terrified that other straight men will mock them for being insufficiently manly, or mocking other straight men for not being sufficiently manly. They're constantly wondering "if I do this will the other guys think it's gay?" But I've already ceded that argument. The horse is out of the barn, and I don't have to think about it any more. If people think what I'm doing is gay, that's a pill I swallowed when I came out in 1984. If they say I'm acting gay I'll say "close, but actually, I'm bi, and go choke on a bag of eels, jackass."

You don't have to be queer to opt out of straight-guy bullshit, and I think I would eventually have done it anyway, but being queer made opting out unavoidable. When I was first figuring out being queer I spent a lot of time rethinking my relationship to society and its gender constructions, and I felt that I was going to have to construct my own gender from now and that I no longer had the option of taking the default. I wasn't ever going to follow Rule Number One of Being a Man (“do not under any circumstances touch, look at, mention, or think about any dick other than your own”), so what rules was I going to follow? Whenever someone tried to pull “men don't” on me, (or whenever I tried to pull it on myself) I'd immediately think of all the Rule Number One stuff I did that “men don't” and it would all go in the same trash bin. Where (did I say this already?) it belongs.

Opting out frees up a lot of mental energy that I might otherwise waste worrying about what other people think of stuff that is none of their business, leaving me more space to think about how I feel about it and whether I think it's morally or ethically right and whether it's what I want. It means that if someone is puzzled or startled by my pink sneakers, I don't have to care, except I might congratulate myself a little for making them think about gender construction for a moment. Or the same if people find out I have a favorite flower (CROCUSES YEAH!) or if I wash the dishes or if I play with my daughters or watch the ‘wrong’ TV programs or cry or apologize for something I did wrong or whatever bullshit they're uncomfortable about this time.

Opting out frees me up to be a feminist; I don't have to worry that a bunch of men think I'm betraying The Team, because I was never on their lousy team in the first place.

And it frees me up to bake cookies for my kid's birthday party, to make a lot of little kids happy, and to know that that can only add to, not subtract from, my identity. I'm Dominus, who loves programming and mathematics and practicing the piano and playing with toy octopuses and decorating cookies with a bunch of delightful girls.

This doesn't have to be a big deal. Nobody is likely to be shocked or even much startled by Dad baking cookies. But these tiny actions, chipping away at these vile rules, are one way we take tiny steps toward a better world. Every kid at that party will know, if they didn't before, that men can and do decorate cookies.

And perhaps I can give someone else courage to ignore some of that same bullshit that prevents all of us from being as great as we could and should be, all those rules about stuff men aren't supposed to do and other stuff women aren't supposed to do, that make everyone less. I decided about twenty years ago that that was the best reason for coming out at all. People are afraid to be different. If I can be different, maybe I can give other people courage and comfort when they need to be different too. As a smart guy once said, you can be a light to the world, like a city on a hilltop that cannot be hidden.

And to anyone who doesn't like it, I say:

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