The Last Line Effect and Extralinguistic Mechanisms

Since the release of the Racket Manifesto by Felleisen, et al. it has become a thesis of mine that normal programmers face, on a regular basis, real problems which could be alleviated by the sorts of macro mechanisms offered by Racket (and to a lesser extent, other Lisps). I’ve got in mind some in-depth investigations I’d like to perform to really back this argument up, but until I have time for that, a recent article will serve as a sort of teaser for what I’ve got in mind.

Andrey Karpov wrote in the “The Last Line Effect”:

When writing program code, programmers often have to write a series of similar constructs. Typing the same code several times is boring and inefficient. That’s why they use the Copy-Paste method: a code fragment is copied and pasted several times with further editing. Everyone knows what is bad about this method: you risk easily forgetting to change something in the pasted lines and thus giving birth to errors. Unfortunately, there is often no better alternative to be found.

Now let’s speak of the pattern I discovered. I figured out that mistakes are most often made in the last pasted block of code.

Andrey goes on to provide some evidence for his assertion. I think he’s on to something, and it makes sense. You do your pasting, then you set your hands in motion with some repeated delete delete delete type type type down arrow, repeat. By the time you’ve done that three or four times, your mind is numb, your eyes are glazed, and the muscles aren’t being supervised anymore.

I’d like to point out and discuss two of his examples:

Example, Chromium

if (access & FILE_WRITE_DATA)
if (access & FILE_WRITE_EA)
if (access & FILE_WRITE_EA)

They’ve got some bitmasks that are referenced via variables (they’re probably #defines, but it doesn’t matter), and when the relevant bits are set, they’d like to write out the name of the bitmask that was set. Turns out this could actually be implemented as a function:

void check_and_append(access, mask, output, string)
  if (access & mask)
    output.append(ASCIIToUTF16(concat("\t", string, "\n")));
check_and_append(access, FILE_WRITE_ATTRIBUTES,
                 output, "FILE_WRITE_ATTRIBUTES");

Hmm. Not really an improvement. We’re still duplicating things. And the repetition of access and output is tedious. I’d expect most programmers to copy and paste the function call. Back where we started.

The C macro system happens to be powerful enough to address this case:

#define C_A(a,o,s) \
  if (a & s) \
      o.append(ASCIIToUTF16("\t" #s "\n"));

C_A(access, output, FILE_WRITE_ATTRIBUTES)

will expand to what we want.

This accomplishes our goal, because the macro is able to see the symbol FILE_WRITE_ATTRIBUTES, rather than just the value of 0x00100 or whatever it might happen to be.

We could shorten the uses of C_A by having the macro definition refer directly to access and output. But sooner or later, that will trip over standard macro hygiene problems.

Example, Multi Theft Auto

CWaterPoly* CWaterManagerSA::CreateQuad (....)
  pInterface->m_wVertexIDs [ 0 ] = pV1->GetID ();
  pInterface->m_wVertexIDs [ 1 ] = pV2->GetID ();
  pInterface->m_wVertexIDs [ 2 ] = pV3->GetID ();
  pInterface->m_wVertexIDs [ 3 ] = pV4->GetID ();

Ok, again, a C macro could help:

#define setter(i) \
  {pInterface->m_wVertexIDs [ (i-1) ] = (pV ## i)->GetID ();}

But, come on, this begs to be in a for loop:

#define setter(i) \
  {pInterface->m_wVertexIDs [ (i-1) ] = (pV ## i)->GetID ();}
   for(int i=1; i<=4; i++) {

And, unsurprisingly, that doesn’t work. Perhaps a more clever C preprocessor could notice that the for loop is static, and generate all of the appropriate code. Maybe there’s even some option I don’t remember floating around that would work with existing preprocessors. Frankly, I’m pleased I remember how to do the stringification and concatenation.

We could write a Racket macro that covers this:

(define-syntax (assign stx)
  (define (trailing-digits expr)
      (regexp-replace* #px"^.*?(\\d+)$"
                       (symbol->string (syntax->datum expr))
  (syntax-case stx ()
    [(_ target source ...)
     (with-syntax ([(i ...)
                    (map trailing-digits
		         (syntax->list #'(source ...)))])
           (vector-set! target (sub1 i) source)

That’s not short. But if you’re going to be writing a lot of code where you want to pull values out of variables with numbers in their names, and stuff those values into corresponding positions in arrays, wouldn’t it be nice to have? How many times would you need to use it before it became worth the effort, in order to avoid the copying and pasting?

How much effort would it be worth to eliminate a class of errors?


Refer back to the bold portion of Andrey’s text: “there is often no better alternative to be found”. What? We’re programming! We’re providing instructions to a machine, which will be faithfully executed on our behalf, regardless of how tedious and repetitive they may be, and there’s no better alternative than copying and pasting?

When the Racket manifesto discusses Racket’s design principles, the authors write:

When programmers must resort to extra-linguistic mechanisms to solve a problem, the chosen language has failed them.

I assert that copying and pasting code is an extra-linguistic mechanism for expressing repetition the language is incapable of. When you are left with no choice but to copy and paste code, your language has failed you.

Musical Interlude 1

Sorry about all the fancy embedded things.

More SciCast-related CDF’s coming soon.

LMSR Made Interactive

The Logarithmic Market Scoring Rule (LMSR) is the rule which governs the price of trades on SciCast. SciCast’s predecessor, DAGGRE, produced a tutorial video on the LMSR.

I thought it might help if people could play with the LMSR directly, to get a feel for how it behaves. I looked around at my options for making interactive mathematical plots, and decided that the best option for the amount of time I was willing to put in was Mathematica CDF files. You’ll need the CDF player installed.

You should have two sliders, “start” and “tradeto”, as well as a graph with two curves on it, one red, the other green. If not, something has gone awry. Sorry.

The value of “start” represents where the trade is starting from; the current state of the market. “tradeto” is the value of the trade you’re considering making (this assumes power mode). “start” and “tradeto” are depicted visually on the graph by vertical lines labelled “S” and “T”. “start” controls a second line, labelled ▽.

Where T intersects with the Yes (green) and No (red) lines, there is a number which is the points you’ll gain (positive) or lose (negative), if the question resolves as Yes, or No, respectively. Between the S and ▽ lines, any trade you make has the potential to earn more than it risks. Everywhere between those lines has decimal odds of ≥2.0.

I have a number of other interactive graphs I’m planning to put up, but I’m still tweaking them. If you’ve got access to Mathematica, and you’d care to root around in my scicasting repo you can find my more-or-less in progress Mathematica notebook where I’m developing them.

On a programming note, these graphs are the most complicated things I’ve ever bothered to do in Mathematica. I’m pleased to discover that a lot of the horrifying looking Mathematica code I’ve seen on their blog, and elsewhere on the Internet over the years, was much worse than it had to be. Apparently nobody believes in adding whitespace to format things nicely. On the downside, it still hurts to read.

GitHub repository for my forecasting code

I’ve put up a GitHub repository of the code I’ve used to aid my forecasting.

Right now I’ve just got the previously discussed Bluefin-21 model, a model for the “Will NASA land a telepresence robot on Venus by 2020?” question, and some code that pulls down citation information from Google Scholar and crunches it for “Will there be at least one female winner of a Fields Medal in 2014?”.

venus launch windows

My Venus model presupposes there are three launch windows (2015, 2016 and 2018) for Venus. I also mention that in my comment on SciCast, though I don’t mention how I determined that.

I’m going to assume you have a reasonable mental model of planets, and their orbits around the Sun. Further, you believe that with current propulsion techniques, there are better and worse times to launch when you want to get to another planet.

Ok, first step, you hit up JPL’s HORIZONS web site. Set your Target Body to Venus (it’ll be 299; that’s the NAIF SPICE ID for Venus), the Time Span to run from 2014-7-1 to 2020-1-1 with a step size of 1 calendar month. Pull up the table settings and turn off everything except #20 Observer range & range-rate. Hit Generate Ephemeris. Bam, you’ve got a table of how far apart Venus and the Earth are, on a monthly basis, for the chunk of time under consideration in the question.

You could plot it, and you’d get something looking suspiciously like this:

Earth-Venus Distance 2014-06 to 2020-01

Hey look, three dips, almost in the three years I mention. Those correspond to the points when the planets are closest, and are roughly the best time to try and get from the Earth to Venus. Since Venus zips around the Sun quite a bit faster than the Earth, every close approach is Venus “catching up” with us.

The best plan for launching, then, is to kick it out of orbit into the direction of Venus, and let Venus catch up to it. The transit then just takes a few months. You can work out a good time to launch by shaving a few months off each of the minimums on the graph. That’s why the 2017 dip corresponds to a (late) 2016 launch window.

SciCast, Bluefin-21, and GeNIe

I’m going to assume you’re familiar with SciCast; if you aren’t, that link is the place to start. Or maybe Wikipedia.

There has been a open question on SciCast, “Will Bluefin-21 locate the black box from Malaysian Airlines flight MH-370?”, since mid-April. (If you don’t know what MH370 is, I envy you.) It dropped fairly quickly to predicting that there was a 10% chance of Bluefin-21 locating MH370. Early on, that was reasonable enough. There was evidence pings from the black box had been detected in the region, so the entire Indian Ocean had been narrowed down to a relatively small area.

Unfortunately weeks passed and on May 29th Bluefin-21’s mission was completed, unsuccessfully. Bluefin-21 then stopped looking. At this point, I (and others) expected the forecast to plummet. But folks kept pushing it back up. In fact I count about 5 or 6 distinct individuals who moved the probability up after completion of the mission. There are perfectly good reasons related to the nature of the prediction market for some of those adjustments.

I’m interested in the bad reasons.

One of the bad reasons that could apply to any question, but seems to be a very likely candidate for this question, is being unaware of the state of the world. Forecasting the future when you don’t know the present just isn’t going to turn out very well. In the best case, you’ve got really good priors, and you’re stuck working off of those. Realistically, your priors aren’t that great. Evidence could help them out, but you’re not updating.

In the specific case of the Bluefin-21, it was suggested (by someone with whom I have no beef, and am not trying to single out!) that there was still a chance MH370 would be discovered by going back and reanalyzing the data.

This didn’t seem at all likely to me. I attempted to explain why, though in retrospect I think my explanation was poor, and incomplete.

To formalize my reasoning, I hunted around for a software tool that would let me crunch the numbers and make a diagram. I turned up GeNIe, which looked appropriate for the task. GeNIe is a GUI on top of SMILE, which is a software library for (hand wave) modeling Bayesian networks.

I ended up producing this model and looks like this:

Before Evidence

(If you want to fiddle with the model, you can download it here.)

This model assumes Bluefin-21 is 90% effective on the first pass, and reanalysis has a 20% chance to find MH370 if it was missed. It also assumes that reanalysis will almost certainly happen, but that there is only a 10% chance that MH370 is even in the search area. I feel that’s generous given the search area, and nature of the acoustic evidence. (I never thought the surface debris was likely to be related. There’s a lot of crap in the ocean. That they found so little is just evidence of the size of the ocean.)

Based on all of that, the model says there’s a 9% chance Bluefin-21 would find MH370. Which is about where the SciCast prediction was at. The other forecasters might’ve been giving the pings more credit. Anyways, we’re kind of in the ballpark. So let’s now tell the model that Bluefin-21 didn’t find MH370 on the first pass, and see what it says:

After Evidence

Weee, the chance that Bluefin-21 will find anything has now dropped to basically zero. And that’s because not finding the MH370 on the first pass is evidence that it isn’t there, even if Bluefin-21 is imperfect. In fact, the better Bluefin-21 is, the stronger the evidence of absence. For any chance of detection to remain, Bluefin-21 would have to be good at collecting data, but the analysis would have to be awful and error prone, unless you repeated it. I didn’t attempt to model that in, because it would be so weird.

First, sonar is well understood, and Bluefin-21 was using COTS units that basically produce a elevation model of the ocean floor. Second, if “trying harder” upped the odds of detection, then the operators would throw more CPU or man power at it until they’d raised the likelihood of detection as high as it was reasonably going to go. The cost of the search had hit at least $44 million before Bluefin-21 got wet; some additional data analysis isn’t going to put a dent in that. (Maybe you could find a signal processing researcher to apply some cutting edge stuff, but if that didn’t happen at the start of the search, I don’t think it’ll happen before the question expires.)

This question is pretty much over, but if you have suggestions for making the model better, I’d be interested in hearing them. This is the first thing I’ve done with this sort of modeling tool. So I’m sure some things could be done better.


I plan to make further use of GeNIe for my SciCasting. For starters, the models make it easier to break apart the separate components of my belief. I can also tweak nodes and have the machine handle propagating the change, which will be much more accurate than fuzzily doing it in my head.

Using some of the fancier(?) features should also allow me to focus my forecasting efforts and points on the questions where I can get the best returns. (Basically by answering the obvious question of “Am I sure enough to make it worth it?”)

Finally, I’d like to put my models up like this, along with some explanation. That way I can demonstrate that I’m predicting the future on the basis of cunning and wit, not piles of luck. ☺