I Patched R to Solve an Exercism Problem

With a serious yak shaving deviation, I have a really short “cheat” solution to one of the featured Exercism problems. It’s been a really insightful journey getting to this point, and as always I’ve learned a lot along the way. The fact that I was able to understand the required changes and propose them is thanks to the open-source nature of programming languages.

This all started out innocently enough - I’ve been using Exercism.org to get a feel for a lot of other programming languages. Last year they hosted the 12in23 challenge where they invited users to try out one programming language each month, solving 5 non-trivial (beyond printing "Hello, World!") exercises in any of a handful of featured languages, with a different ‘theme’ (functional, object-oriented, low-level, …) each month.

I ended up earning the “polyglot” badge for completing this, but I actually ended up doing a lot more

I found this to be an extremely useful exercise in finding out what I did and didn’t like about different languages, and I’m diving a lot deeper into the ones I enjoyed the most. Worthwhile, but certainly an involved challenge as I solved all the exercises locally on my own machine, which means I had to install each of the languages and get their test suites working. My laptop is now capable of running code in more than 20 languages.

This year, Exercism are hosting a 48in24 challenge where rather than just needing to use one language to solve several problems in a month, it’s one problem in three languages in a week. Spending a month to get up to speed with a language I’d never seen (or installed) was one thing, but now I potentially need to do that three times over in a week when the languages are all new to me. Still, I persist, and so far I’m keeping up.

One of the featured problems was ‘Roman Numerals’ where we need to write an algorithm to convert integers into Roman numerals. The featured languages were Julia (I’m already familiar, nice), Elixir (fine, I get to learn what that looks like), and (Pharo) smalltalk, which needs its own IDE and is a workflow entirely distinct from anything I’ve done before. I won’t be willingly writing any more smalltalk in the near future.

When I saw that this was the challenge problem I was delighted - I know that R already has as.roman() in the standard library!

## [1] MMXXIV

It even does math with these values - I base most of my real-life conversations with my wife around Simpsons quotes so I’m all-too familiar with this bit

Bart enters the backstage of the lion enclosure at a zoo and sees a sign:

“Caution: exit through door 7 only. All other rooms contain man-eating tigers.”

“Think, Bart. Where have you seen Roman numerals before? I know… Rocky V! That was the fifth one. So, Rocky five plus Rocky two equals… Rocky VII!”Adrian’s Revenge”!”

R can do this math just fine

(five <- as.roman(5))
## [1] V
(two <- as.roman(2))
## [1] II
five + two
## [1] VII

It’s more than just concatenating those, of course

(five + two) * 2
## [1] XIV

I downloaded the exercise and fired up RStudio. I edited the exercise template to just pass the input argument to as.roman() and hit ‘Run Tests’… all tests failed 😢

Ah, as.roman() returns an object of class roman

## [1] "roman"

and the tests have an expect_equal() (not expect_equivalent())

test_that("48 is XLVIII", {
  expect_equal(roman(48), "XLVIII")

Okay, so it won’t be perfectly clean, but it just need to be re-classed

## [1] "MMXXIV"

Re-run the tests, and…

── Failure ('test_roman-numerals.R:105:3'): 3999 is MMMCMXCIX ──────────────────
roman(3999) not equal to "MMMCMXCIX".
1/1 mismatches
x[1]: NA

[ FAIL 1 | WARN 0 | SKIP 0 | PASS 25 ]

What? twenty-five tests pass, one fails? And produces NA?

Back to the problem statement, it says

For this exercise, we are only concerned about traditional Roman numerals, in which the largest number is MMMCMXCIX (or 3,999).

So, why does that fail? I checked the docs for as.roman() and it says

Only numbers between 1 and 3899 have a unique representation as roman numbers, and hence others result in as.roman(NA).


I checked out some other languages. The intro video for the problem mentions another “cheat” solution which is to use Common Lisp, because that has a formatter. Using GNU Common Lisp

(format nil "~@R" 2024)

Trying out this last value works

(format nil "~@R" 3999)

and a too-large value produces an informative error

(format nil "~@R" 4000)
 The ~@R format directive requires an integer in the range 1 - 3999, not 4000

So, the limit does seem to be 3999, not 3899. Checking Wikipedia, that also seems to be the case. I also found a python implementation which goes up to 3999.

So, why won’t R convert those last 100 numbers?

I dug around the source code (an accessible copy is on GitHub but the “real” copy is on SVN) and found several references to the 3899 limit, none of which could be circumvented.

I posted a quick summary of the issue I saw on my side-blog hoping that it was something I just misunderstood.

I asked around on Mastodon, and no one was sure why this was the case. The next step was to email the R-devel mailing list. I don’t do this lightly, as it has a reputation for not being the friendliest place. Kurt Hornik, who originally wrote the implementation back in 2006 replied and agreed that it was probably just an oversight.

He asked if I could file the bug in Bugzilla the R bug tracker … and asked if I could also add a patch. At this point, I remembered what I was doing in the first place - solving an Exercism problem.

The term “Yak Shaving” means performing some task that appears entirely unrelated to what you were originally trying to do, usually as a result of several other related tasks. It’s summarised well in this clip from the TV show ‘Malcolm in the Middle’

where the character goes to change a lightbulb but finds that the shelf is coming loose, so he gets a screwdriver from a drawer and notices that the drawer isn’t running smoothly, so he gets some lubricant but it’s empty so he gets in his car to go to the store to buy more but the car sounds funny, so he tries fixing that… When his wife asks him “are you going to replace that lightbulb” he yells - appearing from underneath the car covered in grease - “what does it look like I’m doing???”.

The absurdity being that “before you can complete this task, you need to shave a yak.”

In this scenario, someone is asking me “are you solving that Exercism problem?” and I’m emerging from a console typing make check-devel shouting “what does it look like I’m doing?”.

So, how do I submit a patch to R itself? I recalled that I detailed some instructions for that the last time I was trying to resolve an R-related bug. I pulled the bleeding-edge SVN commit and built R locally, which means

./configure --with-x=no --without-recommended-packages --disable-byte-compiled-packages --disable-java  


make -j4

to build the source (using 4 cores).

Then, I searched for the values I might need to change by running

grep -R 3899 

This turns up a bunch of files

src/nmath/qnorm.c:                         2.04426310338993978564e-15 + 1.4215117583164458887e-7)*
src/library/utils/all.R:        if(upper > 3899L)
src/library/utils/all.R:            stop(gettextf("too many list items (at most up to %d)", 3899L),
src/library/utils/man/roman.Rd:  Only numbers between 1 and 3899 have a unique representation as roman
src/library/utils/man/roman.Rd:## simple consistency checks -- arithmetic when result is in  {1,2,..,3899} :
src/library/utils/man/format.Rd:    numerals (with the number of the last item maximally 3899).
src/library/utils/R/format.R:        if(upper > 3899L)
src/library/utils/R/format.R:            stop(gettextf("too many list items (at most up to %d)", 3899L),
src/library/utils/po/R-de.po:#~ msgid "too many list items (at most up to number 3899)"
src/library/utils/po/R-de.po:#~ msgstr "zu viele Listenelemente (höchstens 3899!)"
src/library/utils/po/R-fr.po:#~ msgid "too many list items (at most up to number 3899)"
src/library/utils/po/R-fr.po:#~ msgstr "trop d'entrées de liste (3899 maximum)"
src/library/utils/po/R-ja.po:#~ msgid "too many list items (at most up to number 3899)"
src/library/utils/po/R-ja.po:#~ msgstr " リストの項目が多すぎます(最大でも3899です) "
src/library/utils/po/R-zh_CN.po:#~ msgid "too many list items (at most up to number 3899)"
src/library/utils/po/R-zh_CN.po:#~ msgstr "串列项太多(最多只能有3899项("
src/library/utils/po/R-ru.po:#~ msgid "too many list items (at most up to number 3899)"
grep: src/library/utils/po/R-ru.po: binary file matches
src/main/g_her_glyph.c:  /******** Hershey Glyphs 3800 to 3899 ********/
src/main/g_her_glyph.c:  /******** Oriental Hershey Glyphs 3800 to 3899 ********/
tests/d-p-q-r-tests.Rout: [1] 0.340823726 0.100413165 0.293668976 0.389968983 0.124439520 0.207270198
tests/reg-examples2.Rout:Zambia          0.16361 -0.07917 -0.33899  0.09406  0.228232  0.7482 0.512
tests/lm-tests.Rout.save:Zambia          0.16361 -0.07917 -0.33899  0.09406  0.228232  0.7482 0.512
tests/lm-tests.Rout:Zambia          0.16361 -0.07917 -0.33899  0.09406  0.228232  0.7482 0.512
tests/d-p-q-r-tests.Rout.save: [1] 0.340823726 0.100413165 0.293668976 0.389968983 0.124439520 0.207270198
tests/Examples/stats-Ex.Rout.save:[49]  0.29918521  0.05938999 -0.20355761 -0.02439309 -1.14548572 -0.94045141
tests/Examples/stats-Ex.Rout.save: 0.05510098  0.59958858  1.14407618  1.68856379  2.23305139  2.77753899 
tests/Examples/stats-Ex.Rout.save:9   0.7335126 -1.3468740  2.8138992
tests/Examples/stats-Ex.Rout:[49]  0.29918521  0.05938999 -0.20355761 -0.02439309 -1.14548572 -0.94045141
tests/Examples/stats-Ex.Rout: 0.05510098  0.59958858  1.14407618  1.68856379  2.23305139  2.77753899 
tests/Examples/stats-Ex.Rout:9   0.7335126 -1.3468740  2.8138992

Many of these I could ignore - float values which happen to match. But many were familiar from when I was digging through the source.

I took the first of these src/library/utils/all.R and edited the value from 3899 to 3999 then ran svn status and a lot of files were marked ? (untracked) while none were M (modified). This confused me, until I realised that as part of the build process, R creates these all.R files which aren’t the source-of-truth for the code. Changing, instead, src/library/utils/R/format.R the changes were reflected in svn status.

The format changes were from formatting functions I wasn’t previously familiar with, namely formatOL for “format ordered list” which prefixes a list of items

formatOL(paste0("Chapter", 1:5))
## [1] "1. Chapter1" "2. Chapter2" "3. Chapter3" "4. Chapter4" "5. Chapter5"

and can do the conversion

formatOL(paste0("Chapter", 1:5), type = "Roman")
## [1] "  I. Chapter1" " II. Chapter2" "III. Chapter3" " IV. Chapter4"
## [5] "  V. Chapter5"

and which produces an error if you try to pass in too many items (more than the critical value)

formatOL(10:4010, type = "Roman")
Error in formatOL(10:4010, type = "roman") : 
  too many list items (at most up to 3899)

In addition to the logic of the code itself, the .Rd files needed updating (base R does not use Roxygen and automatic generation of man files) and the .po files (language translations) with the translated error messages (which I hopefully corrected accurately; my French is okay-ish and that looks to be correct, and my Japanese is very beginner-level and I haven’t yet learned those words).

Since I had dug through the source, I knew that “3899” was not the only critical value; there were some tests for x >= 3900 I needed to also take care of. This was a useful point to note - if you’re going to change behaviour at a numeric boundary, it’s probably a good idea to stick to either strictly greater than or greater-than-or-equal if you want to make searching for that value straightforward.

Searching for 3900 again turns up a lot of false positives, but also

src/library/utils/R/roman.R:    if(check.range) x[x <= 0L | x >= 3900L] <- NA
src/library/utils/R/roman.R:    ind <- is.na(x) | (x <= 0L) | (x >= 3900L)

so I edited those as well.

Producing the requisite patch is then just

svn diff > as.roman-upper-limit.patch

creating the file I eventually attached to my Bugzilla report.

Kurt Hornik approved these changes and merged them on my behalf (only R-core members can merge into the source) so my changes should be reflected in the next release.

To prove to myself that this was done and dusted, I wanted to pull the latest version again, but I didn’t want to risk that my local changes were what I was seeing. Docker is a good option in this case (and may have been for the development itself).

Beyond my previous instructions, I figured there were more up-to-date instructions out there. This is a slightly newer post about the process, but I eventually just copied the Dockerfile from rocker’s drd image and built the most up-to-date image locally. This means that I was definitely running an independent installation of R (in docker) and not my local copy (still distinct from my installed copy that RStudio sees).

Using this docker image, I could confirm that my changes had been merged


as well as the necessary translations; I noticed that a simple Sys.setenv(LANGUAGE="ru") worked once, but after that I couldn’t change the language again. The {and} package seemed to do the trick

formatOL(1:4000, type = "roman")
Error in formatOL(1:4000, type = "roman") : 
  too many list items (at most up to 3999)
formatOL(1:4000, type = "roman")
Ошибка в formatOL(1:4000, type = "roman") :
  слишком много элементов в списке (самое большее 3999)
formatOL(1:4000, type = "roman")
Erreur dans formatOL(1:4000, type = "roman") : 
  trop d'entrées de listes (3999 maximum)
formatOL(1:4000, type = "roman")
 formatOL(1:4000, type = "roman") でエラー: 
   リストの項目が多すぎます (最大でも 3999 です) 
formatOL(1:4000, type = "roman")
Fehler in formatOL(1:4000, type = "roman") : 
  zu viele Listenpunkte (höchstens 3999)
formatOL(1:4000, type = "roman")
Error in formatOL(1:4000, type = "roman") : 列表项太多(最多只能有3999项)

With all looking good on the R side, now I just need to wait for the changes to be officially released and then for Exercism to update their version to that so that I can submit my “cheat” one-line solution to the Roman Numerals exercise.

I’ll be waiting!

I’m hoping that the process described here can be of use to someone else who finds a similar issue in the language; the process can be fairly straightforward, but I believe success was dependent on a few key points;

  • don’t assume that the language is broken, it may be something you did wrong or misunderstood

  • look through the source to confirm what you’re seeing and do your best to understand it

  • ask around; someone may be able to resolve the confusion or point you in the right direction

  • “it takes a village” - if you want it to get fixed, you might need to put in some work.

And who knows, maybe it is a bug that’s been there for the last 15+ years?

This definitely ended up being more of an exercise than I planned, but I’m grateful for Exercism prompting me to try out both languages I do and don’t know; for R being open-source so I could find this bug; and for the wealth of knowledge out there to figure out how to do all this.

If you have any comments, feel free to use the comment section below, or hit me up on Mastodon.

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See also