Pune Rains Lemme start with a slightly funny picture that tells as much about Pune, my city as anything else does.

Pune- Leave your attitude behind, we have our own

This and similar tags, puns and whatnot you will find if you are entering Pune from the road highway. You can also find similar similar symbols and Puns all over the city and they are partly sarcasm and ironic and partly the truth. Puneities work from the attitude that they know everything rather than nothing, including yours truly . What is the basis of that or why is there such kind of confidence I have no clue or idea, it is what it is. Approximately 24 hrs. ago, apparently we had a cloudburst. What I came to know later is that we got 100 mm of rain. Sharing from local news site. Much more interesting was a thread made on Reddit where many people half-seriously asked where they can buy a boat. One of the reasons being even if it s October, in fact, we passed middle of October and it s still raining. Even today in the evening, it rained for quite a while. As I had shared in a few blog posts before, June where rains should have started, it didn t, it actually started late July or even August, so something has shifted. The current leadership does not believe in Anthropogenic Climate Change or human activity induced climate change even though that is a reality. I could share many links and even using the term above should give links to various studies. Most of the people who are opposed to it are either misinformed or influenced from the fossil fuel industry. Again, could share many links, but will share just one atm. I have talked to quite a few people about it but nobody has ever been able to give a convincing answer as to why GM had to crush the cars. Let s even take the argument that it was the worst manufactured car in history and there have been quite a few, have the others been crushed? If not, then the reason shared or given by most people sounds hollow. And if you look into it, they had an opportunity that they let go, and now most of them are scrambling and yet most of the legacy auto manufacturers will be out of existence if they don t get back into the game in the next 2-3 years. There have been a bunch of announcements but we are yet to see. The Chinese though have moved far ahead, although one has to remark that they have been doing that for the last decade, so they have a 10-year head start, hardly surprising then. But I need to get back to the subject, another gentleman on Reddit remarked that if you start to use boat, and others start to use boat, then the Govt. will tax it. In fact, somebody had shared the below the other day

Different types of taxes collected by GOI

Many of the taxes that I have shared above are by the Modi Govt. who came on the platform, manifesto that once they come to power they will reduce taxes for the common man, they have reduced taxes but only for the Corporates. For the common man, the taxes have only gone up, both direct tax and indirect tax. Any reference to the Tory party who have also done similar things and have also shared that it is labor who had done large expenditures even though they have been 8 years in power, I am sure for most is purely coincidental. Incidentally, that is the same tack that was taken even by the Republican party. They all like to give tax benefits to the 1% while for the rest is austerity claiming some reason, even if it has been proven to be false.

Corporate Tax Rate, Revenue Loss to Govt.

The figures mentioned above are findings of parliamentary panel so nobody can accuse of anybody having a bias. Also, I probably had shared this but still feel the need to re-share it as people still believe that 2G scam happened even though there are plenty of arguments I can share to prove how it was all a fabricated lie.

Vinod Rai Mafinama in Uttarakhand High Court.

Part 2 of the same Mafinama.

How pathetic Mr. Rai s understanding of economics is can be gauged from the fact that he was made Chairman of IDFC and subsequently had to be thrown out. That whole lie was engineered to throw UPA out and it worked. There are and have been many such coincidences happening over the last 8 years, parallel stories happening in India and UK. This was just yesterday, about a year back Air India was given back to the Tatas, There was controversy about the supposed auctions as only Indigo was the only other party allowed to be at auction but not allowed to buy but more as a spectator as they already have 60% of the Indian civil aviation market. And there was lot of cheering from the Govt. side that finally Air India has been bought home to its true owners, the Tatas. The Tatas too started cheering and sharing how they will take down all the workers, worker unions and everything will be happy glory within a year. In fact, just couple of days back they shared new plans. Btw for the takeover of Air India, they had bought loans from the Banks and they are in the category of too big to fail. As I have shared couple of times before, RBI has not shared any inspection reports of nationalized or private banks after 2013/14. While by law, RBI is supposed to do inspection reports every 3 months and share them in the Public domain. And if you ask any of their supporter, for everything they will say UPA did x or y, which only goes to show morally bankrupt the present Govt. is. Coming back to the topic, before I forget, the idea of sharing their plans is so that they can again borrow money from the banks. But that is not the only story. Just one day back, Smita Prakash, one of the biggest cheerleaders of the present Govt. (she is the boss at Asia News International (ANI)) posted how Air India had treated her sister and other 21 passengers. Basically, they had bought business tickets but the whole cabin was dirty, they complained and they were forced to sit in economy class, not just her cousin sister but the other 21 odd passengers too. Of course Ms Smita became calm as her sister was given free air tickets on Vistara and other goodies. Of course, after that she didn t post anything about the other 21 odd passengers after that. And yes, I understand she is supposed to be a reporter but as can be seen from the twitter thread, there is or was no follow up. Incidentally, she is one of many who has calling others about Revdi culture (freebies to the masses.) but guess that only applies to other people not her or her sister. Again, if there are any coincidences of similar nature in the UK or when Trump was P.M. of the U.S. they are just coincidental .

The Uncosted budget India and the UK have many parallels, it s mind boggling. Before we get into the nitty-gritties, saw something that would be of some interest to the people here.

For those who might not be able to see above, apparently there is place in UK called Tufton Street where there are quite a few organizations that are shadowy and whose finances are not known as to how they are financed. Ms. Truss and quite a few of the people in the cabinet are from the same shadowy organizations. Mr. Kwasi Kwarteng, the just-departed chancellor is and was part of the same group. Now even for me it was a new term to learn and understand what is an uncosted budget is. To make it much more easier I share the example using a common person who goes to the bank for a loan

Now M/S X wants a loan of say Rs. 1000/- for whatever reason. He/she/they go to the bank and asks give me a loan of say INR 1000/- The banks asks them to produce a statement of accounts to show what their financial position is. They produce a somewhat half-filled statement of accounts In which all liabilities are shown, but incomes are not. The bank says you already have so much liabilities, how are you gonna pay those, accounts have to be matched otherwise you are not solvent. M/s X adamantly refuses to do any changes citing that they don t need to. At this point, M/s X credit rating goes down and nobody in the market will give them a loan. At the same time, the assets they had held, their value also depreciates because it became known that they can t act responsibly. So whose to say whether or not M/s X has those number of assets and priced them accurately. But the drama doesn t end there, M/s X says this is the responsibility of actually Mr. Z ( cue The Bank of England) as they are my accountant/lawyer etc. M/s Z says as any lawyer/accountant should. This is not under my remit. If the clients either gives incomplete information or false information or whatever then it is their responsibility not mine. And in fact, the Chancellor is supposed to be the one who is given the responsibility of making the budget. The Chancellor is very similar to our Finance Minister. Because the UK has constitutional monarchy, I am guessing the terms are slightly different, otherwise the functionality seems to be the same. For two weeks, there was lot of chaos, lot of pension funds lost quite a bit in the market and in the end Mr. Kwasi Kwarteng was ousted out of the job. Incredibly, the same media and newspapers who had praised Mr. Kwarteng just few weeks back as the best Tory budget, they couldn t wait to bury him. And while I have attempted to simplify what happened, the best explanation of what has happened can be found in an article from the guardian. Speculation is rife in the UK as to who s ruling atm as the new Chancellor has reversed almost all the policies that Ms. Truss had bought and she is now more or less a figurehead. Mr. Hunt, the new chancellor doesn t have anybody behind him. Apparently, the gentleman wanted to throw his hat the ring in the Tory leadership contest that was held about a month back and he couldn t get 20 MP s to support him. Another thing that is different between UK and India is that in UK by law the PM has to answer questions put up to him or her by the opposition leaders. That is the way accountability is measured there. This is known as PMQ s or Prime Minister Questions and Answers. One can just go to YouTube or any streaming service and give Liz Truss and PMQ s and if they are interested of a certain date, give a date and they can see how she answered the questions thrown at her. Unfortunately, all she could do in both times were non-answers. In fact, the Tories seem to be using some of Labor s policies after they had bad-mouthed the same policies. Politics of right-wing both in the UK and the US seems so out of touch with the people whom they are supposed to protect and administer. An article about cyclists which is sort of half-truth, half irony shows how screwed up the policies are of the RW (right-wing). Now they are questions about the pensions triple-lock. Sadly, it is the working class who would suffer the most, most of the rich have already moved their money abroad several years ago. The Financial Times, did share a video about how things have been unfolding

Seems Ms. Truss forgot to add Financial Times in the list of anti-growth coalition she is so fond of. Also, the Tory party seems to want to create more tax havens in the UK and calling them investment zones. Of course, most of the top tax havens are situated around the UK itself. I wouldn t go more into as that would probably require its own article, although most of that information is all in public domain. Fracking I don t really want to take much time as the blog post has become long. There have been many articles written why Fracking is bad and that is why even the Tories had put in their Manifesto that they won t allow Fracking but apparently, today they are trying to reopen Fracking. And again how bad it is and can be found out by the article in Guardian.

Changes in version 0.1.8 (2022-10-18)

• Use the ' ' operator instead of ' ' on a set of booleans to appease 'clang-14'.

This post by Dirk Eddelbuettel originated on his Thinking inside the box blog. Please report excessive re-aggregation in third-party for-profit settings.

This post by Dirk Eddelbuettel originated on his Thinking inside the box blog. Please report excessive re-aggregation in third-party for-profit settings.

• Kalzium: Molecular editor now works!
• Artikulate: Now works on arm64 ( eg. Rasberry Pi )
• Dragon: Now works on arm64
• Minuet: Now works on arm64

• Picmi
• Kturtle
• Ksudoku
• Konquest

QSoas> load /comments='"'

QSoas> edit

QSoas> set-perp /from-row=0

QSoas> expand /style=red-to-blue /flags=kinetics

QSoas> mfit-exponential-decay flagged:kinetics

• look at the depence of the tau_1 parameter as a function of the azide concentration;
• try fitting more than one exponential, using for instance:

  QSoas> mfit-exponential-decay /exponentials=2 flagged:kinetics
  

source=("git+https://github.com/vimeo/psalm.git#commit=$ _commit "
        "composer.lock")

 prepare()  
   cd $ pkgname 
   cp ../composer.lock .
 
updlockfiles()  
  cd $ pkgname 
  rm -f composer.lock
  composer update
  cp composer.lock "$ outdir /"
 

updlockfiles

Thanks This work is currently crowd-funded on github sponsors. I d like to thank @SantiagoTorres, @repi and @rgacogne for their support in particular.

Running the docker CLI under a process supervisor only results in supervising the CLI process. This has a variety of consequences for users of these tools. For example, any attempt to limit a container's memory usage by running the CLI as a systemd service will fail; the limits will only apply to the CLI and its non-existent children. In addition, attempts to terminate a client process may not result in terminating all of the processes in the container.

As a result, Podman plays nicely with tools like systemd; using podman run with a process supervisor works as expected, because the processes inside the container are children of podman run. The developers of Podman encourage people to use it in this way by a command to generate systemd units for Podman containers.

sudo skopeo copy  docker-daemon,containers-storage :octoprint/octoprint:latest

# podman run -d --rm \
    --name octoprint \
    -p 8080:80 \
    -v /archive/octoprint:/octoprint \
    --device /dev/ttyACM0:/dev/ttyACM0 \
    octoprint/octoprint

# cd /etc/systemd/system
# podman generate systemd octoprint \
    --new \
    --name \
    --files

Dowry Few days back, had posted about the movie Raksha Bandhan and whatever I felt about it. Sadly, just couple of days back, somebody shared this link. Part of me was shocked and part of me was not. Couple of acquaintances of mine in the past had said the same thing for their daughters. And in such situations you are generally left speechless because you don t know what the right thing to do is. If he has shared it with you being an outsider, how many times he must have told the same to their wife and daughters? And from what little I have gathered in life, many people have justified it on similar lines. And while the protests were there, sadly the book was not removed. Now if nurses are reading such literature, how their thought process might be forming, you can tell :(. And these are the ones whom we call for when we are sick and tired :(. And I have not taken into account how the girls/women themselves might be feeling. There are similar things in another country but probably not the same, nor the same motivations though although feeling helplessness in both would be a common thing. But such statements are not alone. Another gentleman in slightly different context shared this as well

The above is a statement shared in a book recommended for CTET (Central Teacher s Eligibility Test that became mandatory to be taken as the RTE (Right To Education) Act came in.). The statement says People from cold places are white, beautiful, well-built, healthy and wise. And people from hot places are black, irritable and of violent nature. Now while I can agree with one part of the statement that people residing in colder regions are more fair than others but there are loads of other factors that determine fairness or skin color/skin pigmentation. After a bit of search came to know that this and similar articulation have been made in an idea/work called Environmental Determinism . Now if you look at that page, you would realize this was what colonialism is and was all about. The idea that the white man had god-given right to rule over others. Similarly, if you are fair, you can lord over others. Seems simplistic, but yet it has a powerful hold on many people in India. Forget the common man, this thinking is and was applicable to some of our better-known Freedom fighters. Pune s own Bal Gangadhar Tilak The Artic Home to the Vedas. It sort of talks about Aryans and how they invaded India and became settled here. I haven t read or have access to the book so have to rely on third-party sources. The reason I m sharing all this is that the right-wing has been doing this myth-making for sometime now and unless and until you put a light on it, it will continue to perpetuate . For those who have read this blog, do know that India is and has been in casteism from ever. They even took the fair comment and applied it to all Brahmins. According to them, all Brahmins are fair and hence have god-given right to lord over others. What is called the Eton boy s network serves the same in this casteism. The only solution is those idea under limelight and investigate. To take the above, how does one prove that all fair people are wise and peaceful while all people black and brown are violent. If that is so, how does one count for Mahatma Gandhi, Martin Luther King Junior, Nelson Mandela, Michael Jackson the list is probably endless. And not to forget that when Mahatma Gandhiji did his nonviolent movements either in India or in South Africa, both black and brown people in millions took part. Similar examples of Martin Luther King Jr. I know and read of so many non-violent civl movements that took place in the U.S. For e.g. Rosa Parks and the Montgomery Bus Boycott. So just based on these examples, one can conclude that at least the part about the fair having exclusive rights to being fair and noble is not correct. Now as far as violence goes, while every race, every community has had done violence in the past or been a victim of the same. So no one is and can be blameless, although in light of the above statement, the question can argumentated as to who were the Vikings? Both popular imagination and serious history shares stories about Vikings. The Vikings were somewhat nomadic in nature even though they had permanent settlements but even then they went on raids, raped women, captured both men and women and sold them at slaves. So they are what pirates came to be, but not the kind Hollywood romanticizes about. Europe in itself has been a tale in conflict since time immemorial. It is only after the formation of EU that most of these countries stopped fighting each other From a historical point perspective, it is too new. So even the part of fair being non-violent dies in face of this evidence. I could go on but this is enough on that topic.

Railways and Industrial Action around the World. While I have shared about Railways so many times on this blog, it continues to fascinate me that how people don t understand the first things about Railways. For e.g. Railways is a natural monopoly. What that means is and you can look at all and any type of privatization around the world, you will see it is a monopoly. Unlike the road or Skies, Railways is and would always be limited by infrastructure and the ability to have new infrastructure. Unlike in road or Skies (even they have their limits) you cannot run train services on a whim. At any particular point in time, only a single train could and should occupy a stretch of Railway network. You could have more trains on one line, but then the likelihood of front or rear-end collisions becomes a real possibility. You also need all sorts of good and reliable communications, redundant infrastructure so if one thing fails then you have something in place. The reason being a single train can carry anywhere from 2000 to 5000 passengers or more. While this is true of Indian Railways, Railways around the world would probably have some sort of similar numbers.It is in this light that I share the below videos.

To be more precise, see the fuller video

Now to give context to the recording above, Mike Lynch is the general secretary at RMT. For those who came in late, both UK and the U.S. have been threatened by railway strikes. And the reason for the strikes or threat of strikes is similar. Now from the company perspective, all they care is to invest less and make the most profits that can be given to equity shareholders. At the same time, they have freezed the salaries of railway workers for the last 3 years. While the politicians who were asking the questions, apparently gave themselves raise twice this year. They are asking them to negotiate at 8% while inflation in the UK has been 12.3% and projected to go higher. And it is not only the money. Since the 1980s when UK privatized the Railways, they stopped investing in the infrastructure. And that meant that the UK Railway infrastructure over period of time started getting behind and is even behind say Indian Railways which used to provide most bang for the buck. And Indian Railways is far from ideal. Ironically, most of the operators on UK are nationalized Railways of France, Germany etc. but after the hard Brexit, they too are mulling to cut their operations short, they have too There is also the EU Entry/Exit system that would come next year. Why am I sharing about what is happening in UK Rail, because the Indian Government wants to follow the same thing, and fooling the public into saying we would do it better. What inevitably will happen is that ticket prices go up, people no longer use the service, the number of services go down and eventually they are cancelled. This has happened both in Indian Railways as well as Airlines. In fact, GOI just recently announced a credit scheme just a few days back to help Airlines stay afloat. I was chatting with a friend who had come down to Pune from Chennai and the round-trip cost him INR 15k/- on that single trip alone. We reminisced how a few years ago, 8 years to be precise, we could buy an Air ticket for 2.5k/- just a few days before the trip and did it. I remember doing/experiencing at least a dozen odd trips via air in the years before 2014. My friend used to come to Pune, almost every weekend because he could afford it, now he can t do that. And these are people who are in the above 5-10% of the population. And this is not just in UK, but also in the United States. There is one big difference though, the U.S. is mainly a freight carrier while the UK Railway Operations are mostly passenger based. What was and is interesting that Scotland had to nationalize their services as they realized the Operators cannot or will not function when they were most needed. Most of the public even in the UK seem to want a nationalized rail service, at least their polls say so. So, it would definitely be interesting to see what happens in the UK next year. In the end, I know I promised to share about books, but the above incidents have just been too fascinating to not just share the news but also share what I think about them. Free markets function good where there is competition, for example what is and has been happening in China for EV s but not where you have natural monopolies. In all Railway privatization, you have to handover the area to one person, then they have no motivation. If you have multiple operators, then there would always be haggling as to who will run the train and at what time. In either scenario, it doesn t work and raises prices while not delivering anything better I do take examples from UK because lot of things are India are still the legacy of the British. The whole civil department that was created in 1953 is/was a copy of the British civil department at that time and it is to this day. P.S. Just came to know that the UK Chancellor Kwasi Kwarteng was just sacked as UK Chancellor. I do commend Truss for facing the press even though she might be dumped a week later unlike our PM who hasn t faced a single press conference in the last 8 odd years.

https://www.youtube.com/watch?v=oTP6ogBqU7of The difference in Indian and UK politics seems to be that the English are now asking questions while here in India, most people are still sleeping without a care in the world. Another thing to note Minidebconf Palakkad is gonna happen 12-13th November 2022. I am probably not gonna go but would request everyone who wants to do something in free software to attend it. I am not sure whether I would be of any use like this and also when I get back, it would be an empty house. But for people young and old, who want to do anything with free/open source software it is a chance not to be missed. Registration of the same closes on 1st of November 2022. All the best, break a leg Just read this, beautifully done.

$ git clone https://gitlab.com/gnutls/gnutls.git guile-gnutls
$ cd guile-gnutls/
$ git checkout f5dcbdb46df52458e3756193c2a23bf558a3ecfd
$ git-filter-repo --path guile/ --path m4/guile.m4 --path doc/gnutls-guile.texi --path doc/extract-guile-c-doc.scm --path doc/cha-copying.texi --path doc/fdl-1.3.texi

1. Does cross-language interference occur?
2. How do experienced programmers learn new languages?
3. What do experienced programmers find confusing in new languages?

This is sometimes called Jerry Seinfeld s secret to productivity: Just keep at it. Don t break the streak.

github activity october 2013 to october 2014

github activity october 2014 to october 2015

github activity october 2015 to october 2016

github activity october 2016 to october 2017

github activity october 2017 to october 2018

github activity october 2018 to october 2019

github activity october 2019 to october 2020

github activity october 2020 to october 2021

github activity october 2021 to october 2022

This post by Dirk Eddelbuettel originated on his Thinking inside the box blog. Please report excessive re-aggregation in third-party for-profit settings.

Microsoft Minesweeper

Minesweeper setup Let s start with defining a suitable data type for the grid of the minesweeper board. I ll use the Array data type, it s Ix-based indexing is quite useful for grids:

import Data.Array

type C = (Int,Int)
type Grid a = Array C a

The library lacks a function to generate an array from a generating function, but it is easy to add:

genArray :: Ix i => (i,i) -> (i -> e) -> Array i e
genArray r f = listArray r $ map f $ range r

Let s also fix the size of the board, as a pair of lower and upper bounds (this is the format that the Ix type class needs):

size :: (C,C)
size = ((0,0), (3,3))

Now board is simply a grid of boolean values, with True indicating that a bomb is there:

type Board = Grid Bool

board1 :: Board
board1 = listArray size
  [ False, False, False, False
  , True,  False, False, False
  , True,  False, False, False
  , False, False, False, False
  ]

It would be nice to be able to see these board in a nicer way. So let us write A function that prints a grid, including a frame, given a function that prints something for each coordinate. Together with a function that prints a bomb (as *), we can print the board:

pGrid :: (C -> String) -> String
pGrid p = unlines
    [ concat [ p' (y,x)   x <- [lx-1 .. ux+1] ]
      y  <- [ly-1 .. uy+1] ]
  where
    ((lx,ly),(ux,uy)) = size

    p' c   inRange size c = p c
    p'  _ = "#"

pBombs :: Board -> String
pBombs b = pGrid $ \c -> if b ! c then "*" else " "

The expression b ! c looks up a the coordinate in the array, and is True when there is a bomb at that coordinate. So here is our board, with two bombs:

ghci> putStrLn $ pBombs board1
######
#    #
#*   #
#*   #
#    #
######

But that s not what we want to show to the user: Every field should have have a number that indicates the number of bombs in the surrounding fields. To that end, we first define a function that takes a coordinate, and returns all adjacent coordinates. This also takes care of the border, using inRange:

neighbors :: C -> [C]
neighbors (x,y) =
    [ c
      (dx, dy) <- range ((-1,-1), (1,1))
    , (dx, dy) /= (0,0)
    , let c = (x + dx, y + dy)
    , inRange size c
    ]

With that, we can calculate what to display in each cell a bomb, or a number:

data H = Bomb   Hint Int deriving Eq

hint :: Board -> C -> H
hint b c   b ! c = Bomb
hint b c = Hint $ sum [ 1   c' <- neighbors c, b ! c' ]

With a suitable printing function, we can now see the full board:

hint :: Board -> C -> H
hint b c   b ! c = Bomb
hint b c = Hint $ sum [ 1   c' <- neighbors c, b ! c' ]

pCell :: Board -> C -> String
pCell b c = case hint b c of
    Bomb -> "*"
    Hint 0 -> " "
    Hint n -> show n

pBoard :: Board -> String
pBoard b = pGrid (pCell b)

And here it is:

ghci> putStrLn $ pBoard board1
######
#11  #
#*2  #
#*2  #
#11  #
######

Next we have to add masks: We need to keep track of which fields the user already sees. We again use a grid of booleans, and define a function to print a board with the masked fields hidden behind ?:

type Mask = Grid Bool

mask1 :: Mask
mask1 = listArray size
  [ True,  True,  True,  False
  , False, False, False, False
  , False, False, False, False
  , False, False, False, False
  ]

pMasked :: Board -> Mask -> String
pMasked b m = pGrid $ \c -> if m ! c then pCell b c else "?"

So this is what the user would see

ghci> putStrLn $ pMasked board1 mask1
######
#11 ?#
#????#
#????#
#????#
######

Uncovering some fields With that setup in place, we now implement the piece of logic we care about: Uncovering all fields that are next to a neutral field. Here is the first attempt:

solve0 :: Board -> Mask -> Mask
solve0 b m0 = m1
  where
    m1 :: Mask
    m1 = genArray size $ \c ->
      m0 ! c   or [ m0 ! c'   c' <- neighbors c, hint b c' == Hint 0 ]

The idea is that we calculate the new mask m1 from the old one m0 by the following logic: A field is visible if it was visible before (m0 ! c), or if any of its neighboring, neutral fields are visible. This works so far: I uncovered the three fields next to the one neutral visible field:

ghci> putStrLn $ pMasked board1 $ solve0 board1 mask1
######
#11  #
#?2  #
#????#
#????#
######

But that s not quite what we want: We want to keep doing that to uncover all fields.

Uncovering all fields So what happens if we change the logic to: A field is visible if it was visible before (m0 ! c), or if any of its neighboring, neutral fields will be visible. In the code, this is just a single character change: Instead of looking at m0 to see if a neighbor is visible, we look at m1:

solve1 :: Board -> Mask -> Mask
solve1 b m0 = m1
  where
    m1 :: Mask
    m1 = genArray size $ \c ->
      m0 ! c   or [ m1 ! c'   c' <- neighbors c, hint b c' == Hint 0 ]

(This is roughly what happened when Franz started to use the kfix comonadic fixed-point operator in his code, I believe.) Does it work? It seems so:

ghci> putStrLn $ pMasked board1 $ solve1 board1 mask1
######
#11  #
#?2  #
#?2  #
#?1  #
######

Amazing, isn t it! Unfortunately, it seems to work by accident. If I start with a different mask:

mask2 :: Mask
mask2 = listArray size
  [ True,  True,  False, False
  , False, False, False, False
  , False, False, False, False
  , False, False, False, True
  ]

which looks as follows:

ghci> putStrLn $ pMasked board1 mask2
######
#11??#
#????#
#????#
#??? #
######

Then our solve1 function does not work, and just sits there:

ghci> putStrLn $ pMasked board1 $ solve1 board1 mask2
######
#11^CInterrupted.

Why did it work before, but now now? It fails to work because as the code tries to figure out if a field, it needs to know if the next field will be uncovered. But to figure that out, it needs to know if the present field will be uncovered. With the normal boolean connectives ( and or), this does not make progress. It worked with mask1 more or less by accident: None of the fields on in the first column don t have neutral neighbors, so nothing happens there. And for all the fields in the third and forth column, the code will know for sure that they will be uncovered based on their upper neighbors, which come first in the neighbors list, and due to the short-circuting properties of , it doesn t have to look at the later cells, and the vicious cycle is avoided.

rec-def to the rescue This is where rec-def comes in: By using the RBool type in m1 instead of plain Bool, the recursive self-reference is not a problem, and it simply works:

import qualified Data.Recursive.Bool as RB

solve2 :: Board -> Mask -> Mask
solve2 b m0 = fmap RB.get m1
  where
    m1 :: Grid RB.RBool
    m1 = genArray size $ \c ->
      RB.mk (m0 ! c) RB.  RB.or [ m1 ! c'   c' <- neighbors c, hint b c' == Hint 0 ]

Note that I did not change the algorithm, or the self-reference through m1; I just replaced Bool with RBool, with RB. and or with RB.or. And used RB.get at the end to get a normal boolean out. And , here we go:

ghci> putStrLn $ pMasked board1 $ solve2 board1 mask2
######
#11  #
#?2  #
#?2  #
#?1  #
######

That s the end of this repetition of let s look at a tying-the-knot-problem and see how rec-def helps , which always end up a bit anti-climatic because it just works , at least in these cases. Hope you enjoyed it nevertheless.