Brave New Hacktoberfest — Learn4Haskell

Date: November 1, 2020
Author: Kowainik

This blog post is our experience report and summary after working on Learn4Haskell — an educational project that took place within Hacktoberfest2020.

It was an exhilarating time for us, and we hope that participants feel the same way. We learned valuable lessons about the event mechanics with our new approach to it. Moreover, we discovered lots of interesting insights that could be helpful and interesting to everyone involved or those who are planning to start similar courses for Hacktoberfest.

In this writing, we are going to describe the GitHub course project we offered to work at as a part of Hacktoberfest, share our journey as maintainers and mentors, identify problems that beginners experience with the course based on our practice of reviewing their solutions. And we will also discuss some moral sides of experiencing such an adventure.

Additionally, we describe pros and cons of using Haskell in the educational field: high-grade parts that make developing and teaching in it enjoyable, as well as some language shortcomings that make newcomers (and sometimes educators) struggle with the language.

The blog post might be interesting for people who are

  • curious about open-source education, GitHub project courses
  • looking for ways to make “known as difficult-to-learn” tools more accessible
  • probably compiler developers and language designers who would love to make the programming languages more approachable
  • interested in creating similar courses in the upcoming Hacktoberfests
  • interested to see statistics (lots of numbers and charts!)
  • and everyone else who would love to hear the story from our side of the barricade!

Brave New Hacktoberfest🔗

Hacktoberfest is an annual event to encourage open-source contributions. This year it is presented by DigitalOcean, Intel and

We participate in Hacktoberfest steadily four years in a row, and this year is no exception! For 2020 Kowainik prepared a special project that puts together a lot of thoughts and goals we have for our organisation. We are happy to present to you Learn4Haskell — a beginner-friendly Haskell course.

The idea behind Learn4Haskell is simple yet absolutely novel for Hacktoberfest. Usually, for this event, lots of incredible people donate their time and forces to improve some open-source projects. And this is a beautiful initiative!

Most of the time during the year developers work on private work projects, and Hacktoberfest is a perfect opportunity to pay back to communities that produce open-source stuff that is used anywhere. However, this year there were fewer opportunities to do that as Hacktoberfest became an opt-in event. Moreover, contributing to other projects is not an easy activity and could require much more time and mental efforts to get a job done.

So we came up with the idea to utilise the hype around Hacktoberfest and use it for a good deed. Instead of trying to make people contribute to our projects, we’ve offered them a way to spend that time on their self-improvement and education.

To successfully complete Hacktoberfest, you need to submit four valid pull requests (PRs). Thus we designed a 4-step course where you can learn Haskell basics and finish Hacktoberfest simultaneously.

The course is a complete working Haskell project. At the same time, it is split into four independent chapters — four source code files, which makes it perfect for GitHub PR system. Each part is a working module that contains learning material and tasks you need to solve to pump your knowledge. Our chapters describe Haskell from basic syntax and simple functions to Monads. The course doesn’t require reading any other materials or guides. So you can just open a source code file in your favourite editor and start learning. Nothing else is needed except the willingness to explore Haskell!


While working on education material, it is easy to get lost and to overcomplicate it (or make it uninteresting to follow). We aimed to find the perfect balance between the minimal necessary content to fulfil the basic essential knowledge for Functional Programming (FP) paradigms and the right amount of work for people to be able to finish it by working on the course only a few hours a day.

Learning Haskell should be easy and straightforward.

Keeping all that in mind, we created the Learn4Haskell project in pursuit of the following goals:

  • Help to get into the Haskell language.
  • Help to think in the Functional Programming way.
  • Challenge people to get out of their comfort zone of imperative programming and tweak their minds to accept FP paradigms.
  • Give a beginner-friendly and self-consistent course with theory and practice in the same place.
  • Explain Haskell topics before each task, but strive to be concise and useful at the same time. It’s a tough balance!
  • Help people who are eager to participate in Hacktoberfest and Open-Source, but also want to learn new things during this process.
  • Provide review and feedback to solutions, so people are never alone in this challenging yet exciting journey.
  • Give people who completed this course all the necessary understandings to be able to work with basic projects that use standard features. We also intend to give a strong basis that would be enough to continue their functional programming studies.
  • Show the wonderful world of FP to people outside and prove that everyone can learn those concepts in no time.
  • Engage people to the Haskell community and make them feel welcome.

Preparations and Execution🔗

Though the idea and goals look straightforward, it required a lot of time and effort from our side to get the project done. Besides the only apparent part that we were doing during the Hacktoberfest itself — reviewing —- we spent quite some time creating a competent project that would fulfil our goals. Our work on Learn4Haskell was, in fact, made in two giant, independent and essential steps:

  1. Preparation: What needed to be done Before the course.
  2. Execution: What we need to do everyday During the course (and after as well).


First of all, we started with the course plan, materials and logistic arrangements. We want people to be able to follow all the guides easily. Moreover, the course itself must be easy to set-up. And we also want it to bring something beneficial and valuable for people.

So, our preparations included the following actions:

  • Course agenda. This is the toughest and the most time-consuming part. We’ve spent a lot of time structuring our tutorials, rethinking the topics that need to be covered, brainstorming various ideas and evaluating the difficulty levels.
  • Write the course content. After deciding on the alignment of topics, we needed actually to write down explanations of different Haskell concepts. As we provide all information in code comments, we want to make it easy to follow visually. After writing the guides, we came up with the tasks that reflect the course materials. We’ve spent around a week only purely writing the content of the course.
  • Test the course. Next step involved a lot of reviewing rounds, putting ourselves into the shoes of FP beginners and battle-testing course concepts and tasks against different aspects. We also needed to estimate time that people could afford to spend on the course. We believe that people don’t have to spend too much time reading hundreds of Haskell guides in order to get started with the language, so the challenge was in keeping explanations concise but helpful simultaneously.
  • Configure the automatic testing environment. Although we provide quick and detailed feedback regarding code, solutions, styling, ideas, performance and other aspects of the code structure and general advice about code in Haskell, we wanted to give people the ability to test their solution locally as well. Our course contains a lot of tasks of various difficulty, themes and concepts, and it makes sense to provide simple ways to check solutions for satisfying basic requirements. Since the course is aimed for beginners, who are not expected to know how to use Haskell build tools efficiently (which is tough to learn even for advanced Haskellers), we created two ways of testing solutions: GitHub Actions CI to test solutions when submitting PRs and Makefile with commands to test solutions locally. CI for forks can be enabled in one-click, so it is super easy to add automatic testing for your own repositories.
  • Prepare solutions. As we want Learn4Haskell to be accessible to more people, we also understand that some would be shy to reach out to us, or ask for the review and ask questions. There could be lots of reasons for which people would prefer to work on the course standalone. And we definitely want learners to be able to verify their solutions even if they decide to go this learning journey on their own. So we’ve spent some time writing idiomatic solutions for all tasks that they can use in their learning.
  • Write documentation and instructions. This is a tricky part as Haskell is known as a very-hard-to-get-started-with language. Our goal was to provide batteries-included instructions that are easy to follow, and that would work on all of the most popular platforms with minimal efforts. So we carefully describe get-started installation instructions, as well as the essential information about the course, how it works, what to expect and what participants should do on each step, and all other required metadata.
  • Design work. As we wanted our project to be visible and Hacktoberfest compliant, we’ve spent some time making it look like candy. For that, we created a resonant name and branded around it. The Learn4Haskell naming was perfect for that as it demonstrates everything this project is about and greatly fits the 4-steps course and 4 PR workflow of the event. Fortunately, Hacktoberfest helped to come up with the design, and the pull request sign perfectly represents number 4 in our logo. Hacktoberfest provides some templates, but it is still not an instant process to come up with the project name, unique concept, design and actually illustrate everything.


After we’ve finished the work on the course material and announced it, the second exciting phase comes into play. People submit their pull requests, and we provide our feedback on their solutions. Our duties include:

  • Spreading the word. It doesn’t make sense to spend time creating a course if nobody tries it. So we worked hard encouraging people to give Learn4Haskell a go. This is a separate big job, which we were hoping to get more help from the community, but mostly we were doing it on our own, especially on the later stages of the course. Twitter was our primary tool to promote Learn4Haskell and provide updates. We tried Reddit and Discourse as well, but it did not help a lot. Of course, we can’t force anyone to share things they don’t want, so no hard feelings to anyone in particular! And we are grateful to everyone who decided to support us and share the course with friends and others!
  • Reviewing. This was our killer feature of the course, so we were glad that lots of people who submitted their solutions asked us for a review. And we did our best to provide high-quality reviews for each and every PR. Even though we saw hundreds of solutions to the same tasks, it still was fun and exciting. We noticed that some tasks are more error-prone, and they usually get the same types of mistakes from time to time. But it was important for us to look at every solution carefully, as for some people it’s their first contribution, and we wanted to make sure that they really learn and understand the new concepts! We didn’t want to let them down. Thus even our thousandth review kept the same high bar of quality as the first one.
  • Answering questions. Sometimes people struggled with some topics, and that is exactly what we expected and wanted to help with when providing our helpline anytime anywhere. People asked their questions through the PRs, code or as the replies to our comments, so we could elaborate more. Lots of questions were unique and thoughtful, and we were happy to see that, as it indicated that people are really eager to learn new ideas and don’t mind listening to some advice. So we spent the time explaining Haskell concepts, sometimes even with ASCII-art and helpful code examples.
  • Side notes and additional Haskell concepts. When we saw opportunities for that, we tried to add some additional information in the review in the form of links to external resources or more elaborate explanations. Sometimes things were not covered by the course itself, but if we notice that it could be interesting for a person or could benefit people’s ideas even more, we shared extra insights into the language usage.
  • Reward good work. Our review was not only about spotting problematic or weak points, but generally evaluating people’s work and motivating them to take all from the course. Luckily for us, there were tons of exciting and thoughtful solutions, so we were not greedy in highlighting people’s fantastic work!
  • Tooling support. Developers, who attempted to solve our course, use different operating systems — Windows, macOS, Ubuntu, Arch, Pop!_OS, NixOS, etc. Even though most of the time our instructions worked as expected, sometimes participants had issues with the Haskell tooling. We tried to do our best to share workarounds and possible solutions, so they can continue the course without problems.
  • Troubleshooting. We did the most we could in a limited amount of time to verify the course beforehand. But sometimes tasks descriptions weren’t clear for everyone, or they contained some typos and a few other minor problems. Also, Hacktoberfest rules changed after several days. So we spend our time polishing all problems, warning people about changes in rules, answering their questions about the course, investigating errors, and much more.
  • Collecting stats. In our first Hacktoberfest, we shared some contribution stats, because a lot of people contributed to our projects, and we wanted to show interesting information. This year we planned stats gathering beforehand, and we’ve been collecting various contribution information to draw pretty plots and visualise contribution stats.
  • Writing this blog post. Compiling an experience report and summarising our thoughts to share our results is also not free in terms of time, notably for non-native English speakers. But we believe that it could help somebody in future, to be aware of all aspects of the event.

You can see that creating a course and teaching it includes completely exceptional tasks that require a diverse set of skills (both technical and soft). Especially if you want not only to produce a decent and marvellous course and leave your users happy but also make it possible to maintain it easily and not go crazy.


Without further ado, let’s move to exciting results of the several months of our work and see some statistics and visualisations!

Short summary of contributions:

  • 419 people forked the Learn4haskell repository
  • 326 stars on GitHub
  • 187 people submitted their solutions to Learn4Haskell tasks and asked us for review
  • 67 people finished all four chapters
  • We reviewed 481 Pull Requests

It was quite a month for us at GitHub, very dark-green. You can see the total number of reviewed PRs for each of us from the following screenshots of GitHub stats:

PRs reviewed 1 PRs reviewed 2

Our reviews contain various suggestions, and people patched their code after our clues. So we re-reviewed PRs for the second, third and sometimes the fourth time. The total number of reviews we provided is approximately the above number of PRs multiplied by three.

We were curious about how many people get started with our course each day, so we’ve been collecting info about the first open Pull Request, and here this information aggregated by days:

Course starters per day

Interestingly enough, we had not a single day without new people. We noticed some more activity on weekends, but generally everyday we received several PRs with different Chapter solutions. Sometimes our notifications were not decreasing even after clearing up some of the PRs, because people were already submitting a few more, while we were focused on one.

Some people rapidly were nailing all four Chapters, others were active only periodically. However, not everyone finished all chapters. Here is the diagram of the PRs opening distribution per chapter.

Solutions per chapter

As you see, there is a more significant gap between the second (lists and streaming functions) and third Chapters (ADTs, typeclasses), so we need to work on the problem to make it flatter. That is why we would appreciate the feedback from people who haven’t finished all Chapters yet whether it is because of the lack of interest, time or difficulty, we want to hear from you!

In our course, we recommended people to open PRs to their own forks of our course, but many people submitted PRs to our repository (that was not a problem for us, though we will talk about downsides later). Here is a pie chart for this information:

Fork vs Repo

You can see from the chart that more than half of the people opened pull requests to their own forks, though some people submitted their solutions directly to the course repository. We are going to talk later about the difference in review workflow between forks and our repository.

A lot of Learn4Haskell participants helped us make the course better by improving documentation and adding more test cases. Here is a chart of the relationship between all participants and active contributors:

Contributors diagram

It’s interesting to see charts of stars growth and distribution of stars per location.

Stars trend
Stars location

From the stats, you can see that many people tried Learn4Haskell. Some people helped us to improve the course both course participants and non-participants.

Almost 200 people tried our course (and these are only people who asked us for review, the total number of forks is twice as big!). But because of GitHub UI, it wasn’t easy for people to open pull requests to their forks, so many of the PRs were open to our course’s repository.

Not all people finished all four chapters. According to the distributions of submitted solutions per chapter, the third chapter seems to be the most challenging (the gap between chapters two and three is bigger). In Chapter Three, we explain custom data types, sum types, newtypes and typeclasses. And, indeed, from our observations, we conclude that people struggle the most with the type system itself, not with specific typeclasses.

We hope that our stats are useful (or at least interesting) to everyone, and there’s a lot to learn from them!

Tools advantages🔗

To get more out of the platform and tools we are using, we take all we can from the nicely provided features of the Haskell language, Haskell compiler and GitHub platform.

Haskell and its ecosystem have several exceptional qualities that make such project-based all-inclusive courses shine. Some remarkable Haskell features, wonderful tooling around the language created by a lot of enthusiasts and GitHub ecosystem perks made it very easy for people to start coding in Haskell and not even thinking that this is elitism or privilege!

Basically, to get started with Haskell via our course, you just

  1. Install compiler and the build tool with a few commands.
  2. Open your favourite editor (or install VS Code with the brilliant all-inclusive Haskell plugin).
  3. Run Haskell REPL and load a module you’re currently editing.

That’s all!

We highlight the main benefits of various tools we used in our course to provide a smooth experience for participants:

  1. GHCi. REPL is an incredibly powerful tool when learning a new programming language. You don’t need to create a complete project or even produce a binary file to run your code. You can just write a function and test it immediately in REPL. The feedback cycle is very short!

  2. Haskell tooling. The state of IDE in Haskell improved a lot recently! It is very easy to have a full-featured, stable and working Haskell development environment anywhere nowadays. Configuring the Haskell tooling is relatively straightforward everywhere: install (1) ghcup (ghcups on Windows), (2) VSCode, (3) the Haskell plugin, and that’s all! The VSCode’s Haskell plugin automatically downloads all extra tools for you, so, if you have a good internet connection, you can start coding in Haskell in minutes. In our course, we provided instructions on how to install Haskell, and so far, a small list of steps was enough for people to get started!

  3. Documentation testing. We use comments to structure our tutorial and all presented information. Our explanations contain multiple code snippets, and we want to make sure they are correct. We used the brilliant Haskell testing library doctest for this. It allows testing such documentation examples. So, on the one hand, we are sure that we don’t provide bogus code. Additionally, learners can verify that their implementations pass basic documentation examples. Moreover, we added playing in REPL as the first exercise, and people can verify what they see through our tests. Even though doctest is not perfect in some aspects, it turned out to be very helpful!

  4. Hintman. In Kowainik, we developed a GitHub Application that runs the Haskell linter HLint on source code diff of the PR and provides an automatic instant review. Some people told us that our app already provided helpful suggestions for writing better Haskell without even our manual review! Just a screenshot of one such example:

  5. CI in one click. Having an easy way to run various tests and notify participants of the results without manual review is a great advantage. This is possible due to the wonderful GitHub Actions and Haskell setups for that.

  6. Haskell libraries for the unit- and property-testing. We use Haskell libraries hspec for unit testing and hedgehog for property testing. It turns out, tests in Haskell are easily readable and writable, and multiple people were able to add their own tests without even learning about Monads, do-notation and many other things.

Beginner struggles🔗

Our course is intended for beginners in Haskell or open-source in general. Despite all advantages of used technologies, people still struggle with some parts due to the legit reasons and actual problems. Let’s see them and think about how it could be improved.


Our course is located on GitHub, as we prepared it for Hacktoberfest. Notwithstanding all the nice bonuses it gave to boost our course, we spotted a few things that could have been improved in order to make the experience of first-time contributors better, as well as help maintainers with more comfortable and more agreeable processes.

  1. Fork pull requests. We recommend people to open pull requests to their forks. This way, they have more freedom, can merge their solutions and enjoy all green CI in the end when all checks pass. However, a lot of people (as you saw in stats) opened PRs to our repository. And many were confused about how to open PRs to their forks. Not to mention that hundreds of PRs were just open by mistake in the wrong place due to the non-convenient design of branch/repo choice when opening PRs!

    To elaborate more on why we prefer the forking workflow for the course, here are some troubles with opening PRs to our repository:

    1. Confusion/additional work for our participants on how to separate solutions chapters into different PRs; it is actually just another thing to watch out for or be worried about.
    2. Our automatic reviewer hintman sometimes produces noisy suggestions that we don’t actually want to expose to people on that point of the study journey (as they are a bit more advanced, or it expels the solutions). Hintman also warns about trailing whitespaces, so it was more difficult and time-consuming to review people who haven’t set up trailing whitespace cleanup in their IDEs.
    3. We had to close PRs afterwards, which is always mentally hard and sometimes can feel rude.
    4. We had to deal with labels. The price of us forgetting to put them on PRs (which is very easy with that many PRs) is someone’s Hacktoberfest prize, and we definitely don’t want that!
    5. It is usually double work for us in case of the ‘oops’ pull requests: one in our repo + one in their fork. And people do not always close the outdated ones, so we need to double-check notifications to make sure this is not the case, which is also time-consuming.
  2. Visibly hidden review comments. When a review contains a lot of messages, some of them become hidden by the GitHub UI. They are often hard to find, so people sometimes don’t even see parts of our review.

  3. Line-breaks. Some people develop on Windows, and when they save files, editors change line-breaks on every single line. This makes diff hard to read at first. Fortunately, there’s an option to ignore whitespace changes.

  4. Misleading update notifications. When you need to review dozens of PRs daily, the UI around notifications and updates plays a crucial role. When jumping to a PR from the notifications tab, it’s often not clear immediately what’s actually changed: is it a new comment, a new commit or just a label addition? Especially in hidden or resolved comments, there is no way to see if anything was changed in there right away. So, re-reviewing already reviewed work took more time that could be. It would be much more convenient if new changes from the last review were highlighted somehow in a way that you don’t need to search for them manually.

  5. Hacktoberfest topics and labels. For pull requests to be accepted during Hacktoberfest, you need to add the Hacktoberfest topic to your repository and add the hacktoberfest-accepted labels to PRs. This is extra work, and sometimes it’s not even clear from the UI how to do this. Not to mention that it’s possible to write those words with typos. It would be nice if you identify your Hacktoberfest participation in one click.

  6. GitHub Actions failure visibility. Recently, the GitHub Actions UI was changed, and it doesn’t use coloured labels for indicating failed steps. It takes more time to spot if tests passed or failed. In our course, we have some advanced tasks, but people are not required to solve them. So we add the continue-on-error flag for tests on advanced tasks. And, again, from the UI, it’s not clear whether the step passed because it actually passed or because we skip errors. Using different indicators for different outcomes would help to spot such cases faster. Otherwise, we had to spend a lot of time jumping to the CI view and expanding steps to view the actual result.

  7. Hard to promote. Because the course language is Haskell, it’s more challenging to promote the course. The GitHub and Hacktoberfest UI don’t have Haskell by default, and it is a bit more advanced to set up the search by Haskell. The trending repositories in Haskell also almost always contain the same popular 3-4 repositories and no way for new projects to be highlighted there due to the GitHub algorithms. Learn4Haskell was almost never highlighted despite the relatively rapid growth of stars, forks and PRs.


The Haskell language and its ecosystem also have several shortcomings that made learning Haskell more difficult than it should be. Specifically:

  1. Operating systems support. Installing a working Haskell environment on Windows is still more difficult than it should be. macOS users also have weird linker errors sometimes when they try to build the project. And users of more exotic OSs like Arch, NixOS or Pop!_OS have troubles building the projects due to different linker issues as well.

  2. Excessive polymorphism in the standard library. Polymorphic numeric constants scare people when they type the :t 42 command and see the result. Also, types of innocently-looking functions like length and concat have the Foldable constraint instead of working with simple lists. People have hard times finding proper functions to solve their problems. Not to mention, that typeclasses and higher-kinded types are too advanced topics for beginners, and it would be much better if they can start coding without having “too many things they should ignore for now”.

  3. GHC doesn’t warn on common things by default. You need to turn on the -Wall flag to enable the pattern matching checker. And if you add all common flags in the .cabal file, GHCi doesn’t see them. So people often have common bugs in their solutions that GHC catches during compile-time if they simply run the ghci command. Adding such flags in the top of each module to make the ghci workflow smoother is awkward and less maintainable.

  4. Not enough extensions by default. Similar to the previous point, some handy for education and explanation extensions (e.g. InstanceSigs) are not enabled by default. So you need to introduce people to the extensions too early, which could be easily avoided until later. However, the new GHC20XX initiative seems to address this issue.

  5. Haskell formatters inconsistency. We encountered a lot of issues due to people using formatters that change all the existing code. We use block-style comments to explain various topics. This helps avoid unnecessary noise and makes it easier to read, as we have really substantial comment blocks of explanations. But some people add Haskell formatters in their IDEs. As a consequence, formatters automatically change the style of comments to use line comments everywhere. This made reviewing such PRs more difficult and time-consuming, as we cannot see the code diff clearly.

  6. Let-in vs Where. We’ve seen so many ways of people writing let-in expressions and where, it’s not even funny… Haskell syntax rules are too flexible and allow too many syntactic constructions. At the same time, parser errors are not always helpful. Not to mention that scoping rules for let-in and where, especially in the presence of guards and multiple top-level pattern-matchings are far from obvious. Defining a variable shouldn’t be that hard.

  7. Parentheses. It’s not so big of a problem, but in many places, it’s not clear where you should use () or not. The rules around () are also inconsistent sometimes. So, people end up using a lot of () in their code.

  8. Spaces in function-arguments. We do understand that lots of people have habits from their primary languages, where you use function calls with parentheses in the syntax like fun(x) or even fun(x, y). Unfortunately, this is accepted in Haskell as well, though it could be misleading and should not be allowed.

  9. Spaces with operators. Most of the people omitted spacing between operators, e.g. 1-2. While this is valid in Haskell, it is better to avoid such writing. Mainly because some future changes in GHC (e.g. the RecordDot extension) could change the meaning of operators without spaces.

  10. Operator precedence. Mixing different operators can be very frustrating sometimes. For instance, people sometimes are confused in the following situations, where similar expressions produce completely different results:

    -1 `mod` 7


    mod (-1) 7
  11. GHC error messages. GHC speaks for itself:

    • Couldn't match expected type 'b' with actual type 'Monster'
      'b' is a rigid type variable bound by
        the type signature for:
          fight :: forall b. Monster -> b -> String -> (Monster, b)

    People were not able to understand what is the problem in their code on their own by only looking at the compiler output. If the main benefit of a powerful compiler is to catch errors earlier, then the messages could be friendlier as well.

  12. Haskell syntax sometimes allows weird constructions. For example, you can put ; at the end of the line or even write the whole function (including type signature) on a single-line (apparently, some code in GHC uses this style). Turns out, you also can define typeclasses with curly brackets {} (similar to ordinary records). Did you also know that the else keyword in the if-then-else expression can be written from the start of the line?? Haskell is layout-sensitive language, but this mixing of layout-oriented and separator-oriented rules can be confusing.

  13. Inconsistent GHCi behaviour. Sometimes, when people type the :t 42 command in GHCi, they see the line 42 :: Num a => a, sometimes 42 :: Num t => t, and sometimes they see 42 :: Num p => p. We use doctest to check the output of this command, but still for some people output is different from doctest expectations, they become frustrated, and we spend more time explaining the problem and solution.


Learn4Haskell is our first experience of conducting courses of this particular shape. We haven’t been working on anything similar before, though Dmitrii has experience teaching Haskell in university and we were mentoring people within our workplaces.

As we enjoyed both learning and prosecuting the project, we want to improve the course more based on the feedback of both participants and experienced Haskellers, as we proceed with the project. We did get lots of different notes and opinions on the course from the former; however, we didn’t hear a lot from Haskellers.

Many people who tried our course were helping us by providing their thoughts on the fly or filling our anonymous feedback form afterwards. We’ve put together the results and want to share them with everyone.

Not every person finished all four chapters, though, the majority among those who left feedback did so.

Completed chapters

If you are reading this blog post, but you did Learn4Haskell and haven’t submitted your feedback yet, feel free to do this afterwards! We would appreciate your opinion.

As for the course authors, it is crucial for us to keep the learning interesting, so people would stay captivated and follow our course for fun, and don’t feel that it’s a daily job or some mandatory university course. It turns out that people really enjoy our content!

Course interest

We have been in Haskell for too long to be able to soberly assess the level of difficulty of Learn4Haskell tasks from the beginners’ point of view. That’s why we were interested in learners’ opinions about our course difficulty. It looks like we’ve managed to achieve quite the right balance!

Course difficulty

We spend an enormous amount of our time on review as we hoped it is key to the successful learning process. 100% of people who filled the form seem to agree that they find reviews helpful, so no diagram this time!

Our goal for this course was to make it accessible to everyone. So it was vital for us to know about people’s backgrounds to see if we could attract and help very diverse experienced folks. It turns out that developers who tried our course have a diverse experience! They come from languages like C#, C, C++, Java, Scala, Erlang, PHP, JavaScript, Ruby, Python, Go, Rust, Common Lisp, R, Scheme, Prolog, Elm, Solidity, as full-stack, backend or frontend engineers!

In the end, a few highlights from people’s feedback in the general form:

  1. Progressive feedback on the solutions and unbelievable rapid reviews on the PR’s.
  2. Awesome mentors and fascinating exercises.

A job well done! Congratulations.

It is a real bummer that there is no Chapter 5… ;)

The course was structured well, TBH literally like a game :), for a Haskell and FP newbie, it’s well organized and If I compare my knowledge before and after, the delta is HUGE

Exercises with dragons, castles, … too distracting Also probably a little too much dense…

I thoroughly enjoyed the course material and exercises. I really appreciate the maintainers taking time to review each and every line of every single PR. That’s really great. Usually we read some blog post or book and may be try to implement some examples or exercises. But, getting immediate feedback and suggestions for the code implementation and getting a chance to work on improving it just took the whole learning experience to the next level. Thanks a lot for creating such wonderful tutorial. Brilliant idea.

Thanks everyone who took their time to help us understand how we can improve the course. We are already thinking about the further ideas on how to make it better, and we will definitely want to continue with that as it seems many people actually liked it!

Lessons learned🔗

While we were teaching Haskell and helping others, we ourselves gained invaluable experience in areas of project management and starting such initiatives.

Motivation and Gratuitousness🔗

This free Haskell course is the result of hard work. We’ve put a lot of our experience and knowledge to make it accessible to as many people as possible. The excitement was the main feeling we had when we were launching the course. And imagine how hard it was to keep that mood during the whole month with all the ups and downs of the different participants’ attitude.

It is hard to motivate ourselves to continue working on such a project each day and doing the best we could each time, especially without any external support. The only thing that kept us moving is people enjoying the course, fascinated folks learning new things and sharing their adventure steps. Every reaction even via simple emoji to our comments mattered, every “thank you” gave us strength and filled with the meaning all our days and nights when we stayed late, skipped personal matters to sit and help people with learning.

Fun fact: to keep our engines working, we’ve created memos with all the exciting, funny or just grateful comments people shared with us. Just something to remind ourselves that at least somebody enjoys our course, appreciates what we do, and it wasn’t all for nothing :) And we don’t regret having it right now!

However, not all people were interacting with us (even though they specifically asked for our review). Imagine spending half an hour reviewing somebody’s code very carefully and providing elaborate comments with possible improvements and suggestions, and realising that all that time you were speaking into the void. Or what is worth, people completely disrespecting your opinions and knowledge and attacking you instead. Also, some people just copy-pasted all our official solutions, presented them as their own solutions and wasted some of our time.

It is not possible to create projects like this if you don’t have enough motivation and support. It is not going to be a bed of roses, so be ready mentally before starting something similar.

The Power of Diversity🔗

The main “WOW” moment for us was the diversity of people’s angles on various topics.

We already had a chance to work with different people during our time in open source, and that was incredible to see very novel and unexpected ideas that were boosting the projects a lot.

But here, as we were looking at the solutions to the same issues, we could feel the power of diversity even more! Many people were implementing approaches we didn’t even think about. And it was very innovative! Sometimes it was more elegant, other times more performant, or short, or expressive, or more readable, flexible or extensive!

So, the lesson that we knew already as we ourselves are a team of people with different backgrounds (both cultural and technical), but rediscovered clearly again – DIVERSITY is the key to successful projects!

To not be unfounded on our conclusions and to illustrate the diversity of views, let’s look at various implementations for some of our tasks.

Even a simple function that returns the next integer was implemented in many ways by different people:

next x = x + 1
next x = 1 + x
next x = x+1
next = (+1)
next = (+ 1)
next = (1 +)
next = succ

By the way, believe it or not, but the type signature to the next was implemented in that many ways as well!

Or, to give an example of a bigger function, we were asking people to implement the subList function that crops list elements starting from a given index and ending under the given index, returning an empty list on negative indexes.

Here is our official solution:

subList :: Int -> Int -> [a] -> [a]
subList from to l
    | from < 0 || to < 0 || to < from = []
    | otherwise = take (to - from + 1) (drop from l)

But almost every person came out with their own version of subList, sometimes even more efficient than ours! To highlight a few examples:

subList :: Int -> Int -> [a] -> [a]
subList f t xs | f >= 0 && t >= f = take ((t + 1) - f) . drop f $ xs
               | otherwise = []
subList :: Int -> Int -> [a] -> [a]
subList start end list = if start > end || start < 0 || end < 0
                            then []
                            else take (end - start + 1) (drop start list)
subList :: Int -> Int -> [a] -> [a]
subList a b xs
  | a < 0 || b < 0 || b < a = []
  | otherwise = drop a (take (b+1) xs)
subList :: Int -> Int -> [a] -> [a]
subList x y [] = []
subList x y z
             | (x < 0) || (y < 0) || (x > y) = []
             | (x == 0) && (y == 0) = take 1 z
             | (x == 0) = take y z
             | otherwise = take (y-x+1) (drop (x) z)
subList :: Int -> Int -> [a] -> [a]
subList firstPoint secondPoint mainList = if firstPoint > secondPoint then [] else mainList!!firstPoint:subList (firstPoint + 1) secondPoint mainList
subList i j l
    | i > length l || i < 0 || j < 0 = []
    | i <= j && j - 1 > length l = (l !! i) : subList (i + 1) (length l - 1) l
    | i <= j = (l !! i) : subList (i + 1) j l
    | otherwise = []

All people read the same text, but they have different backgrounds, views, and, as a consequence, come up with different solutions. You can’t pretend that “people should think the same way if they use the same language” is possible. We are all different, and we should accept our differences, embrace them and empower each other for their unique qualities!

Lonely journey🔗

We love Haskell, and we are doing a lot of work to show the real power of the language to people outside of the community as well.

Even though we believe that such a course is a tremendous boost to Haskell popularity and accessibility, we can’t say that this initiative got a welcoming attitude or support from the Haskell community itself. There may be some legitimate reasons why, however, it is hard to say if there were strong and fundamental beliefs or just personal dislike, as we didn’t receive any feedback regarding it. The only real reaction on the course “content” we got was that our logo sucks and we are not good at design, which is maybe fair but not that relevant to the course (and a bit rude, to be honest).

Though we got help from some Haskellers and participants through (re)tweets, it was not very spreading. Our announcements on Reddit even got downvoted a lot. The main problem for us was that we didn’t feel bolstered up.

The Learn4Haskell project per se is hard to sell to people because of

  1. Haskell itself (not many people consider as something to invest their time in)
  2. Hacktoberfest that was spoiled this year by some dishonest people and got a terrible reputation and anti-trend around it
  3. many other reasons such as lack of promoting skills and complicated topics.

So we had to promote, maintain and improve the project mostly on our own. Just the two of us in our free time, which was hard to deal with.

We want to highlight that we are grateful to everyone who helped us to promote and showed their support! You rock! We do not belittle your help; we just wanted to highlight the unexpected struggle of fighting this battle almost alone that we need to deal with doing such projects in Haskell as a community, which often feels like:

Everyone in the community works on their own thing and collaboration is very rare.

You can not make everyone happy🔗

No matter what you do and how good you are at it, there will always be people unsatisfied with your work. This is a hard pill to swallow, but this is just the way it is. Even if you do something good, with good intentions, for free.

Be ready to tune up some things, but also use your judgement to make things you believe in better. Be true to yourself and to who you are. For instance, we are not going to stop highlighting good moments in the review even if not all people like it, especially if they don’t write explicitly what type of review they want.

Just to show a few aspects where different people have different opinions:

  • Some people enjoyed the course, some found it not satisfying in content or the way the materials were given
  • Some said that some particular chapter was too difficult, others were particularly excited about the Chapter and noted that it was the best in the course
  • Some enjoyed Open-ended tasks, others wanted more strict descriptions
  • Some people liked the gaming fantasy theme, others found it distracting
  • Some people find our feedback motivating, other unnecessary and noisy
  • Some people thought that some of our advice makes code less readable, others were happy to see how more elegant and understandable it becomes (e.g. Pattern matching VS recursive go with the accumulators, removal of redundant parentheses vs keeping them)

As you see, there are too many opinions that could not just live together. We tried to take the feedback in such opinionated topics with a grain of salt. Our goals are the main orienteer for the direction of the course, so we are happy to apply all suggestions that align with it, but would be too careful to accept all the wishes of all people.


Our course proves that Haskell is not a difficult language. It turns out that you can learn Haskell basics in several days, and start coding in it very quickly. Haskell has a few distinguishing features that make it easy for learning despite all language drawbacks (and it’s even possible to improve some aspects of the language to make it more beginner-friendly without losing the power!). The critical point of the teaching process of Haskell is to avoid the mindset of making things look hard and complicating topics unnecessarily. Quite the opposite, it is feasible to make difficult concepts look approachable!

At the same time, we want to mention that mentoring is key to successful learning. It is one thing when you are learning a completely new technology on your own, and it’s entirely another thing when there’s somebody to guide you.

As the project creators, maintainers and mentors, we have mixed feelings in the end.

On the one hand, a lot of people tried Haskell. Some of them were scared to learn Haskell for years, but our course helped them to overcome their fears. And for some people contributions to Learn4Haskell were even their first pull requests ever! We welcomed 187 new people into the Haskell world, and we are eager to hear about their successes more in the future!

However, leading such projects is hard work, and without any appreciation and support, it’s tough to find the motivation to continue, and sometimes very depressing to keep doing it. We’ve spent all our free time on review and support during the last month sacrificing resting, reading, learning new stuff, watching TV series and walking. We also have full-time jobs, so it was quite exhausting to spend each free minute reviewing dozens of PRs. Approximately, we’ve been spending about 40 hours per week only on Learn4Haskell, besides other open-source stuff we need to do.

However, if you ask us, was it worth it, our answer is YES. We have learned a lot, met wonderful people and got the feeling that even just two of us can bring very significant improvements to this world!

Important Note:

Even though Hacktoberfest is over, we are planning to review everyone’s solutions to Learn4Haskell tasks as long as we can. We are also going to improve the course after our observations and feedback left by people who tried our course. So don’t despair if you didn’t manage to learn Haskell during Hacktoberfest. It is never too late to learn something new, and we are here to help!

We hope our Learn4Haskell experience can inspire somebody, and who knows, maybe next year it will be time to Learn4PutYourExcitingLanguageOrToolHere!


Thanks to Taylor Fausak and Cameron Gera for mentioning Learn4Haskell on Haskell Weekly podcasts.

Thanks cmdvtv and jailandrade for streaming Learn4Haskell and sharing the content on the YouTube as well.

We want to say thank you to everyone involved in the course, all participants, contributors, people who spread a word about the course anywhere (special thanks to kazup0n for providing Learn4Haskell instructions in Japanese), and those who supported us.

Several people supported us on Ko-Fi: Gints Dremains, Donna, Joe Lucas, Abby S, @tfplug, Penny, cspan, wookeny, Orca, Maxim, Naga Charan Meda, swamp_agr, Eric Moritz, Nicolás Gargano and 5 other anonymous supporters on Ko-Fi ☕️ Thank you all!