Writing an Android app in Haskell


I have been willing to learn FRP for a long time. Since I had the chance to have a little more time than usual these days, I thought it was the right time for me to dive into this paradigm and learn this stuff for good.

Among the different implementations of FRP in Haskell, I was especially interested in Reflex. What I find interesting in this implementation is that the very same code can be deployed on the web, as an Android app, or as an iPhone app which is spectacular when you think about it.

The idea of being capable of running Haskell code on a phone is so appealing that I could not resist. Once I heard about it, I knew that I needed to write an app at some point in my Haskell journey. And that promise of being able to run the very same code on different platforms. How true can that be? I needed to find out.

Unfortunately, examples of such apps in the wild are not that common, if not non-existent1. I spent a fair amount of time looking for examples. Only tutorials are sometimes deployed on the web but nothing on Google Play (I did not dig in the app store as I don’t own an iPhone). And as a professional developer, I know there is often a big gap between tutorials and real-life applications.

I needed to find out by myself. And that is what I did with this project.

The project

As I like to do it, I needed an idea of something simple yet nontrivial to get my hands dirty. I found something that might be a good fit.

I am a big note taker. I take notes all the time about just anything, technical or not. I have tried many different tools. But what works best for me is plain markdown files in a private git repository.

What I am missing in my workflow is the ability to browse and search my notes from my phone. I could use the official GitHub app for that purpose. However, that would spoil the fun. I also know that there is a lot of value in owning the code of a tool you use. Once the code is written and works, it becomes easy to fix bugs and add features as needed.

The project would then be an app able to browse and search in a GitHub repository. It should also be able to render markdown files. For simplicity’s sake, I decided to use the GitHub API to talk to the Git repository.

Here is the summary of the things the app needs to do:

  • have some settings
  • validate and store the settings
  • sends requests to the GitHub API
  • react to responses, parse them, and show the results
  • render markdown
  • handle errors
  • wrap everything in an easy-to-use UI


I had great fun learning FRP. As a developer, this is really the kind of experience I am looking at. A new concept that, once grokked, gives a deeper understanding of the domain, here, reactive UI development.

People usually say that FRP is hard to learn. I would not say that. But it is true that one has to change mindset to be able to be productive with it. And that is exactly the interesting point. My opinion is that it helps to understand the interactions of the widgets and user actions as a whole. It makes it clear that if something is difficult to implement in FRP that usually means the interactions are themselves complicated.

I put the app online and on the Play Store for you to try out (and me to use). As advertised by Obsidian Systems folks, it does work the same on the web and on Android. The whole code is available on GitHub.

Missing pieces and dark corners

I want to take advantage of this post to shed light on some dark corners I encountered during the development of this project.

Developer tools

For me, Obelisk is a developer tool, such as ghc, cabal, haskell-language-server, or even gcc. The documentation states that it must be installed.

But I don’t usually install the developer tools I use. I pull all the ones I need from a pinned version of nixpkgs and put them in scope with a nix-shell.

Unfortunately, the ob command is not in nixpkgs. So I came up with that shell.nix:

  obeliskSrc = fetchGit {
    url = "https://github.com/obsidiansystems/obelisk.git";
    ref = "refs/tags/v1.2.0.0";
(import obeliskSrc { }).shell

One can see, that for this shell, we pin obelisk version v1.2.0.0. Yet the ob command, whatever version, doesn’t care about this and uses the rev pinned there instead. It fetches it, compiles it, and passes the commands to it. This is undocumented and can be super confusing if you don’t know about it.

Now I also like to have haskell-language-server available in my editor. This is not very hard to add it in the development shell, yet quite difficult if you don’t know how to do it. These are the relevant lines to be added to default.nix.

  shellToolOverrides = self: super: {
    haskell-language-server = pkgs.haskell.packages.ghc8107.haskell-language-server;
    implicit-hie = pkgs.haskell.packages.ghc8107.implicit-hie;

At the end, working around those weird uses of nix, my workflow to work on this project is:

$ nix-shell

brings ob into scope. I can now use ob run or ob watch and develop. Then:

$ ob shell

brings haskell-language-server into scope. From this shell, I start my editor to have type hovers, auto-formatting and all the niceties that haskell-language-server provides.

Build and upload the Android app

The Play Store expects a signed AAB file. That file can be built with:

$ nix-build -A android.frontend --arg androidIsRelease true

The AAB file is then available in ./result/android-app-release.aab.

It must be signed with some java tools. Let’s start a shell with them:

$ nix-shell -p jdk17_headless

Now create a key store for storing the keys that will be used to sign the file.

$ keytool -genkey -v -keystore diverk.keystore -alias diverk -keyalg RSA -keysize 2048 -validity 10000

It will ask a few questions, along with a password. That one must be kept in a safe place. It will be needed for the next commands.

$ cp ./result/android-app-release.aab .
$ jarsigner -verbose -sigalg SHA256withRSA -digestalg SHA-256 -storepass the-passord-given-to-keytool -keystore diverk.keystore ./android-app-release.aab diverk

And that’s it! The AAB file is now signed and ready to be uploaded on the Play Store.

Deploy the app on the web

To deploy the app on the web, one must first build the derivation:

$ nix-build -A linuxExe
$ ls -L result
backend  frontend.jsexe.assets  static.assets  version

It contains the following files:

  • backend is the executable of the HTTP server itself
  • frontend.jsexe.assets is the web app compiled by ghcjs
  • static.assets are the static assets used by the frontend

Now you can copy that derivation on any system having nix, reverse proxy it with nginx if you want, starts the backend, and it will serve your app on the web.

The backend supports many command line options. Find out which ones with backend --help.

Learning resources

Here you can find the best resources I have found on the subject:

  1. Except for tenjinreader on the Play Store↩︎

November 7, 2023