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Exploring Nix & Haskell Part 3: Less Nix, More Reflex

Christian Henry

13 May 2019

12 min read

Exploring Nix & Haskell Part 3: Less Nix, More Reflex
  • Haskell

In the last post, we ended off with a Nix-based environment with some basic workflow commands to build and test your Haskell code. I mentioned that my reason for investigating Nix environments in the first place was because it's the most supported way to build a frontend Haskell application with Reflex / GHCJS.

In this post we'll start with a project based off reflex-project-skeleton and remove/add bits and pieces to it until we have a deployable (frontend only, for now) application with a prescribed IDE experience and workflow. It shouldn't be strictly necessary to read the previous posts before this one, but it could help to put things in context.

To that end, this post will go over:

  • other projects that have similar end goals
  • editing reflex-project-skeleton to make it more streamlined for the frontend portion of a webapp
  • adding some workflow pieces, like hot reloading and IDE config
  • command line operations you'll use often when developing on this type of project
  • common issues you'll run into when adding more dependencies

A quick disclaimer on alternatives...

A lot of the information in this post will parallel what Obelisk gives you. For my own project, I ended up using the setup described here rather than Obelisk for the following reasons:

  • Most of Obelisk's commands are pass-throughs to nix-shell and nix-build operations. While it is a fairly thin wrapper on top of these commands, as a Nix noob it can be another layer of indirection that just adds complexity, especially when things go wrong. In its current state, you sometimes have to end up knowing what Obelisk is doing as well as the underlying Nix commands (which involves digging through a lot of source code). For this reason, I think reading through this post could help you even if you do decide that going through Obelisk is better for you.
  • I mostly develop on Mac and lots of the commands would not work for a variety of reasons. Usually this was because the underlying Nix operation wasn't working, but for the above reason, this was slightly obscured from me.
  • Android and iOS builds would be theoretically nice but I didn't care about it that much. I'd rather just get some website/web-app up, and just make it so that it has a nice mobile format.
  • I didn't care about server-side rendering. I wanted my backend to be a fairly simple Haskell CRUD server and able to be on a more modern version of GHC if needed.
  • I didn't want to pay for the frontend aspect of my project. My solution to this (which I'll include in this post) is to use GitHub Pages, which makes makes some parts of Obelisk unnecessary.
  • Probably related to my lack of server-side rendering, the routing update broke my project. Since my backend was an independent REST service, I didn't gain any benefit from whatever routing things it introduced, and there wasn't much documentation on it so I didn't really understand it either.

That being said, a lot of these things have been fixed over the last few months, and you may want different things than me. The Obelisk folks have been very responsive to my questions, and I think the project has a good goal. Make sure to check it out to see if it'll work for you; if it does, hopefully the information in this post will still help you there too!

Setting up the basic skeleton

The goal for this section is to get the skeleton to a spot where we can comfortably learn the workflow commands and make future changes. We'll start with the reflex-project-skeleton, wiping history and re-committing:

git clone --recurse-submodules https://github.com/ElvishJerricco/reflex-project-skeleton my-haskell-reflex-project
cd my-haskell-reflex-project
rm -rf .git
git init
git add .
git commit -m "Initial commit after cloning skeleton"

You probably want to run ./reflex-platform/try-reflex to set up and populate your caches. Some parts of this can take a while; if you've never tried reflex before, the try-reflex script will:

  1. Download / install Nix (fairly quick, it's around 22MB)
  2. Ask you to manually input whether to configure binary caches. Definitely say yes.
  3. Download / build whatever is needed for Reflex / GHCJS environment. This part will take a while. To give you a general idea, I tried this on the default "hashicorp/precise64" Vagrant box with an internet connection of ~100 Mbit/s (fixed to match my usual speed with this) running on my 2016 Mac and it took about 30 minutes. I'd recommend letting this run in the background while you follow along with the rest of the setup. Keep in mind that this is roughly the lead time to expect if you ever update reflex-platform in the future.

One thing you should notice is that the default.nix file is re-using the Nix expression from the reflex-platform submodule. We're going to keep this, but know that this means that our entry point will be fundamentally different from the previous posts, where we built it up from scratch. Some settings are pre-filled in this default.nix; to see the full list (it's not too long) check the reflex-platform/project/default.nix here.

Constraining to a frontend web-app

First, I want to simplify the scope here and not consider a backend and android/ios builds. Those can be a topic for another time. We'll still keep the shells section though, so that this can be added in sometime later. So:

rm -rf backend
# edit default.nix to remove backend from packages and shells
# edit default.nix to remove android/ios sections

and you should end up with

{ reflex-platform ? import ./reflex-platform {} }:
reflex-platform.project ({ pkgs, ... }: {
  packages = {
    common = ./common;
    frontend = ./frontend;

  shells = {
    ghc = ["common" "frontend"];
    ghcjs = ["common" "frontend"];

Using Nix instead of git submodules

The source skeleton uses a git submodule to "import" the reflex-platform nix file. I had some issues getting this to work in my cloned repo (probably because I wiped the history), but that's fine -- by now we should be pros at importing Nix things! We know (from part 1 of this series) that we can use fetchFromGitHub to get a repository, and nix-prefetch-git to find the correct sha256 value. Looking at the skeleton again, we see that it's pinning a reflex-platform commin that starts with 7e002c5, so we can run:

nix-prefetch-git https://github.com/reflex-frp/reflex-platform.git 7e002c5

to get the full rev and sha256 fields, then put it into a nix/reflex-platform.nix file that binds all this together:

{ bootstrap ? import <nixpkgs> {} }:
  reflex-platform = bootstrap.fetchFromGitHub {
    owner = "reflex-frp";
    repo  = "reflex-platform";
    rev = "7e002c573a3d7d3224eb2154ae55fc898e67d211";
    sha256 = "1adhzvw32zahybwd6hn1fmqm0ky2x252mshscgq2g1qlks915436";
  import reflex-platform {}

Use this in our default.nix by changing line 1 to import this file instead of looking at a sub-folder:

{ reflex-platform ? import ./nix/reflex-platform.nix {} }:

and remove the now unnecessary submodule pieces:

rm -rf reflex-platform
rm .gitmodules

Adding Hoogle toggle

Let's also take as input whether hoogle is enabled, so that we don't have to wait for Hoogle to build when we don't need it:

{ reflex-platform ? import ./reflex-platform.nix {}
, withHoogle ? false
reflex-platform.project ({ pkgs, ... }: {
  inherit withHoogle;


Wiring in Warp

Warp is what will provide us with a hot-reloading browser window running GHC when we save the source code. This will be very useful in our workflow, so let's enable that by default. To do that, just set useWarp = true; in default.nix:

reflex-platform.project ({ pkgs, ... }: {
  withHoogle = false;
  useWarp = true;

which will start it on port 3003 by default. You can change this port by setting the JSADDLE_WARP_PORT environment variable. You'll know it's working since it will print out

Running jsaddle-warp server on port 3003

when main is ran.

Note: you previously had to manually import jsaddle-warp, import Reflex.Dom.Core instead of Reflex.Dom, and wrap widgetMain with run 3003 $ but this shouldn't be necessary anymore.

Miscellaneous cleanup and additions

Reflex-platform will give you cabal and other dev tools by default, so we don't need to touch shellToolOverrides in default.nix, which pretty much does what the buildInputs param did in the previous post. However, let's just add in empty sections for that and overrides in case we do need it sometime:

  shellToolOverrides = self: super: {

  overrides = self: super: {

As you'll see in the next section, I don't really use the cabal helper scripts, so I'm going to go ahead and remove them (if you find them helpful, feel free to keep them):

rm cabal
rm cabal-ghcjs
rm cabal-ghcjs.project # without a backend, we only have one "project"

and ignore intermediate things in the .gitignore:


Setting up Visual Studio Code

At this point, this guide is pretty committed to using Visual Studio Code so we'll make a settings override that will work with our project structure. The default settings set you up pretty well, but we'll also use :set -i (a ghci command) to include the frontend/src and common/src folders. The plugin is usually pretty good about figuring out your project type but I have had it incorrectly default to Stack, so we'll specify that directly. Note that, for now, this is non-new-style cabal. All together your .vscode/settings.json file should look like:

  "ghcSimple.startupCommands.custom": [
    ":set -fno-diagnostics-show-caret -fdiagnostics-color=never -ferror-spans",
    ":set -fdefer-type-errors -fdefer-typed-holes -fdefer-out-of-scope-variables",
    ":set -ifrontend/src:common/src"
  "ghcSimple.workspaceType": "cabal"

Now we have a skeleton that will work with everything in the next section. To see the code tagged now, see here.

Workflow commands

I mentioned some of these in the past, but let's get a complete list of commands you need to know while you're working in this environment. Most of these need to be ran in the GHC nix-shell, which you launch with:

nix-shell -A shells.ghc

and then run the command in it. If you want to run it in one single-fire command, enter it in the form:

nix-shell -A shells.ghc --run "<command>"

Start up IDE

As an example of above: since we want to primarily develop on GHC (it's faster), we start up the GHC shell:

nix-shell -A shells.ghc

and launch Visual Studio Code from there:

code .

Or as one command:

nix-shell -A shells.ghc --run "code ."

Fire up ghcid

Note: this is basically what Obelisk's ob run does.

The main benefit of ghcid here is that we can hot-reload our app a few seconds after saving our source file. Another benefit is that you can see the full compile error message (sometimes it gets formatted weird on the IDE), and you can have a "sanity test" for your IDE if it freezes or something. We start ghcid like this by running (in the GHC shell):

ghcid -W -c "cabal new-repl frontend" -T Main.main

To break that down:

  • -W lets you run tests even if there's warnings
  • -T runs that method whenever a re-compile is done
  • -c specifies the shell/repl to run that command in. We want new-repl since we have a cabal.project file in our repo.

Since this is something you want to fire once and keep running, you probably want the single command to copy-paste:

nix-shell -A shells.ghc --run 'ghcid -W -c "cabal new-repl frontend" -T Main.main'

And then you can go to localhost:3003 (or whatever port you have set in your entry point) to see your application.

Fire up a repl

Note: this is basically what Obelisk's ob repl does.

You shouldn't be using this for compile errors, but sometimes you want a repl to test out small sections of code. Do this by firing up the same repl ghcid does, i.e:

nix-shell -A shells.ghc --run "cabal new-repl frontend"

Start Hoogle

nix-shell -A shells.ghc --run --arg withHoogle true "hoogle server --local --port=8080"

Note that we could have also done what we did in previous posts where we define a nix-shell in a new file that passes in withHoogle = true and run the hoogle command in there. Either works.

Build frontend application

You probably don't want to do this very often, since ghcjs will compile much slower than ghc. However, I'd recommend doing this whenever you edit your cabal files, since your ghcjs dependencies could require some updating too, and it can be frustrating to do this in one push when you want to deploy. So to build the final thing, we'll use nix-build with:

nix-build -o frontend-result -A ghcjs.frontend

"Deploy" your application

In your github project settings, make sure you have GitHub Pages enabled with the option to build from the /docs folder. With this, all you need to do is copy the stuff in frontend-result to docs:

rm -rf docs; cp -r frontend-result/bin/frontend.jsexe docs

and push the updated docs folder to github.

Before pushing, you may want to test out the final ghcjs app by opening it in your browser:

open docs/index.html

Once you push the docs/ folder, your Haskell GHCJS application will be available at https://name.github.io/repo-name. In the case of this repository, that would be https://cah6.github.io/haskell-nix-skeleton-2/.

I know this method isn't for everybody, but if you have strong counter-opinions to doing this you probably have enough knowledge or desire to host the docs folder elsewhere.

All together now

All this means that I usually have a number of terminals (in my case iTerm split windows) open:

  1. One terminal for a consistent Hoogle db running in foreground
  2. One terminal for a consistent ghcid running in foreground
  3. One terminal sitting in nix-shell -A shells.ghc to re-launch the IDE or pop into a repl.
  4. One terminal sitting in nix-shell -A shells.ghcjs to rebuild the final frontend.

Organizing them in the foreground like this means that it's easier to restart all 4 when you change a package in your cabal file / Nix expression.

Adding some dependencies

Now that we have these core commands out of the way, let's make some changes to the project to illustrate common types of environment changes you'll need to make. This is pretty similar to the info in https://github.com/Gabriel439/haskell-nix, but this can be really confusing for newcomers, so I think it's worth re-showing.

dontCheck to skip tests

Let's say our frontend needed the package http-media. First, add that package to the frontend.cabal file. Re-entering our nix-shell -A shells.ghc works quickly but re-entering nix-shell -A shells.ghcjs makes it build and test the package from scratch, since it's not in the cached set of packages for reflex-platform.

If we don't care about running the tests for this package, whether because it takes too long, can't compile, or can't pass with the package combination we have, we can override it with the dontCheck function in our default.nix:

  overrides = self: super: {
    http-media = pkgs.haskell.lib.dontCheck super.http-media;

Another reason to use dontCheck is if the test suites depend on incompatible packages. The biggest culprit here is the doctest package, which can't build on GHCJS. For example, the lens-aeson library has a doctests test-suite, so you'll need to run it through dontCheck in order to use it.

Pinning a library through cabal2nix

Let's say we need the servant-reflex package now. Adding this to our frontend cabal file and re-entering the GHC shell will fail with:

Setup: Encountered missing dependencies:
base >=4.8 && <4.10,
exceptions ==0.8.*,
http-media ==0.6.*,
servant >=0.8 && <0.13

because this package uses strict bounds on dependencies and the version it decided to use (v0.3.3) doesn't mesh with what we have. The latest commit (v0.3.4) does though, so we can generate a Nix file pointing to the latest commit with:

cabal2nix https://github.com/imalsogreg/servant-reflex > nix/servant-reflex.nix

or for a specific commit:

cabal2nix https://github.com/imalsogreg/servant-reflex --revision ba8d4f8 > nix/servant-reflex.nix

Then call it in our default.nix overrides:

  overrides = self: super: {
    servant-reflex = self.callPackage ./nix/servant-reflex.nix { };

Make sure to use callPackage from self ([the final package set](https://nixos.org/nixpkgs/manual# sec-overlays-definition)) instead of pkgs.haskellPackages to make sure the package you're calling is getting any other overridden packages.

Pinning through the libary's default.nix file

The servant-reflex library has a Nix file defined in its repository, so you might be wondering why we generated our own instead of just using theirs. I tried this first, but encountered a strange issue where including it as a dependency via their Nix file somehow turned off jsaddle-warp when my app was launched with GHC. I'm not sure why this happened, but did notice that using their Nix expression pulled in more dependencies since it also tried to satisfy and build the executable example section. On the other hand, cabal2nix knows that we only want the package as a library, and so will only build the library section of the cabal file.

My suggestion would be to be careful using the repository's Nix derivation for packages that provide more than just a libary build in the cabal file, and maybe only try the below method if using cabal2nix doesn't work with that library.

If you do need to use their custom Nix file, do that by getting the full rev/sha256 with:

nix-prefetch-git https://github.com/imalsogreg/servant-reflex

putting that info into the nix/servant-reflex.nix file:

{ bootstrap ? import <nixpkgs> {} }:
bootstrap.fetchFromGitHub {
  owner = "imalsogreg";
  repo  = "servant-reflex";
  rev = "44595630e2d1597911ecb204e792d17db7e4a4ee";
  sha256 = "009d8vr6mxfm9czywhb8haq8pwvnl9ha2cdmaagk1hp6q4yhfq1n";

and then putting the same callPackage as the above section in your default.nix overrides.

Ending Notes

If you want to play around with the barebones skeleton in your own repository, just do something similar to the initial skeleton clone:

git clone https://github.com/cah6/haskell-nix-skeleton-2/tree/barebones_webapp my-haskell-widgets
cd my-haskell-reflex-project
rm -rf .git
git init
git add .
git commit -m "Initial commit after cloning skeleton"

Hopefully this post has helped you get started in making widgets in Haskell and providing an easy way to show them off to your friends. Friend send friends widgets made in Haskell, right?

If you want to learn more Reflex now that you have a good test environment, I would recommend the qfpl tutorial for concepts, reflex-dom-inbits for helpful bits of knowledge, and the quickref for a function cheat sheet.

As always, if you'd like to comment on this post please do so in the associated reddit post here.

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