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Multi-package Reflex example

This repo is an example of combining cabal.project, Nix, reflex-platform, and jsaddle to drastically improve the developer experience.

Either clone this repo with --recurse-submodules, or run git submodule update --init --recursive in this directory after cloning to make sure reflex-platform is checked out.

First, run ./reflex-platform/try-reflex at least once. We won't use it at all in this project, but it does some extra work to setup your system requirements automatically, namely installing Nix and configuring the Reflex binary cache.

Once Nix is installed, everything else is mostly handled for you. To build the project's backend and jsaddle-webkit2gtk frontend app, use the ./cabal script:

$ ./cabal new-build all

To build the GHCJS app, use the ./cabal-ghcjs script:

$ ./cabal-ghcjs new-build all

You can use GHCi with the jsaddle-webkit2gtk app for much better dev cycles:

$ ./cabal new-repl frontend


Nix is useful for creating deterministic, production ready build products. You can use the nix-build command to build all the parts of the project with Nix.

  • Build everything

    $ nix-build
    Skipping ios apps; system is x86_64-linux, but x86_64-darwin is needed.
    Use `nix-build -A all` to build with remote machines.
    $ tree result
    ├── android
    │   └── frontend -> /nix/store/4w62ly3hi75zpdmiq52lk1m4kir660vc-android-app
    ├── ghc
    │   ├── backend -> /nix/store/bgraikacjv68lfcghkprj3mspwx9f2bn-backend-
    │   ├── common -> /nix/store/lcgz36j77y6w7jyd39b14zp00hfaxn3s-common-
    │   └── frontend -> /nix/store/h9dbc2dvh11g1saj52ndn8ys3kj6f03l-frontend-
    └── ghcjs
        ├── common -> /nix/store/fgbmn6mjgh7gfdbgnb7a21fsb9175gmv-common-
        └── frontend -> /nix/store/hfpq2580jbvgm20p992v8qjdczvr20gm-frontend-
    9 directories, 0 files
  • Build the backend

    $ nix-build -o backend-result -A ghc.backend
  • Build the JS app

    $ nix-build -o frontend-result -A ghcjs.frontend
  • Build the native frontend

    $ nix-build -o native-frontend-result -A ghc.frontend


Building a multi-package project with Nix can be a pain because of Nix's lack of incremental building. A small change to a common package will require Nix to rebuild that package from scratch, causing a huge interruption during development. Although this is usually where Stack would shine, Stack doesn't officially support using Nix for Haskell derivations, and has zero support for Nix with GHCJS. You can build Reflex apps using only Stack and no Nix, but you lose a lot of benefits that reflex-platform provides, like the curated set of package versions that Reflex works best with (including a GHCJS-optimized textlibrary), binary caches of all the Haskell derivations, and zero-effort cross compilation for native mobile apps.

How it works



  • Actually implement a backend / frontend that uses the common library to show that even cross-package dependencies are built incrementally.
  • new-build doesn't yet support any means of programmatically finding build products. It would be nice to have some kind of solution for this, especially so that the backend could serve the dist-ghcjs products without some hardcoded path.


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