Simple flake for multi cabal home

Uses haskell-flake-utils (experimental) to provide build and devShell with nix flakes:
```
nix build .#rhine-gloss
result/bin/rhine-gloss-gears
```

```
nix develop
cabal build rhine
```

.github/workflows/ci.yml

@@ -37,3 +37,24 @@ jobs:
       - name: Cabal test
         run: |
           cabal test all --enable-tests --test-show-details=Always --project-file=cabal.project.${{ matrix.ghc }}
+  build-flake:
+    name: Nix Flake
+    strategy:
+      matrix:
+        os:
+          - ubuntu-20.04
+          - macos-12
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/[email protected]
+      - uses: cachix/install-nix-action@v15
+      - name: Build default package (rhine)
+        run: nix build
+      - name: Build default package (rhine-examples)
+        run: nix build .#rhine-examples
+      - name: Build default package (rhine-gloss)
+        run: nix build .#rhine-gloss
+      - name: Run tests
+        run: nix develop -c cabal test all
+      # nix flake check breaks on IFD in multi-platform flake https://github.com/NixOS/nix/issues/4265
+      # - run: nix flake check

.gitignore

@@ -17,3 +17,4 @@ cabal.sandbox.config
 *.eventlog
 .stack-work/
 cabal.project.local
+result

Contributing.md

@@ -23,8 +23,8 @@
 
 ### GHC, cabal, stack, nix
 
-* Rhine should be compilable with `cabal`, `nix` and `stack`.
-  If in doubt, use `cabal` and `nix`.
+* Rhine should be compilable with `cabal`, `nix` (including flakes) and `stack`.
+  If in doubt, use `cabal` and `nix` (preferably flakes with `nix develop`).
 * Rhine does not make an effort to support old GHC versions,
   but it does make an effort to support _new_ versions.
   Check `.travis.yml` for supported GHC versions.

README.md

@@ -38,6 +38,7 @@ constraints on clocks come into effect, enforcing clock safety.
 
 A typical example,
 which can be run as `cd rhine-examples/ && cabal run Demonstration`,
+(or using nix flakes with `nix develop` followed `cabal run Demonstration`),
 would be:
 
 ```haskell

default.nix

@@ -1,12 +1,9 @@
-attrs@{ ... }:
-
 let
-  localPackages = import ./nix/localPackages.nix attrs;
+  lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+  flake-compat-src = fetchTarball {
+    url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+    sha256 = lock.nodes.flake-compat.locked.narHash;
+  };
+  flake-compat = import flake-compat-src { src =  ./.; };
 in
-{
-  inherit (localPackages)
-    rhine
-    rhine-gloss
-    rhine-examples
-  ;
-}
+flake-compat.defaultNix

flake.nix

@@ -0,0 +1,36 @@
+{
+  description = "rhine";
+  nixConfig.bash-prompt = "\[rhine\]$ ";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs";
+    flake-utils.url = "github:numtide/flake-utils";
+
+    flake-compat = {
+      url = "github:edolstra/flake-compat";
+      flake = false;
+    };
+
+    haskell-flake-utils.url = "github:ivanovs-4/haskell-flake-utils";
+    haskell-flake-utils.inputs.nixpkgs.follows = "nixpkgs";
+    haskell-flake-utils.inputs.flake-utils.follows = "flake-utils";
+  };
+
+outputs = { self, nixpkgs, flake-utils, haskell-flake-utils, flake-compat, ... }:
+  haskell-flake-utils.lib.simpleCabalProject2flake {
+    inherit self nixpkgs;
+
+    systems = [
+      # Tested in CI
+      "x86_64-linux"
+      "x86_64-darwin"
+      # Tested by maintainers
+      "aarch64-darwin"
+      # Not sure we can test this in CI
+      "i686-linux"
+    ];
+
+    name = "rhine";
+    packageNames = [ "rhine-gloss" "rhine-examples" ];
+  };
+}

shell.nix

@@ -1,4 +1,9 @@
 let
-  mypkgs = import ./. {};
-  shellFrom = import ./nix/shellFrom.nix {};
-in shellFrom mypkgs.rhine-gloss
+  lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+  flake-compat-src = fetchTarball {
+    url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+    sha256 = lock.nodes.flake-compat.locked.narHash;
+  };
+  flake-compat = import flake-compat-src { src =  ./.; };
+in
+flake-compat.shellNix