[OPS-958] Add template for a haskell.nix project

haskell.nix/.buildkite/pipeline.yml

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+steps:
+
+# build haskell components
+- label: build
+  commands:
+  - nix-build -A checks.x86_64-linux.build-all
+
+# don't run tests until all components are built
+- wait
+
+# run the tests
+- label: test
+  commands:
+  - nix-build -A checks.x86_64-linux.test
+
+# run hlint
+##- label: hlint
+##  commands:
+##  - nix run -f. legacyPackages.x86_64-linux.haskellPackages.hlint
+##      -c hlint -j --hint .hlint.yaml */
+
+# run weeder
+##- label: weeder
+##  commands:
+##  - nix-build -A weeder-script
+##    # weeder needs .cabal file:
+##  - nix run -f. legacyPackages.x86_64-linux.haskellPackages.hpack -c hpack
+##  - ./result

haskell.nix/.gitlab-ci.yml

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+default:
+  # specify nix tag to select runner
+  tags: [nix]
+
+stages:
+  - build
+  - test
+
+# build haskell components
+build-all:
+  stage: build
+  script:
+  - nix-build -A checks.x86_64-linux.build-all
+
+# run the tests
+test:
+  stage: test
+  script:
+  - nix-build -A checks.x86_64-linux.test
+
+# run hlint
+##hlint:
+##  stage: test
+##  script:
+##  - nix run -f. legacyPackages.x86_64-linux.haskellPackages.hlint
+##      -c hlint --hint .hlint.yaml */
+
+# run weeder
+##weeder:
+##  stage: test
+##  script:
+##  - nix-build -A weeder-script
+##    # weeder needs .cabal file:
+##  - nix run -f. legacyPackages.x86_64-linux.haskellPackages.hpack -c hpack
+##  - ./result

haskell.nix/README.md

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+Template for haskell.nix configuration for Buildkite or Gitlab CI
+
+## How to use this template
+
+1. Copy `flake.nix` file to you repository, it contains all nix dependencies and definitions.
+
+2. Copy `default.nix` and `shell.nix` files which provide compatibility with non-flake nix interface.
+
+3. Copy pipeline configuration: `.gitlab-ci.yml` for Gitlab, or `.buildkite/pipeline.yml` for Buildkite.
+
+4. Replace `pataq-package` in `flake.nix` with your haskell package name (usually specified in `package.yaml`).
+
+5. If you want to run weeder or hlint in your CI, uncomment related lines in `flake.nix` and in the pipeline configuration. If you don't need weeder or hlint, remove those lines.
+
+6. Get unstable nix with `nix-command flakes` experimental features enabled, and run `nix flake update` to generate `flake.lock` file. This file pins latest versions of the nix dependencies.
+
+7. Enjoy working CI.

haskell.nix/default.nix

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+(import
+  (
+    let
+      lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+    in
+    fetchTarball {
+      url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+      sha256 = lock.nodes.flake-compat.locked.narHash;
+    }
+  )
+  {
+    src = ./.;
+  }).defaultNix

haskell.nix/flake.nix

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+{
+  inputs = {
+    flake-compat = {
+      url = "github:edolstra/flake-compat";
+      flake = false;
+    };
+    nixpkgs.url = "github:serokell/nixpkgs";
+    haskell-nix.url = "github:serokell/haskell.nix/serokell-latest";
+    hackage-nix = {
+      url = "github:input-output-hk/hackage.nix";
+      flake = false;
+    };
+    stackage-nix = {
+      url = "github:input-output-hk/stackage.nix";
+      flake = false;
+    };
+
+    # if you want to run weeder:
+    ##haskell-nix-weeder = {
+    ##  url = "github:serokell/haskell-nix-weeder";
+    ##  flake = false;
+    ##};
+  };
+
+  outputs = { self, nixpkgs, haskell-nix, hackage-nix, stackage-nix, ... }:
+
+  # for weeder:
+  ##outputs = { self, nixpkgs, haskell-nix, hackage-nix, stackage-nix, haskell-nix-weeder, ... }:
+
+    let
+      haskellNix = import haskell-nix {
+        sourcesOverride = { hackage = hackage-nix; stackage = stackage-nix; };
+      };
+
+      pkgs = import nixpkgs haskellNix.nixpkgsArgs;
+      lib = pkgs.lib;
+
+      # invoke haskell.nix
+      hs-pkgs = pkgs.haskell-nix.stackProject {
+        src = pkgs.haskell-nix.haskellLib.cleanGit {
+          name = "pataq-package";
+          src = ./.;
+        };
+
+        # haskell.nix configuration
+        modules = [{
+          packages.pataq-package = {
+            ghcOptions = [
+              # fail on warnings
+              "-Werror"
+              # disable optimisations to speed up builds
+              "-O0"
+
+              # for weeder: produce *.dump-hi files
+              ##"-ddump-to-file" "-ddump-hi"
+            ];
+
+            # for weeder: collect all *.dump-hi files
+            ##postInstall = weeder-hacks.collect-dump-hi-files;
+          };
+
+        }];
+      };
+
+      hs-pkg = hs-pkgs.pataq-package;
+
+      # returns the list of all components for a package
+      get-package-components = pkg:
+        # library
+        lib.optional (pkg ? library) pkg.library
+        # haddock
+        ++ lib.optional (pkg ? library) pkg.library.haddock
+        # exes, tests and benchmarks
+        ++ lib.attrValues pkg.exes
+        ++ lib.attrValues pkg.tests
+        ++ lib.attrValues pkg.benchmarks;
+
+      # all components for the current haskell package
+      all-components = get-package-components hs-pkg.components;
+
+      # for weeder:
+      ##weeder-hacks = import haskell-nix-weeder { inherit pkgs; };
+
+      # nixpkgs has weeder 2, but we use weeder 1
+      ##weeder-legacy = pkgs.haskellPackages.callHackageDirect {
+      ##  pkg = "weeder";
+      ##  ver = "1.0.9";
+      ##  sha256 = "0gfvhw7n8g2274k74g8gnv1y19alr1yig618capiyaix6i9wnmpa";
+      ##} {};
+
+      # a derivation which generates a script for running weeder
+      ##weeder-script = weeder-hacks.weeder-script {
+      ##  weeder = weeder-legacy;
+      ##  hs-pkgs = hs-pkgs;
+      ##  local-packages = [
+      ##    { name = "pataq-package"; subdirectory = "."; }
+      ##  ];
+      ##};
+
+    in {
+      # nixpkgs revision pinned by this flake
+      legacyPackages.x86_64-linux = pkgs;
+
+      # derivations that we can run from CI
+      checks.x86_64-linux = {
+        # builds all haskell components
+        build-all = all-components;
+
+        # runs the test
+        test = hs-pkg.checks.pataq-test;
+      };
+
+      # script for running weeder
+      ##weeder-script = weeder-script;
+    };
+}

haskell.nix/shell.nix

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+(import
+  (
+    let
+      lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+    in
+    fetchTarball {
+      url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+      sha256 = lock.nodes.flake-compat.locked.narHash;
+    }
+  )
+  {
+    src = ./.;
+  }).shellNix.default