Add haskell-project

haskell-project/Main.hs

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+module Main where
+
+import HaskellSay (haskellSay)
+import ExampleLibrary
+
+main :: IO ()
+main = haskellSay $ "If inverted, False is " ++ writeBool (invert False)
+

haskell-project/README.md

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+**Nix flake for an example haskell package**
+
+To get started:
+- In the `package.yaml`, set any names beginning with `example` to your chosen name - for example, change the `name` setting on the first line from `example-package` to the nameof your new package.
+- Run `hpack` from the terminal; this will generate a cabal file for your project.
+- Run `cabal test` or `cabal run ${executable-name}`, where `${executable-name}` is the name of your executable, defined under `executables` in the `package.yaml`.
+
+**Overview of what's here**
+This flake has a simple executable, a library that that executable depends on, and a test suite which tests the library.
+
+It uses cabal to build, hpack as the format for the cabal project specification, and hspec to run tests.
+
+Remember to run hpack after any changes to the `package.yaml`, in order to re-create the `${project-name}.cabal` file, as well as the first time you try to use this.
+
+The executable can be run via cabal using `cabal run example-executable`, whereas tests can be run via `cabal test`.
+

haskell-project/flake.lock

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+{
+  "nodes": {
+    "flake-utils": {
+      "locked": {
+        "lastModified": 1659877975,
+        "narHash": "sha256-zllb8aq3YO3h8B/U0/J1WBgAL8EX5yWf5pMj3G0NAmc=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "c0e246b9b83f637f4681389ecabcb2681b4f3af0",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1628427351,
+        "narHash": "sha256-WuZUIQ07AvRw+T9wvQ3qFf8MXmKZ+ktZz9drNgWXDbs=",
+        "path": "/nix/store/aqinic6h77nrsrzwdsq2mxihw0kd87ml-source",
+        "rev": "348bc5de8bca09c624f5c4975f538684da4713d2",
+        "type": "path"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

haskell-project/flake.nix

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+{
+  description = "Example haskell package with library, executable and tests";
+  inputs = {
+    nixpkgs.url = "nixpkgs";
+    flake-utils.url = github:numtide/flake-utils;
+  };
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        overlay = (final: prev: {
+          inherit example-package;
+        });
+        pkgs = import nixpkgs {
+          inherit system;
+          overlays = [ overlay ];
+        };
+        example-package = pkgs.haskellPackages.callCabal2nix "example-package" ./. {};
+      in
+      {
+        packages = { inherit example-package; };
+        defaultPackage = example-package;
+        devShell = pkgs.haskellPackages.shellFor {
+            name = "example-package";
+            packages = p: [ example-package ];
+            withHoogle = true;
+            buildInputs = with pkgs; with pkgs.haskellPackages; [
+              haskell-language-server
+              ghcid
+              cabal-install
+              hpack
+            ];
+          shellHook = "command -v fish &> /dev/null && fish";
+          };
+       }
+    );
+}

haskell-project/package.yaml

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+name: example-package
+
+dependencies:
+  - base == 4.*
+  - haskell-say
+
+library:
+  source-dirs: src
+  exposed-modules: ExampleLibrary
+
+executables:
+  example-executable:
+    main: Main.hs
+    dependencies:
+      - example-package
+
+tests:
+  spec:
+    main: Spec.hs
+    source-dirs:
+      - test
+      - src
+    dependencies:
+      - hspec == 2.*
+      - HUnit >= 1.6.0.0
+    build-tools: hspec-discover

haskell-project/src/ExampleLibrary.hs

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+module ExampleLibrary where
+
+invert :: Bool -> Bool
+invert True = False
+invert False = True
+
+writeBool :: Bool -> String
+writeBool True = "True"
+writeBool False = "False"

haskell-project/test/Spec.hs

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+import Test.Hspec
+import ExampleLibrary(invert)
+
+main :: IO ()
+main = hspec $ do
+    describe "test simple functions" $ do
+        it "ensures invert True is False" $ do
+            invert True `shouldBe` False
+