add basic nix build

.github/workflows/nix-build.yml

@@ -0,0 +1,18 @@
+name: Build Polysemy with Nix
+on:
+  push:
+    branches:
+      - nix-build
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+    - uses: cachix/install-nix-action@v12
+      with:
+        install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-2.4pre20201221_9fab14a/install
+        extra_nix_config: |
+          experimental-features = nix-command flakes
+    - run: nix flake check

README.md

@@ -220,6 +220,50 @@ You're going to want to stick all of this into your `package.yaml` file.
     - TypeFamilies
 ```
 
+## Building with Nix
+
+The project provides a basic nix config for building in development.
+It is defined as a [flake] with backwards compatibility stubs in `default.nix` and `shell.nix`.
+
+To build the main library or plugin:
+
+```bash
+nix-build -A polysemy
+nix-build -A polysemy-plugin
+```
+
+Flake version:
+
+```bash
+nix build
+nix build '.#polysemy-plugin'
+```
+
+To inspect a dependency:
+
+```bash
+nix repl
+
+> p = import ./.
+> p.unagi-chan
+```
+
+To run a shell command with all dependencies in the environment:
+
+```bash
+nix-shell --pure
+nix-shell --pure --run 'cabal v2-haddock polysemy'
+nix-shell --pure --run ghcid
+```
+
+Flake version:
+
+```bash
+nix develop -i # just enter a shell
+nix develop -i -c cabal v2-haddock polysemy
+nix develop -i -c haskell-language-server-wrapper # start HLS for your IDE
+```
+
 ## *What about performance?* ([TL;DR](#tldr))
 
 Previous versions of this `README` mentioned **the library being**
@@ -311,3 +355,4 @@ implementation:
 [oleg:more]: http://okmij.org/ftp/Haskell/extensible/more.pdf
 [schrijvers:fusion]: https://people.cs.kuleuven.be/~tom.schrijvers/Research/papers/mpc2015.pdf
 [wu:scope]: https://www.cs.ox.ac.uk/people/nicolas.wu/papers/Scope.pdf
+[flake]: https://nixos.wiki/wiki/Flakes

default.nix

@@ -0,0 +1 @@
+{...}: (import ./nix/compat.nix).defaultNix

flake.nix

@@ -0,0 +1,37 @@
+{
+  description = "Higher-order, low-boilerplate free monads.";
+
+  inputs = {
+    nixpkgs.url = github:NixOs/nixpkgs/nixos-20.09;
+    flake-utils.url = github:numtide/flake-utils;
+  };
+
+  outputs = { nixpkgs, flake-utils, ... }:
+  flake-utils.lib.eachSystem ["x86_64-linux"] (system:
+  let
+    overlay = import ./nix/overlay.nix;
+
+    pkgs = import nixpkgs {
+      inherit system;
+      overlays = [overlay];
+    };
+  in {
+    packages = { inherit (pkgs.haskellPackages) polysemy polysemy-plugin; };
+
+    defaultPackage = pkgs.haskellPackages.polysemy;
+
+    devShell = pkgs.haskellPackages.shellFor {
+      packages = _: [];
+      buildInputs = with pkgs.haskellPackages; [
+        cabal-install
+        haskell-language-server
+        ghcid
+      ];
+      withHoogle = true;
+    };
+
+    checks = {
+      inherit (pkgs.haskellPackages) polysemy polysemy-plugin;
+    };
+  });
+}

hie.yaml

@@ -0,0 +1,13 @@
+cradle:
+  cabal:
+    - path: "."
+      component: "lib:polysemy"
+
+    - path: "./test"
+      component: "polysemy:test:polysemy-test"
+
+    - path: "./polysemy-plugin"
+      component: "lib:polysemy-plugin"
+
+    - path: "./polysemy-plugin/test"
+      component: "polysemy-plugin:test:polysemy-plugin-test"

nix/compat.nix

@@ -0,0 +1,7 @@
+let
+  compat = fetchTarball {
+    url = "https://github.com/edolstra/flake-compat/archive/99f1c2157fba4bfe6211a321fd0ee43199025dbf.tar.gz";
+    sha256 = "0x2jn3vrawwv9xp15674wjz9pixwjyj3j771izayl962zziivbx2";
+  };
+in
+  import compat { src = ../.; }

nix/overlay.nix

@@ -0,0 +1,33 @@
+let
+  pkgOverrides = pkgs: self: super:
+  let
+    filter = pkgs.nix-gitignore.gitignoreSourcePure [./source-filter];
+    c2n = name: src: self.callCabal2nixWithOptions name (filter src) "" {};
+    hackage = pkg: ver: sha256: self.callHackageDirect { inherit pkg ver sha256; } {};
+  in {
+    polysemy = c2n "polysemy" ../.;
+    polysemy-plugin = c2n "polysemy-plugin" ../polysemy-plugin;
+    first-class-families =
+      hackage "first-class-families" "0.8.0.1" "0h1rxbc7zsxrlhx5xcl58wjx3qi2wny8wb3sk7c1qnydf4ckcckz";
+    ghc-lib = hackage "ghc-lib" "0.20190204" "1i2zjlr30bxpabsm8f995x455v847gbpzczm2w4089fqmsnzpzfd";
+    dump-core = hackage "dump-core" "0.1.3.2" "1mi8p736yn00z549pwnjv4ydwbs8mwg6dla3ly447c027nq8py6g";
+    monadLib = hackage "monadLib" "3.10" "1v4ynjcb963s3lfw3v71qdzvld1mmz1faf8swhvicma5jbvwchy2";
+  };
+in
+  self: super: {
+    haskell = super.haskell // (
+      let 
+        p = super.haskell.packages;
+        overrides = pkgOverrides self;
+      in {
+        packages = p // {
+          ghc865 = p.ghc865.override { inherit overrides; };
+          ghc883 = p.ghc883.override { inherit overrides; };
+          ghc884 = p.ghc884.override { inherit overrides; };
+          ghc8101 = p.ghc8101.override { inherit overrides; };
+          ghc8102 = p.ghc8102.override { inherit overrides; };
+        };
+      }
+    );
+    haskellPackages = self.haskell.packages.ghc884;
+  }

nix/source-filter

@@ -0,0 +1,11 @@
+/*
+.*
+!*.cabal
+!*.nix
+!*.hs
+!LICENSE
+!/flake.lock
+!/nix/
+!/src/
+!/test/
+!/bench/

polysemy-plugin/polysemy-plugin.cabal

@@ -1,10 +1,8 @@
 cabal-version: 2.0
 
--- This file has been generated from package.yaml by hpack version 0.33.0.
+-- This file has been generated from package.yaml by hpack version 0.34.4.
 --
 -- see: https://github.com/sol/hpack
---
--- hash: 0e9b200907c62e6df486f7512ceffff92f0941cf55d6ef49a581324d50201d44
 
 name:           polysemy-plugin
 version:        0.2.5.2
@@ -51,14 +49,27 @@ library
       Paths_polysemy_plugin
   hs-source-dirs:
       src
-  default-extensions: DataKinds DeriveFunctor FlexibleContexts GADTs LambdaCase PolyKinds RankNTypes ScopedTypeVariables StandaloneDeriving TypeApplications TypeOperators TypeFamilies UnicodeSyntax
+  default-extensions:
+      DataKinds
+      DeriveFunctor
+      FlexibleContexts
+      GADTs
+      LambdaCase
+      PolyKinds
+      RankNTypes
+      ScopedTypeVariables
+      StandaloneDeriving
+      TypeApplications
+      TypeOperators
+      TypeFamilies
+      UnicodeSyntax
   build-depends:
       base >=4.9 && <5
     , containers >=0.5 && <0.7
     , ghc >=8.4.4 && <9
     , ghc-tcplugins-extra >=0.3 && <0.5
     , polysemy >=1.3
-    , syb >=0.7 && <0.8
+    , syb ==0.7.*
     , transformers >=0.5.2.0 && <0.6
   default-language: Haskell2010
 
@@ -80,7 +91,20 @@ test-suite polysemy-plugin-test
       Build_doctests
   hs-source-dirs:
       test
-  default-extensions: DataKinds DeriveFunctor FlexibleContexts GADTs LambdaCase PolyKinds RankNTypes ScopedTypeVariables StandaloneDeriving TypeApplications TypeOperators TypeFamilies UnicodeSyntax
+  default-extensions:
+      DataKinds
+      DeriveFunctor
+      FlexibleContexts
+      GADTs
+      LambdaCase
+      PolyKinds
+      RankNTypes
+      ScopedTypeVariables
+      StandaloneDeriving
+      TypeApplications
+      TypeOperators
+      TypeFamilies
+      UnicodeSyntax
   ghc-options: -threaded -rtsopts -with-rtsopts=-N -fplugin=Polysemy.Plugin
   build-tool-depends:
       hspec-discover:hspec-discover
@@ -95,7 +119,7 @@ test-suite polysemy-plugin-test
     , polysemy >=1.3.0.0
     , polysemy-plugin
     , should-not-typecheck >=2.1.0 && <3
-    , syb >=0.7 && <0.8
+    , syb ==0.7.*
     , transformers >=0.5.2.0 && <0.6
   if flag(corelint)
     ghc-options: -dcore-lint -dsuppress-all