add haskell.nix build system

it's an annoying blockchain company, but apparently that thing is the
only usable option to build stack things with nix without having to
redefine all the (outdated) haskell packages in nixpkgs?

.gitignore

 dist-newstyle/*
 .stack-work
 walint.cabal
+result*

default.nix

+let
+  sources = import ./nix/sources.nix {};
+  haskellNix = import sources.haskellNix {};
+
+  # Import nixpkgs and pass the haskell.nix provided nixpkgsArgs
+  pkgs = import
+    # use haskell.nix's nixpkgs, which may (?) have more substitutes available
+    haskellNix.sources.nixpkgs-unstable
+    # args for nixpkgs; includes the haskell.nix overlay
+    (haskellNix.nixpkgsArgs // { system = "x86_64-linux"; });
+
+  drvs = pkgs.haskell-nix.project {
+   # 'cleanGit' cleans a source directory based on the files known by git
+   src = pkgs.haskell-nix.haskellLib.cleanGit {
+     src = ./.;
+     name = "walint";
+   };
+   modules = [{
+     packages.walint.components.exes = {
+       # don't include gcc or ghc in the dependency closure …
+       walint-server.dontStrip = false;
+       walint.dontStrip = false;
+     };
+   }];
+   stack-sha256 = "16ilij2cygmwbdmjdzj6yl4yv7zi4qzwg7rxkxgp0hbjpkz6n42y";
+  };
+in
+{
+  walint = drvs.walint.components.exes.walint;
+  walint-server = pkgs.stdenvNoCC.mkDerivation {
+    name = "walint-server-with-assets";
+    src = drvs.walint.components.exes.walint-server;
+    phases = [ "buildPhase" ];
+    buildPhase = ''
+      mkdir -p $out
+      cp -r $src/* $out
+      cp -r ${pkgs.copyPathToStore ./static} $out/static
+      cp ${pkgs.copyPathToStore ./config.json} $out/config.json
+      cp ${pkgs.copyPathToStore ./config.toml} $out/config.toml
+    '';
+  };
+}

nix/sources.json

 {
+    "haskellNix": {
+        "branch": "master",
+        "description": "Alternative Haskell Infrastructure for Nixpkgs",
+        "homepage": "https://input-output-hk.github.io/haskell.nix",
+        "owner": "input-output-hk",
+        "repo": "haskell.nix",
+        "rev": "c2677deef14f2772110f56b60d74dee25f19902f",
+        "sha256": "0pa5n8lgq2hmsskyd7m4xsfiinq8mfpgw7g7ll3j77l5z4shi264",
+        "type": "tarball",
+        "url": "https://github.com/input-output-hk/haskell.nix/archive/c2677deef14f2772110f56b60d74dee25f19902f.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
     "niv": {
         "branch": "master",
         "description": "Easy dependency management for Nix projects",

stack.yaml

-resolver:
-  url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/18/16.yaml
+resolver: lts-18.16
 
 # User packages to be built.
 # Various formats can be used as shown in the example below.

stack.yaml.lock

@@ -86,5 +86,4 @@ snapshots:
     size: 586286
     url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/18/16.yaml
     sha256: cdead65fca0323144b346c94286186f4969bf85594d649c49c7557295675d8a5
-  original:
-    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/18/16.yaml
+  original: lts-18.16