Revert "Drop flake stuff, I don't understand it" (#46)

This reverts commit 0c3b23b.

Author: chrisdone

flake.lock

@@ -0,0 +1,29 @@
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs    = nixpkgs.legacyPackages.${system};
+        overlay = final: prev: {
+          hell = prev.callCabal2nix "hell" ./. { };
+        };
+        haskellPackages = pkgs.haskellPackages.extend overlay;
+      in {
+        # nix build
+        packages.default = haskellPackages.hell;
+
+        # nix develop
+        devShells.default = haskellPackages.shellFor {
+          packages = p: [ p.hell ];
+          buildInputs = with haskellPackages; [
+            cabal-install
+            haskell-language-server
+          ];
+        };
+      }
+    );
+}