Basics for building PostgREST with Nix (#1489)

* Add GHC options used with stack to postgrest.cabal

* Add a Nix derivation for just the executable.

default.nix

@@ -0,0 +1,56 @@
+let
+  name =
+    "postgrest";
+
+  # PostgREST source files, filtered based on the rules in the .gitignore files
+  # and file extensions. We want to include as litte as possible, as the files
+  # added here will increase the space used in the Nix store and trigger the
+  # build of new Nix derivations when changed.
+  src =
+    pkgs.lib.sourceFilesBySuffices
+      (pkgs.gitignoreSource ./.)
+      [ ".cabal" ".hs" ".lhs" "LICENSE" ];
+
+  nixpkgsVersion =
+    import nix/nixpkgs-version.nix;
+
+  pinnedPkgs =
+    builtins.fetchTarball {
+      url = "https://github.com/nixos/nixpkgs/archive/${nixpkgsVersion.rev}.tar.gz";
+      sha256 = nixpkgsVersion.tarballHash;
+    };
+
+  overlays =
+    [
+      (import nix/overlays/gitignore.nix)
+    ];
+
+  pkgs =
+    import pinnedPkgs { inherit overlays; };
+
+  drv =
+    pkgs.haskellPackages.callCabal2nix name src {};
+in
+rec {
+  inherit pkgs pinnedPkgs;
+
+  # Derivation for the PostgREST Haskell package, including the executable,
+  # libraries and documentation. We disable running the test suite on Nix
+  # builds, as they require a database to be set up.
+  postgrestWithLib =
+    pkgs.haskell.lib.dontCheck drv;
+
+  # Derivation for just the PostgREST binary, where we strip all dynamic
+  # libraries and documentation, leaving only the executable.
+  postgrest =
+    pkgs.haskell.lib.justStaticExecutables postgrestWithLib;
+
+  # Environment in which PostgREST can be built with cabal, useful e.g. for
+  # defining a shell for nix-shell.
+  env =
+    drv.env;
+
+  # Utility for updating the pinned version of Nixpkgs.
+  nixpkgsUpgrade =
+    pkgs.callPackage nix/nixpkgs-upgrade.nix {};
+}