Add static build with Nix (fixes #1295) (#1494)

* Include modifications to nixpkgs and static-haskell-nix as patches.

* use the patched static-haskell-nix version

* Add cachix postgrest

default.nix

@@ -2,6 +2,9 @@ let
   name =
     "postgrest";
 
+  compiler =
+    "ghc883";
+
   # 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
@@ -11,10 +14,13 @@ let
       (pkgs.gitignoreSource ./.)
       [ ".cabal" ".hs" ".lhs" "LICENSE" ];
 
+  # Commit of the Nixpkgs repository that we want to use.
   nixpkgsVersion =
     import nix/nixpkgs-version.nix;
 
-  pinnedPkgs =
+  # Nix files that describe the Nixpkgs repository. We evaluate the expression
+  # using `import` below.
+  nixpkgs =
     builtins.fetchTarball {
       url = "https://github.com/nixos/nixpkgs/archive/${nixpkgsVersion.rev}.tar.gz";
       sha256 = nixpkgsVersion.tarballHash;
@@ -27,8 +33,9 @@ let
       (import nix/overlays/haskell-packages)
     ];
 
+  # Evaluated expression of the Nixpkgs repository.
   pkgs =
-    import pinnedPkgs { inherit overlays; };
+    import nixpkgs { inherit overlays; };
 
   postgresqlVersions =
     [
@@ -40,22 +47,39 @@ let
       pkgs.postgresql_9_4
     ];
 
+  patches =
+    pkgs.callPackage nix/patches {};
+
+  # Base dynamic derivation for the PostgREST package.
   drv =
     pkgs.haskellPackages.callCabal2nix name src {};
+
+  # Static derivation for the PostgREST executable.
+  drvStatic =
+    import nix/static { inherit nixpkgs name src compiler patches; };
+
+  lib =
+    pkgs.haskell.lib;
 in
 rec {
-  inherit pkgs pinnedPkgs;
+  inherit nixpkgs pkgs;
 
   # 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;
+    lib.dontCheck drv;
 
   # Derivation for just the PostgREST binary, where we strip all dynamic
-  # libraries and documentation, leaving only the executable.
+  # libraries and documentation, leaving only the executable. Note that the
+  # executable is static with regards to Haskell libraries, but not system
+  # libraries like glibc and libpq.
   postgrest =
-    pkgs.haskell.lib.justStaticExecutables postgrestWithLib;
+    lib.justStaticExecutables postgrestWithLib;
+
+  # Static executable.
+  postgrestStatic =
+    lib.justStaticExecutables (lib.dontCheck drvStatic);
 
   # Environment in which PostgREST can be built with cabal, useful e.g. for
   # defining a shell for nix-shell.