Add nix package expression

.gitignore

@@ -1,2 +1,3 @@
 /vendor/
 /bangs.go
+/result*

default.nix

@@ -0,0 +1,23 @@
+{ pkgs ? import <nixpkgs> { } }:
+let
+  gitignoreSrc = pkgs.fetchFromGitHub {
+    owner = "hercules-ci";
+    repo = "gitignore";
+    rev = "c4662e662462e7bf3c2a968483478a665d00e717";
+    sha256 = "1npnx0h6bd0d7ql93ka7azhj40zgjp815fw2r6smg8ch9p7mzdlx";
+  };
+  inherit (import gitignoreSrc { inherit (pkgs) lib; }) gitignoreSource;
+in
+pkgs.buildGoModule {
+  name = "bang-evaluator";
+
+  src = gitignoreSource ./.; # FIXME: reproducibility
+
+  subPackages = [ "." ];
+
+  vendorSha256 = "11r1l5lcdfm3wymrkbddl5khpjmr30jln31l40mfyyy9msnqayf3";
+
+  preBuild = ''go generate ./...'';
+
+  doCheck = false; # no tests (yet?)
+}