feat(emacs-pkgs): Introduce nix-util helper library

Adds an Emacs library with so far a grand total of one helper function
that can prefetch and insert a git repository at point.

This is very useful for the various Go repo imports I am doing at the
moment.

tools/emacs-pkgs/nix-util/default.nix

@@ -0,0 +1,7 @@
+{ pkgs, ... }:
+
+pkgs.third_party.emacsPackagesNg.trivialBuild rec {
+  pname = "nix-util";
+  version = "1.0";
+  src = ./nix-util.el;
+}

tools/emacs-pkgs/nix-util/nix-util.el

@@ -0,0 +1,67 @@
+;;; nix-util.el --- Utilities for dealing with Nix code. -*- lexical-binding: true; -*-
+;;
+;; Copyright (C) 2019 Google Inc.
+;;
+;; Author: Vincent Ambo <tazjin@google.com>
+;; Version: 1.0
+;; Package-Requires: (json map)
+;;
+;;; Commentary:
+;;
+;; This package adds some functionality that I find useful when
+;; working in Nix buffers.
+
+(require 'json)
+(require 'map)
+
+(defun nix/prefetch-github (owner repo) ; TODO(tazjin): support different branches
+  "Fetch the master branch of a GitHub repository and insert the
+  call to `fetchFromGitHub' at point."
+
+  (interactive "sOwner: \nsRepository: ")
+
+  (let* (;; Keep these vars around for output insertion
+         (point (point))
+         (buffer (current-buffer))
+         (name (concat "github-fetcher/" owner "/" repo))
+         (outbuf (format "*%s*" name))
+         (errbuf (get-buffer-create "*github-fetcher/errors*"))
+         (cleanup (lambda ()
+                    (kill-buffer outbuf)
+                    (kill-buffer errbuf)
+                    (with-current-buffer buffer
+                      (read-only-mode -1))))
+         (prefetch-handler
+          (lambda (_process event)
+            (unwind-protect
+                (pcase event
+                  ("finished\n"
+                   (let* ((json-string (with-current-buffer outbuf
+                                         (buffer-string)))
+                          (result (json-parse-string json-string)))
+                     (with-current-buffer buffer
+                       (goto-char point)
+                       (map-let (("rev" rev) ("sha256" sha256)) result
+                         (read-only-mode -1)
+                         (insert (format "fetchFromGitHub {
+  owner = \"%s\";
+  repo = \"%s\";
+  rev = \"%s\";
+  sha256 = \"%s\";
+};" owner repo rev sha256))
+                         (indent-region point (point))))))
+                  (_ (with-current-buffer errbuf
+                       (error "Failed to prefetch %s/%s: %s"
+                              owner repo (buffer-string)))))
+              (funcall cleanup)))))
+
+    ;; Fetching happens asynchronously, but we'd like to make sure the
+    ;; point stays in place while that happens.
+    (read-only-mode)
+    (make-process :name name
+                  :buffer outbuf
+                  :command `("nix-prefetch-github" ,owner ,repo)
+                  :stderr errbuf
+                  :sentinel prefetch-handler)))
+
+(provide 'nix-util)

tools/emacs/default.nix

@@ -83,8 +83,12 @@ let
   ]) ++
 
   # Custom packages
-  [ carp-mode localPackages.dottime localPackages.term-switcher ]
-  )));
+  (with localPackages; [
+    carp-mode
+    dottime
+    nix-util
+    term-switcher
+  ]))));
 in lib.fix(self: l: f: third_party.writeShellScriptBin "tazjins-emacs" ''
   exec ${tazjinsEmacs f}/bin/emacs \
     --debug-init \