Add (guix describe) and use it to initialize '%package-search-path'.

* guix/describe.scm: New file.
* Makefile.am (MODULES): Add it.
* gnu/packages.scm (%default-package-module-path): New variable.
(%package-module-path): Honor 'package-path-entries'.
* build-aux/update-NEWS.scm (main): Use %DEFAULT-PACKAGE-MODULE-PATH
instead of (last (%package-module-path)).

Makefile.am

@@ -86,6 +86,7 @@ MODULES =					\
   guix/derivations.scm				\
   guix/grafts.scm				\
   guix/inferior.scm				\
+  guix/describe.scm				\
   guix/gnu-maintenance.scm			\
   guix/self.scm					\
   guix/upstream.scm				\

build-aux/update-NEWS.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2017 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2017, 2018 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -128,11 +128,10 @@ paragraph."
 (define (main . args)
   (match args
     ((news-file data-directory)
-     ;; Don't browse things listed in the user's $GUIX_PACKAGE_PATH.  Here we
+     ;; Don't browse things listed in the user's $GUIX_PACKAGE_PATH and
-     ;; assume that the last item in (%package-module-path) is the distro
+     ;; in external channels.
-     ;; directory.
      (parameterize ((%package-module-path
-                     (list (last (%package-module-path)))))
+                     %default-package-module-path))
        (define (package-file version)
          (string-append data-directory "/packages-"
                         version ".txt"))

gnu/packages.scm

@@ -30,6 +30,7 @@
                 #:select ((package-name->name+version
                            . hyphen-separated-name->name+version)))
   #:autoload   (guix profiles) (packages->manifest)
+  #:use-module (guix describe)
   #:use-module (ice-9 vlist)
   #:use-module (ice-9 match)
   #:use-module (srfi srfi-1)
@@ -46,6 +47,7 @@
             %auxiliary-files-path
             %bootstrap-binaries-path
             %package-module-path
+            %default-package-module-path
 
             fold-packages
 
@@ -130,22 +132,31 @@ for system '~a'")
          ("gnu/packages.scm"      gnu/)
          ("guix.scm"))))
 
+(define %default-package-module-path
+  ;; Default search path for package modules.
+  `((,%distro-root-directory . "gnu/packages")))
+
 (define %package-module-path
   ;; Search path for package modules.  Each item must be either a directory
   ;; name or a pair whose car is a directory and whose cdr is a sub-directory
   ;; to narrow the search.
   (let* ((not-colon   (char-set-complement (char-set #\:)))
          (environment (string-tokenize (or (getenv "GUIX_PACKAGE_PATH") "")
-                                       not-colon)))
+                                       not-colon))
-    ;; Automatically add items from $GUIX_PACKAGE_PATH to Guile's search path.
+         (channels    (package-path-entries)))
-    (for-each (lambda (directory)
+    ;; Automatically add channels and items from $GUIX_PACKAGE_PATH to Guile's
-                (set! %load-path (cons directory %load-path))
+    ;; search path.  For historical reasons, $GUIX_PACKAGE_PATH goes to the
+    ;; front; channels go to the back so that they don't override Guix' own
+    ;; modules.
+    (set! %load-path
+      (append environment %load-path channels))
     (set! %load-compiled-path
-                      (cons directory %load-compiled-path)))
+      (append environment %load-compiled-path channels))
-              environment)
 
     (make-parameter
-     (append environment `((,%distro-root-directory . "gnu/packages"))))))
+     (append environment
+             %default-package-module-path
+             channels))))
 
 (define %patch-path
   ;; Define it after '%package-module-path' so that '%load-path' contains user

guix/describe.scm

@@ -0,0 +1,73 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2018 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (guix describe)
+  #:use-module (guix memoization)
+  #:use-module (guix profiles)
+  #:use-module (srfi srfi-1)
+  #:use-module (ice-9 match)
+  #:export (package-path-entries))
+
+;;; Commentary:
+;;;
+;;; This module provides supporting code to allow a Guix instance to find, at
+;;; run time, which profile it's in (profiles created by 'guix pull').  That
+;;; allows it to read meta-information about itself (e.g., repository URL and
+;;; commit ID) and to find other channels available in the same profile.  It's
+;;; a bit like ELPA's pkg-info.el.
+;;;
+;;; Code:
+
+(define current-profile
+  (mlambda ()
+    "Return the profile (created by 'guix pull') the calling process lives in,
+or #f if this is not applicable."
+    (match (command-line)
+      ((program . _)
+       (and (string-suffix? "/bin/guix" program)
+            ;; Note: We want to do _lexical dot-dot resolution_.  Using ".."
+            ;; for real would instead take us into the /gnu/store directory
+            ;; that ~/.config/guix/current/bin points to, whereas we want to
+            ;; obtain ~/.config/guix/current.
+            (let ((candidate (dirname (dirname program))))
+              (and (file-exists? (string-append candidate "/manifest"))
+                   candidate)))))))
+
+(define current-profile-entries
+  (mlambda ()
+    "Return the list of entries in the 'guix pull' profile the calling process
+lives in, or #f if this is not applicable."
+    (match (current-profile)
+      (#f '())
+      (profile
+       (let ((manifest (profile-manifest profile)))
+         (manifest-entries manifest))))))
+
+(define package-path-entries
+  (mlambda ()
+    "Return a list of package path entries to be added to the package search
+path.  These entries are taken from the 'guix pull' profile the calling
+process lives in, when applicable."
+    ;; Filter out Guix itself.
+    (filter-map (lambda (entry)
+                  (and (not (string=? (manifest-entry-name entry)
+                                      "guix"))
+                       (string-append (manifest-entry-item entry)
+                                      "/share/guile/site/"
+                                      (effective-version))))
+                (current-profile-entries))))