gnu: Improve error reporting of the use-.*modules macros.

Suggested by Julien Lepiller and myglc2
at <https://lists.gnu.org/archive/html/guix-devel/2017-11/msg00106.html>.

* gnu.scm (%try-use-modules): New procedure.
(package-module-hint, service-module-hint): New procedures.
(try-use-modules): New macro.
(use-package-modules, use-service-modules, use-system-modules): Use it.
* tests/guix-system.sh: Test it.

gnu.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014, 2015, 2016 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2015 Joshua S. Grant <jgrant@parenthetical.io>
 ;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;;
@@ -19,6 +19,14 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (gnu)
+  #:use-module (guix i18n)
+  #:use-module (guix utils)
+  #:use-module (srfi srfi-34)
+  #:use-module (srfi srfi-35)
+  #:use-module (ice-9 match)
+  #:use-module (guix packages)
+  #:use-module (gnu packages)
+  #:use-module (gnu services)
   #:export (use-package-modules
             use-service-modules
             use-system-modules))
@@ -52,13 +60,97 @@
                   (module-use! i (resolve-interface m))))
               %public-modules)))
 
+(define (%try-use-modules modules location make-hint)
+  "Attempt to load all of MODULES.  Report errors as coming from LOCATION, a
+<location> record, and use MAKE-HINT to produce a fix hint."
+  (define (location->string loc)
+    (match loc
+      (#f "")
+      (($ <location> file line column)
+       (format #f "~a:~a:~a: " file line column))))
+
+  (for-each (lambda (module)
+              (catch 'misc-error
+                (lambda ()
+                  (process-use-modules `((,module))))
+                (lambda _
+                  (raise
+                   (apply
+                    make-compound-condition
+                    (condition
+                     (&message
+                      (message (format #f (G_ "module ~a not found")
+                                       module))))
+                    (condition
+                     (&error-location (location location)))
+                    (or (and=> (make-hint module) list)
+                        '()))))))
+            modules))
+
+(define (package-module-hint module)
+  (define last-name
+    (match module
+      ((_ ... last)
+       (symbol->string last))))
+
+  (match (find-packages-by-name last-name)
+    (()
+     (condition
+      (&fix-hint
+       (hint (G_ "\
+You may use @command{guix package --show=foo | grep location} to search
+for the location of package @code{foo}.
+If you get the line @code{location: gnu/packages/bar.scm:174:2},
+add @code{bar} to the @code{use-package-modules} form.")))))
+    ((package _ ...)
+     (condition
+      (&fix-hint
+       (hint (format #f (G_ "\
+Try adding @code{(use-package-modules ~a)}.")
+                     (basename (location-file (package-location package))
+                               ".scm"))))))))
+
+(define (service-module-hint module)
+  (define last-name
+    (match module
+      ((_ ... last)
+       last)))
+
+  (match (lookup-service-types last-name)
+    (()
+     (condition
+      (&fix-hint
+       (hint (format #f (G_ "\
+You may use @command{guix system search ~a} to search for a service
+matching @code{~a}.
+If you get the line @code{location: gnu/services/foo.scm:188:2},
+add @code{foo} to the @code{use-service-modules} form.")
+                     last-name last-name)))))
+    ((package _ ...)
+     (condition
+      (&fix-hint
+       (hint (format #f (G_ "\
+Try adding @code{(use-service-modules ~a)}.")
+                     (basename (location-file (service-type-location package))
+                               ".scm"))))))))
+
+(define-syntax-rule (try-use-modules hint modules ...)
+  (eval-when (expand load eval)
+    (%try-use-modules '(modules ...)
+                      (source-properties->location
+                       (current-source-location))
+                      hint)))
+
 (define-syntax-rule (use-package-modules module ...)
-  (use-modules (gnu packages module) ...))
+  (try-use-modules package-module-hint
+                   (gnu packages module) ...))
 
 (define-syntax-rule (use-service-modules module ...)
-  (use-modules (gnu services module) ...))
+  (try-use-modules service-module-hint
+                   (gnu services module) ...))
 
 (define-syntax-rule (use-system-modules module ...)
-  (use-modules (gnu system module) ...))
+  (try-use-modules (const #f)                     ;no hint
+                   (gnu system module) ...))
 
 ;;; gnu.scm ends here

po/guix/POTFILES.in

@@ -1,5 +1,6 @@
 # List of source files which contain translatable strings.
 # This should be source files of the various tools, and not package modules.
+gnu.scm
 gnu/packages.scm
 gnu/services.scm
 gnu/system.scm

tests/guix-system.sh

@@ -68,6 +68,34 @@ else
 fi
 
 
+# Reporting of module not found errors.
+
+cat > "$tmpfile" <<EOF
+;; Line 1.
+(use-modules (gnu))
+  (use-service-modules openssh)
+EOF
+
+if guix system build "$tmpfile" -n 2> "$errorfile"
+then false
+else
+    grep "$tmpfile:3:2: .*module .*openssh.*not found" "$errorfile"
+    grep "Try.*use-service-modules ssh" "$errorfile"
+fi
+
+cat > "$tmpfile" <<EOF
+;; Line 1.
+(use-modules (gnu))
+  (use-package-modules qemu)
+EOF
+
+if guix system build "$tmpfile" -n 2> "$errorfile"
+then false
+else
+    grep "$tmpfile:3:2: .*module .*qemu.*not found" "$errorfile"
+    grep "Try.*use-package-modules virtualization" "$errorfile"
+fi
+
 # Reporting of unbound variables.
 
 cat > "$tmpfile" <<EOF