modules: Raise an error when a dependency could not be found.

* guix/modules.scm (&missing-dependency-error): New error condition.
(source-module-dependencies): Raise it when 'search-path' returns #f.
* tests/modules.scm ("&missing-dependency-error"): New test.

guix/modules.scm

@@ -20,8 +20,13 @@
   #:use-module (guix memoization)
   #:use-module (guix sets)
   #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-34)
+  #:use-module (srfi srfi-35)
   #:use-module (ice-9 match)
-  #:export (source-module-closure
+  #:export (missing-dependency-error?
+            missing-dependency-module
+
+            source-module-closure
             live-module-closure
             guix-module-name?))
 
@@ -35,6 +40,11 @@
 ;;;
 ;;; Code:
 
+;; The error corresponding to a missing module.
+(define-condition-type &missing-dependency-error &error
+  missing-dependency-error?
+  (module  missing-dependency-module))
+
 (define (colon-symbol? obj)
   "Return true if OBJ is a symbol that starts with a colon."
   (and (symbol? obj)
@@ -106,9 +116,12 @@ depends on."
   "Return the modules used by MODULE by looking at its source code."
   (if (member module %source-less-modules)
       '()
-      (module-file-dependencies
-       (search-path load-path
-                    (module-name->file-name module)))))
+      (match (search-path load-path (module-name->file-name module))
+        ((? string? file)
+         (module-file-dependencies file))
+        (#f
+         (raise (condition (&missing-dependency-error
+                            (module module))))))))
 
 (define* (module-closure modules
                          #:key

tests/modules.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2016 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2016, 2017 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -19,7 +19,9 @@
 (define-module (test-modules)
   #:use-module (guix modules)
   #:use-module ((guix build-system gnu) #:select (%gnu-build-system-modules))
+  #:use-module ((guix utils) #:select (call-with-temporary-directory))
   #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-64))
 
 (test-begin "modules")
@@ -42,4 +44,25 @@
          (live-module-closure '((gnu build vm)))
          (source-module-closure '((gnu build vm)))))
 
+(test-equal "&missing-dependency-error"
+  '(something that does not exist)
+  (call-with-temporary-directory
+   (lambda (directory)
+     (call-with-output-file (string-append directory "/foobar.scm")
+       (lambda (port)
+         (write '(define-module (foobar)
+                   #:use-module (something that does not exist))
+                port)))
+
+     (call-with-output-file (string-append directory "/baz.scm")
+       (lambda (port)
+         (write '(define-module (baz)
+                   #:use-module (foobar))
+                port)))
+
+     (guard (c ((missing-dependency-error? c)
+                (missing-dependency-module c)))
+       (source-module-closure '((baz)) (list directory)
+                              #:select? (const #t))))))
+
 (test-end)