packages: Gracefully print packages whose 'location' is #f.

* guix/packages.scm (<package> printer): Check whether LOC is #f.
* tests/packages.scm ("printer with location", "printer without
  location"): New tests.

guix/packages.scm

@@ -229,11 +229,14 @@ (define-record-type* <package>
                           (lambda (package port)
                             (let ((loc    (package-location package))
                                   (format simple-format))
-                              (format port "#<package ~a-~a ~a:~a ~a>"
+                              (format port "#<package ~a-~a ~a~a>"
                                       (package-name package)
                                       (package-version package)
-                                      (location-file loc)
-                                      (location-line loc)
+                                      (if loc
+                                          (format #f "~a:~a "
+                                                  (location-file loc)
+                                                  (location-line loc))
+                                          "")
                                       (number->string (object-address
                                                        package)
                                                       16)))))

tests/packages.scm

@@ -19,7 +19,12 @@
 (define-module (test-packages)
   #:use-module (guix tests)
   #:use-module (guix store)
-  #:use-module (guix utils)
+  #:use-module ((guix utils)
+                ;; Rename the 'location' binding to allow proper syntax
+                ;; matching when setting the 'location' field of a package.
+                #:renamer (lambda (name)
+                            (cond ((eq? name 'location) 'make-location)
+                                  (else name))))
   #:use-module (guix hash)
   #:use-module (guix derivations)
   #:use-module (guix packages)
@@ -34,6 +39,7 @@ (define-module (test-packages)
   #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-64)
   #:use-module (rnrs io ports)
+  #:use-module (ice-9 regex)
   #:use-module (ice-9 match))
 
 ;; Test the high-level packaging layer.
@@ -52,6 +58,21 @@ (define-syntax-rule (dummy-package name* extra-fields ...)
            (home-page #f) (license #f)
            extra-fields ...))
 
+(test-assert "printer with location"
+  (string-match "^#<package foo-0 foo.scm:42 [[:xdigit:]]+>$"
+                (with-output-to-string
+                  (lambda ()
+                    (write
+                     (dummy-package "foo"
+                       (location (make-location "foo.scm" 42 7))))))))
+
+(test-assert "printer without location"
+  (string-match "^#<package foo-0 [[:xdigit:]]+>$"
+                (with-output-to-string
+                  (lambda ()
+                    (write
+                     (dummy-package "foo" (location #f)))))))
+
 (test-assert "package-field-location"
   (let ()
     (define (goto port line column)