import/cran: Add input style "specification".

* guix/import/cran.scm (%input-style): New parameter.
(format-inputs): Use it.
* guix/scripts/import/cran.scm (guix-import-cran): Set the %input-style
parameter.
 (%options): Add "--style" option.
* doc/guix.texi (Invoking guix import): Document it.

doc/guix.texi

@@ -11009,6 +11009,13 @@ When @option{--recursive} is added, the importer will traverse the
 dependency graph of the given upstream package recursively and generate
 package expressions for all those packages that are not yet in Guix.
 
+When @option{--style=specification} is added, the importer will generate
+package definitions whose inputs are package specifications instead of
+references to package variables.  This is useful when generated package
+definitions are to be appended to existing user modules, as the list of
+used package modules need not be changed.  The default is
+@option{--style=variable}.
+
 When @option{--archive=bioconductor} is added, metadata is imported from
 @uref{https://www.bioconductor.org/, Bioconductor}, a repository of R
 packages for the analysis and comprehension of high-throughput

guix/import/cran.scm

@@ -51,7 +51,9 @@
   #:use-module (guix upstream)
   #:use-module (guix packages)
   #:use-module (gnu packages)
-  #:export (cran->guix-package
+  #:export (%input-style
+
+            cran->guix-package
             bioconductor->guix-package
             cran-recursive-import
             %cran-updater
@@ -74,6 +76,9 @@
 ;;;
 ;;; Code:
 
+(define %input-style
+  (make-parameter 'variable)) ; or 'specification
+
 (define string->license
   (match-lambda
    ("AGPL-3" 'agpl3+)
@@ -128,7 +133,11 @@
 (define (format-inputs names)
   "Generate a sorted list of package inputs from a list of package NAMES."
   (map (lambda (name)
-         (list name (list 'unquote (string->symbol name))))
+         (case (%input-style)
+           ((specification)
+            (list name (list 'unquote (list 'specification->package name))))
+           (else
+            (list name (list 'unquote (string->symbol name))))))
        (sort names string-ci<?)))
 
 (define* (maybe-inputs package-inputs #:optional (type 'inputs))

guix/scripts/import/cran.scm

@@ -67,6 +67,10 @@ Import and convert the CRAN package for PACKAGE-NAME.\n"))
                  (lambda (opt name arg result)
                    (alist-cons 'repo (string->symbol arg)
                                (alist-delete 'repo result))))
+         (option '(#\s "style") #t #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'style (string->symbol arg)
+                               (alist-delete 'style result))))
          (option '(#\r "recursive") #f #f
                  (lambda (opt name arg result)
                    (alist-cons 'recursive #t result)))
@@ -93,6 +97,7 @@ Import and convert the CRAN package for PACKAGE-NAME.\n"))
                              value)
                             (_ #f))
                            (reverse opts))))
+    (parameterize ((%input-style (assoc-ref opts 'style)))
       (match args
         ((package-name)
          (if (assoc-ref opts 'recursive)
@@ -112,4 +117,4 @@ Import and convert the CRAN package for PACKAGE-NAME.\n"))
         (()
          (leave (G_ "too few arguments~%")))
         ((many ...)
-       (leave (G_ "too many arguments~%"))))))
+         (leave (G_ "too many arguments~%")))))))