graph: Add '%reverse-package-node-type'.

* guix/scripts/graph.scm (%reverse-package-node-type): New variable.
(%node-types): Add it.
* tests/graph.scm ("reverse package DAG"): New test.
* doc/guix.texi (Invoking guix refresh): Add cross-reference to
"Invoking guix graph".
(Invoking guix graph): Document 'reverse-package'.

1 files changed, 21 insertions, 0 deletions

diff --git a/guix/scripts/graph.scm b/guix/scripts/graph.scm
index d96df5fbaf..79ce503a2e 100644
--- a/guix/scripts/graph.scm
+++ b/guix/scripts/graph.scm
@@ -37,6 +37,7 @@
   #:use-module (srfi srfi-37)
   #:use-module (ice-9 match)
   #:export (%package-node-type
+            %reverse-package-node-type
             %bag-node-type
             %bag-with-origins-node-type
             %bag-emerged-node-type
@@ -103,6 +104,25 @@ name."
 
 
 ;;;
+;;; Reverse package DAG.
+;;;
+
+(define %reverse-package-node-type
+  ;; For this node type we first need to compute the list of packages and the
+  ;; list of back-edges.  Since we want to do it only once, we use the
+  ;; promises below.
+  (let* ((packages   (delay (fold-packages cons '())))
+         (back-edges (delay (run-with-store #f    ;store not actually needed
+                              (node-back-edges %package-node-type
+                                               (force packages))))))
+    (node-type
+     (inherit %package-node-type)
+     (name "reverse-package")
+     (description "the reverse DAG of packages")
+     (edges (lift1 (force back-edges) %store-monad)))))
+
+
+;;;
 ;;; Package DAG using bags.
 ;;;
 
@@ -323,6 +343,7 @@ substitutes."
 (define %node-types
   ;; List of all the node types.
   (list %package-node-type
+        %reverse-package-node-type
         %bag-node-type
         %bag-with-origins-node-type
         %bag-emerged-node-type