packages: Add 'package-mapping' and base 'package-input-rewriting' on it.

* guix/packages.scm (package-mapping): New procedure.
(package-input-rewriting): Rewrite in terms of 'package-mapping'.
* tests/packages.scm ("package-mapping"): New test.
* doc/guix.texi (Defining Packages): Document it.

1 files changed, 37 insertions, 19 deletions

diff --git a/guix/packages.scm b/guix/packages.scm
index b68b3de6d2..44f2c32fb7 100644
--- a/guix/packages.scm
+++ b/guix/packages.scm
@@ -98,6 +98,7 @@
             package-transitive-propagated-inputs
             package-transitive-native-search-paths
             package-transitive-supported-systems
+            package-mapping
             package-input-rewriting
             package-source-derivation
             package-derivation
@@ -741,36 +742,53 @@ dependencies are known to build on SYSTEM."
   "Return the \"target inputs\" of BAG, recursively."
   (transitive-inputs (bag-target-inputs bag)))
 
-(define* (package-input-rewriting replacements
-                                  #:optional (rewrite-name identity))
-  "Return a procedure that, when passed a package, replaces its direct and
-indirect dependencies (but not its implicit inputs) according to REPLACEMENTS.
-REPLACEMENTS is a list of package pairs; the first element of each pair is the
-package to replace, and the second one is the replacement.
-
-Optionally, REWRITE-NAME is a one-argument procedure that takes the name of a
-package and returns its new name after rewrite."
+(define* (package-mapping proc #:optional (cut? (const #f)))
+  "Return a procedure that, given a package, applies PROC to all the packages
+depended on and returns the resulting package.  The procedure stops recursion
+when CUT? returns true for a given package."
   (define (rewrite input)
     (match input
       ((label (? package? package) outputs ...)
-       (match (assq-ref replacements package)
-         (#f  (cons* label (replace package) outputs))
-         (new (cons* label new outputs))))
+       (let ((proc (if (cut? package) proc replace)))
+         (cons* label (proc package) outputs)))
       (_
        input)))
 
   (define replace
     (mlambdaq (p)
-      ;; Return a variant of P with its inputs rewritten.
-      (package
-        (inherit p)
-        (name (rewrite-name (package-name p)))
-        (inputs (map rewrite (package-inputs p)))
-        (native-inputs (map rewrite (package-native-inputs p)))
-        (propagated-inputs (map rewrite (package-propagated-inputs p))))))
+      ;; Return a variant of P with PROC applied to P and its explicit
+      ;; dependencies, recursively.  Memoize the transformations.  Failing to
+      ;; do that, we would build a huge object graph with lots of duplicates,
+      ;; which in turns prevents us from benefiting from memoization in
+      ;; 'package-derivation'.
+      (let ((p (proc p)))
+        (package
+          (inherit p)
+          (location (package-location p))
+          (inputs (map rewrite (package-inputs p)))
+          (native-inputs (map rewrite (package-native-inputs p)))
+          (propagated-inputs (map rewrite (package-propagated-inputs p)))))))
 
   replace)
 
+(define* (package-input-rewriting replacements
+                                  #:optional (rewrite-name identity))
+  "Return a procedure that, when passed a package, replaces its direct and
+indirect dependencies (but not its implicit inputs) according to REPLACEMENTS.
+REPLACEMENTS is a list of package pairs; the first element of each pair is the
+package to replace, and the second one is the replacement.
+
+Optionally, REWRITE-NAME is a one-argument procedure that takes the name of a
+package and returns its new name after rewrite."
+  (define (rewrite p)
+    (match (assq-ref replacements p)
+      (#f  (package
+             (inherit p)
+             (name (rewrite-name (package-name p)))))
+      (new new)))
+
+  (package-mapping rewrite (cut assq <> replacements)))
+
 
 ;;;
 ;;; Package derivations.