[PATCH] Inhibit duplicates in fold-packages

[Mark H Weaver <mhw@netris.org>]

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Here's a patch to inhibit the same package (in the sense of eq?) from
being traversed more than once by fold-packages.  One example where this
helps is the guile-2.0 package in (gnu packages guile), which is
exported in two different variables: guile-2.0 and guile-2.0/fixed.

Note that even after applying this patch, there are still cases where
"guix-package -A ." produces the same output twice, but that's a
different problem.  In the remaining cases, there are actually two
distinct package objects being printed the same way because the
'location' field is inherited from one package to another.  For example
this happens with the 'gcc-boot0' and 'gcc-4.7' packages in (gnu
packages base).

What do you think?

    Mark



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From 1d30931a28af4ed6aa747a05bc3540e335ce1b32 Mon Sep 17 00:00:00 2001
From: Mark H Weaver <mhw@netris.org>
Date: Tue, 12 Feb 2013 20:29:30 -0500
Subject: [PATCH] Inhibit duplicates in fold-packages.

* gnu/packages.scm (fold2): New procedure.
  (fold-packages): Rework to suppress duplicates.
---
 gnu/packages.scm |   36 +++++++++++++++++++++++++-----------
 1 file changed, 25 insertions(+), 11 deletions(-)

diff --git a/gnu/packages.scm b/gnu/packages.scm
index 6a5496e..d783e4c 100644
--- a/gnu/packages.scm
+++ b/gnu/packages.scm
@@ -21,6 +21,7 @@
   #:use-module (guix utils)
   #:use-module (ice-9 ftw)
   #:use-module (ice-9 vlist)
+  #:use-module (ice-9 match)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-39)
@@ -108,20 +109,33 @@
                   (false-if-exception (resolve-interface name))))
               (package-files)))
 
+(define (fold2 f seed1 seed2 lst)
+  (if (null? lst)
+      (values seed1 seed2)
+      (call-with-values
+          (lambda () (f (car lst) seed1 seed2))
+        (lambda (seed1 seed2)
+          (fold2 f seed1 seed2 (cdr lst))))))
+
 (define (fold-packages proc init)
   "Call (PROC PACKAGE RESULT) for each available package, using INIT as
 the initial value of RESULT."
-  (fold (lambda (module result)
-          (fold (lambda (var result)
-                  (if (package? var)
-                      (proc var result)
-                      result))
-                result
-                (module-map (lambda (sym var)
-                              (false-if-exception (variable-ref var)))
-                            module)))
-        init
-        (package-modules)))
+  (identity   ; discard second return value
+   (fold2 (lambda (module result seen)
+            (fold2 (lambda (var result seen)
+                     (if (and (package? var)
+                              (not (vhash-assq var seen)))
+                         (values (proc var result)
+                                 (vhash-consq var #t seen))
+                         (values result seen)))
+                   result
+                   seen
+                   (module-map (lambda (sym var)
+                                 (false-if-exception (variable-ref var)))
+                               module)))
+          init
+          vlist-null
+          (package-modules))))
 
 (define* (find-packages-by-name name #:optional version)
   "Return the list of packages with the given NAME.  If VERSION is not #f,
-- 
1.7.10.4