From cdf9811d24b9c857cb79e0ddd38181862ec34bd3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ludovic=20Court=C3=A8s?= <ludo@gnu.org>
Date: Thu, 3 Oct 2019 22:54:28 +0200
Subject: [PATCH] gexp: 'load-path-expression' produces an expression that
 deletes duplicates.

Fixes <https://bugs.gnu.org/37531>.

"herd eval root '(length %load-path)'" on a freshly-booted bare-bones
system now returns 8 instead of 119 before.

* guix/gexp.scm (load-path-expression): Rewrite expression to that it
deletes duplicates.
---
 guix/gexp.scm | 49 +++++++++++++++++++++++++++++++------------------
 1 file changed, 31 insertions(+), 18 deletions(-)

diff --git a/guix/gexp.scm b/guix/gexp.scm
index e788fc5981..26881ce16c 100644
--- a/guix/gexp.scm
+++ b/guix/gexp.scm
@@ -1527,24 +1527,37 @@ are searched for in PATH.  Return #f when MODULES and EXTENSIONS are empty."
                                                       #:module-path path
                                                       #:system system
                                                       #:target target)))
-        (return (gexp (eval-when (expand load eval)
-                        (set! %load-path
-                          (cons (ungexp modules)
-                                (append (map (lambda (extension)
-                                               (string-append extension
-                                                              "/share/guile/site/"
-                                                              (effective-version)))
-                                             '((ungexp-native-splicing extensions)))
-                                        %load-path)))
-                        (set! %load-compiled-path
-                          (cons (ungexp compiled)
-                                (append (map (lambda (extension)
-                                               (string-append extension
-                                                              "/lib/guile/"
-                                                              (effective-version)
-                                                              "/site-ccache"))
-                                             '((ungexp-native-splicing extensions)))
-                                        %load-compiled-path)))))))))
+        (return
+         (gexp (eval-when (expand load eval)
+                 ;; Augment the load paths and delete duplicates.  Do that
+                 ;; without loading (srfi srfi-1) or anything.
+                 (let ((extensions '((ungexp-native-splicing extensions)))
+                       (prepend (lambda (items lst)
+                                  ;; This is O(N²) but N is typically small.
+                                  (let loop ((items items)
+                                             (lst lst))
+                                    (if (null? items)
+                                        lst
+                                        (loop (cdr items)
+                                              (cons (car items)
+                                                    (delete (car items) lst))))))))
+                   (set! %load-path
+                     (prepend (cons (ungexp modules)
+                                    (map (lambda (extension)
+                                           (string-append extension
+                                                          "/share/guile/site/"
+                                                          (effective-version)))
+                                         extensions))
+                              %load-path))
+                   (set! %load-compiled-path
+                     (prepend (cons (ungexp compiled)
+                                    (map (lambda (extension)
+                                           (string-append extension
+                                                          "/lib/guile/"
+                                                          (effective-version)
+                                                          "/site-ccache"))
+                                         extensions))
+                              %load-compiled-path)))))))))
 
 (define* (gexp->script name exp
                        #:key (guile (default-guile))