doc: cookbook: Update "Build system arguments" section.

* doc/guix-cookbook.texi (Build system arguments)

Remove recommendation on phase return value.
Ensure code examples can run and are up-to-date.

Change-Id: I143babf79983751578d6d4e20b20195ea000656d
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

1 files changed, 23 insertions, 14 deletions

diff --git a/doc/guix-cookbook.texi b/doc/guix-cookbook.texi
index 3a37602018..cbb0163bd3 100644
--- a/doc/guix-cookbook.texi
+++ b/doc/guix-cookbook.texi
@@ -1255,6 +1255,7 @@ definition in @samp{$GUIX_CHECKOUT/guix/build/gnu-build-system.scm}:
             validate-documentation-location
             delete-info-dir-file
             patch-dot-desktop-files
+            make-dynamic-linker-cache
             install-license-files
             reset-gzip-timestamps
             compress-documentation)))
@@ -1265,8 +1266,8 @@ Or from the REPL:
 @lisp
 (add-to-load-path "/path/to/guix/checkout")
 ,use (guix build gnu-build-system)
-(map first %standard-phases)
-@result{} (set-SOURCE-DATE-EPOCH set-paths install-locale unpack bootstrap patch-usr-bin-file patch-source-shebangs configure patch-generated-file-shebangs build check install patch-shebangs strip validate-runpath validate-documentation-location delete-info-dir-file patch-dot-desktop-files install-license-files reset-gzip-timestamps compress-documentation)
+(map car %standard-phases)
+@result{} (set-SOURCE-DATE-EPOCH set-paths install-locale unpack bootstrap patch-usr-bin-file patch-source-shebangs configure patch-generated-file-shebangs build check install patch-shebangs strip validate-runpath validate-documentation-location delete-info-dir-file patch-dot-desktop-files make-dynamic-linker-cache install-license-files reset-gzip-timestamps compress-documentation)
 @end lisp
 
 If you want to know more about what happens during those phases, consult the
@@ -1288,13 +1289,25 @@ working directory."
         ;; Preserve timestamps (set to the Epoch) on the copied tree so that
         ;; things work deterministically.
         (copy-recursively source "."
-                          #:keep-mtime? #true))
+                          #:keep-mtime? #t)
+        ;; Make the source checkout files writable, for convenience.
+        (for-each (lambda (f)
+                    (false-if-exception (make-file-writable f)))
+                  (find-files ".")))
       (begin
-        (if (string-suffix? ".zip" source)
-            (invoke "unzip" source)
-            (invoke "tar" "xvf" source))
-        (chdir (first-subdirectory "."))))
-  #true)
+        (cond
+         ((string-suffix? ".zip" source)
+          (invoke "unzip" source))
+         ((tarball? source)
+          (invoke "tar" "xvf" source))
+         (else
+          (let ((name (strip-store-file-name source))
+                (command (compressor source)))
+            (copy-file source name)
+            (when command
+              (invoke command "--decompress" name)))))
+        ;; Attempt to change into child directory.
+        (and=> (first-subdirectory ".") chdir))))
 @end lisp
 
 Note the @code{chdir} call: it changes the working directory to where the source was
@@ -1330,14 +1343,10 @@ this:
   (let ((bash-directory (assoc-ref inputs "bash"))
         (output-directory (assoc-ref outputs "out"))
         (doc-directory (assoc-ref outputs "doc")))
-    ;; ...
-    #true))
+    ;; ... ))
 @end lisp
 
-The procedure must return @code{#true} on success.  It's brittle to rely on the return
-value of the last expression used to tweak the phase because there is no
-guarantee it would be a @code{#true}.  Hence the trailing @code{#true} to ensure the right value
-is returned on success.
+Its return value is ignored.
 
 @subsubsection Code staging