[bug#50614] [PATCH] build: utils: Add ‘list-when’ macro.

[Xinglu Chen]

* guix/build/utils.scm (list-when): New syntax
* tests/build-utils.scm ("list-when: expr1 is non-#f", list-when: expr1 is #f"):
  Test it.
* guix.texi (Build Utilities): Document it.
---
Changes since v1:

* Rename ‘optional’ to ‘list-when’.

* Make

    (list-when test consequent ...)

  equivalent to

    (if tests
        (list consequent ...)
        '())

 doc/guix.texi         | 35 +++++++++++++++++++++++++++++++++++
 guix/build/utils.scm  | 17 ++++++++++++++++-
 tests/build-utils.scm |  8 ++++++++
 3 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index 9a3e8ae12c..3b469c04c9 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -8801,6 +8801,41 @@ in a build phase of the @code{wireguard-tools} package:
         `("PATH" ":" prefix ,(list coreutils))))))
 @end lisp
 
+@subsection Miscellaneous
+
+@cindex miscellaneous build utilities
+This section documents some miscellaneous utilities that are useful to
+have.
+
+@deffn {Scheme Syntax} list-when @var{test} @var{consequent} @dots{}
+Like @code{when} (@pxref{Conditionals,,, guile, GNU Guile Reference
+Manual}), but if @var{test} evaluates to true, return @code{(list
+@var{consequent} @dots{})}, and if @var{test} evaluates to false, return
+the empty list.  This is replaces the following idiom:
+@end deffn
+
+@lisp
+(if @var{test}
+    (list @var{consequent} @dots{})
+    '())
+@end lisp
+
+with this:
+
+@lisp
+(list-when @var{test} @var{consequent} @dots{})
+@end lisp
+
+It can be useful when certain targets require an additional configure
+flags, e.g.,
+
+@lisp
+(arguments
+ `(#:configure-flags (list "--localstatedir=/var"
+                           "--sysconfdir=/etc"
+                           ,@@(list-when (hurd-target?) "--with-courage"))))
+@end lisp
+
 @subsection Build Phases
 
 @cindex build phases
diff --git a/guix/build/utils.scm b/guix/build/utils.scm
index 3beb7da67a..d3fb207ee5 100644
--- a/guix/build/utils.scm
+++ b/guix/build/utils.scm
@@ -8,6 +8,7 @@
 ;;; Copyright © 2020 Efraim Flashner <efraim@flashner.co.il>
 ;;; Copyright © 2020, 2021 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
+;;; Copyright © 2021 Xinglu Chen <public@yoctocell.xyz>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -123,7 +124,9 @@
 
             make-desktop-entry-file
 
-            locale-category->string))
+            locale-category->string
+
+            list-when))
 

 ;;;
@@ -1613,6 +1616,18 @@ returned."
              LC_NAME LC_NUMERIC LC_PAPER LC_TELEPHONE
              LC_TIME)))
 
+
+;;;
+;;; Misc.
+;;;
+
+;; If EXPR1 evaluates to a non-#f value, return '(EXPR2 ...).  Otherwise,
+;; return an empty list.
+(define-syntax list-when
+  (syntax-rules ()
+    ((_ expr1 expr2 ...)
+     (if expr1 (list expr2 ...) '()))))
+
 ;;; Local Variables:
 ;;; eval: (put 'call-with-output-file/atomic 'scheme-indent-function 1)
 ;;; eval: (put 'call-with-ascii-input-file 'scheme-indent-function 1)
diff --git a/tests/build-utils.scm b/tests/build-utils.scm
index 6b131c0af8..b558feb47d 100644
--- a/tests/build-utils.scm
+++ b/tests/build-utils.scm
@@ -3,6 +3,7 @@
 ;;; Copyright © 2019 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2021 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
+;;; Copyright © 2021 Xinglu Chen <public@yoctocell.xyz>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -287,5 +288,12 @@ print('hello world')"))
           ("guile/bin" . ,(dirname (which "guile"))))
         "guile"))))
 
+(test-equal "list-when: expr1 is non-#f"
+  (list 3 'bar)
+  (list-when 'foo (+ 1 2) 'bar))
+
+(test-equal "list-when: expr1 is #f"
+  '()
+  (list-when #f (+ 2 3) 'bar))
 
 (test-end)

base-commit: 22f7d4bce1e694b7ac38e62410d76a6d46d96c5d
-- 
2.33.0