[bug#50614] [PATCH core-updates] build: utils: Add ‘optional’ macro.

[bug#50614] [PATCH core-updates] build: utils: Add ‘optional’ macro.

[Xinglu Chen]

* guix/build/utils.scm (optional): New syntax
* tests/build-utils.scm ("optional: expr1 is non-#f", optional: expr1 is #f"):
  Test it.
* guix.texi (Build Utilities): Document it.
---
A common idiom I have seen is

  (if EXPR1
      EXPR2
      '())

with the ‘optional’ macro, one can just write

  (optional EXPR1
            EXPR2)

I am not sure if ‘optional’ is the best name (it was inspired by
‘lib.optional’ in Nixpkgs), feedback welcome!

 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..1bb9ddb397 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 build utilities
+
+@cindex miscellaneous build utilities
+This section documents some miscellaneous utilities that are useful to
+have.
+
+@deffn {Scheme Syntax} optional @var{test} @var{consequent}
+Like @code{when} (@pxref{Conditionals,,, guile, GNU Guile Reference
+Manual}), but if @var{test} evaluates to false, return the empty list.
+This is replaces the following idiom:
+@end deffn
+
+@lisp
+(if @var{test}
+    @var{consequent}
+    '())
+@end lisp
+
+with this:
+
+@lisp
+(optional @var{test}
+          @var{consequent})
+@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"
+                           ,@@(optional (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..ecf834461f 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
+
+            optional))
 
 \f
 ;;;
@@ -1613,6 +1616,18 @@ returned."
              LC_NAME LC_NUMERIC LC_PAPER LC_TELEPHONE
              LC_TIME)))
 
+\f
+;;;
+;;; Misc.
+;;;
+
+;; If EXPR1 evaluates to a non-#f value, return EXPR2.  Otherwise, return an
+;; empty list.
+(define-syntax optional
+  (syntax-rules ()
+    ((_ expr1 expr2)
+     (if expr1 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..44ad9bafc0 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 "optional: expr1 is non-#f"
+  'bar
+  (optional 'foo 'bar))
+
+(test-equal "optional: expr1 is #f"
+  '()
+  (optional #f 'bar))
 
 (test-end)

base-commit: 22f7d4bce1e694b7ac38e62410d76a6d46d96c5d
-- 
2.33.0

[bug#50614] [PATCH core-updates] build: utils: Add ‘optional’ macro.

[Liliana Marie Prikler]

Hi,

Am Donnerstag, den 16.09.2021, 09:11 +0200 schrieb Xinglu Chen:
> * guix/build/utils.scm (optional): New syntax
> * tests/build-utils.scm ("optional: expr1 is non-#f", optional: expr1
> is #f"):
>   Test it.
> * guix.texi (Build Utilities): Document it.
> ---
> A common idiom I have seen is
> 
>   (if EXPR1
>       EXPR2
>       '())
> 
> with the ‘optional’ macro, one can just write
> 
>   (optional EXPR1
>             EXPR2)
> 
> I am not sure if ‘optional’ is the best name (it was inspired by
> ‘lib.optional’ in Nixpkgs), feedback welcome!
> 
>  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..1bb9ddb397 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 build utilities
> +
> +@cindex miscellaneous build utilities
> +This section documents some miscellaneous utilities that are useful
> to +have.
> +
> +@deffn {Scheme Syntax} optional @var{test} @var{consequent}
> +Like @code{when} (@pxref{Conditionals,,, guile, GNU Guile Reference
> +Manual}), but if @var{test} evaluates to false, return the empty
> list.
> +This is replaces the following idiom:
> +@end deffn
> +
> +@lisp
> +(if @var{test}
> +    @var{consequent}
> +    '())
> +@end lisp
> +
> +with this:
> +
> +@lisp
> +(optional @var{test}
> +          @var{consequent})
> +@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"
> +                           ,@@(optional (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..ecf834461f 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
> +
> +            optional))
>  
>  
>  ;;;
> @@ -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 optional
> +  (syntax-rules ()
> +    ((_ expr1 expr2)
> +     (if expr1 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..44ad9bafc0 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 "optional: expr1 is non-#f"
> +  'bar
> +  (optional 'foo 'bar))
> +
> +(test-equal "optional: expr1 is #f"
> +  '()
> +  (optional #f 'bar))
>  
>  (test-end)
> 
> base-commit: 22f7d4bce1e694b7ac38e62410d76a6d46d96c5d

Naming choice aside, I don't think I like the asymmetry behind having a
plain symbol on the one side vs. an empty list.  What about 
(list-if condition . rest), which evaluates to rest if condition is
true and otherwise nil?

[bug#50614] [PATCH core-updates] build: utils: Add ‘optional’ macro.

[Sarah Morgensen]

Hi,

Xinglu Chen <public@yoctocell.xyz> writes:

> * guix/build/utils.scm (optional): New syntax
> * tests/build-utils.scm ("optional: expr1 is non-#f", optional: expr1 is #f"):
>   Test it.
> * guix.texi (Build Utilities): Document it.
> ---
> A common idiom I have seen is
>
>   (if EXPR1
>       EXPR2
>       '())
>
> with the ‘optional’ macro, one can just write
>
>   (optional EXPR1
>             EXPR2)
>
> I am not sure if ‘optional’ is the best name (it was inspired by
> ‘lib.optional’ in Nixpkgs), feedback welcome!

I like the idea!  But perhaps if we're getting rid of boilerplate, we
should go all the way and make it so that instead of

  (if EXPR1
      `(EXPR2 EXPR3)
      '())

we can write

  (optional EXPR1 EXPR2 EXPR3)

What do you think?

Also, maybe 'list-when' for the name?

> +@deffn {Scheme Syntax} optional @var{test} @var{consequent}
> +Like @code{when} (@pxref{Conditionals,,, guile, GNU Guile Reference
> +Manual}), but if @var{test} evaluates to false, return the empty list.
> +This is replaces the following idiom:
        ^ no "is"

> +@end deffn
> +
> +@lisp
> +(if @var{test}
> +    @var{consequent}
> +    '())
> +@end lisp
> +
> +with this:
> +
> +@lisp
> +(optional @var{test}
> +          @var{consequent})
> +@end lisp          
> +
> +It can be useful when certain targets require an additional configure
                                                 ^ no "an"

> +flags, e.g.,
> +
> +@lisp
> +(arguments
> + `(#:configure-flags (list "--localstatedir=/var"
> +                           "--sysconfdir=/etc"
> +                           ,@@(optional (hurd-target?) '("--with-courage"))))
                               ^ extra "@"?

--
Sarah

[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))
 
 \f
 ;;;
@@ -1613,6 +1616,18 @@ returned."
              LC_NAME LC_NUMERIC LC_PAPER LC_TELEPHONE
              LC_TIME)))
 
+\f
+;;;
+;;; 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

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

[Liliana Marie Prikler]

Am Freitag, den 17.09.2021, 10:26 +0200 schrieb 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 ...)
>         '())
LGTM, but let's wait on more opinions.  IIRC, changing (guix build)
causes a world rebuild, no?  So this patch might go to core-updates
first.

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

[Sarah Morgensen]

Hi,

Liliana Marie Prikler <liliana.prikler@gmail.com> writes:

> Am Freitag, den 17.09.2021, 10:26 +0200 schrieb 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 ...)
>>         '())
> LGTM, but let's wait on more opinions.  IIRC, changing (guix build)
> causes a world rebuild, no?  So this patch might go to core-updates
> first.

Do we actually use this idiom anywhere build-side?  (If not, does it
belong in (guix utils)?  Or should we stick it in (guix build utils)
just to be safe?)

--
Sarah

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

[Liliana Marie Prikler]

Am Freitag, den 17.09.2021, 12:15 -0700 schrieb Sarah Morgensen:
> Do we actually use this idiom anywhere build-side?  (If not, does it
> belong in (guix utils)?  Or should we stick it in (guix build utils)
> just to be safe?)
I'm pretty sure that this would mostly be used in quasi-quoted contexts
outside of the actual build to build #:configure-flags, #:make-flags
and inputs.  However, I can't really say there'd be no use of it inside
builds, even if builds typically don't need to check for conditionals
other than tests?

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

[Maxime Devos]

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Liliana Marie Prikler schreef op vr 17-09-2021 om 19:55 [+0200]:
> Am Freitag, den 17.09.2021, 10:26 +0200 schrieb 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 ...)
> >         '())
> LGTM, but let's wait on more opinions.

It looks nice to me, though you might want an opinion from someone who
is new to Scheme.

Greetings,
Maxime.

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

[Xinglu Chen]

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On Fri, Sep 17 2021, Liliana Marie Prikler wrote:

> Am Freitag, den 17.09.2021, 10:26 +0200 schrieb 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 ...)
>>         '())
> LGTM, but let's wait on more opinions.  IIRC, changing (guix build)
> causes a world rebuild, no?  So this patch might go to core-updates
> first.

Sorry, I forgot to add ‘core-updates’ to the subject prefix.

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

[Sarah Morgensen]

Hi,

Looking at this again I have a couple more comments.

Xinglu Chen <public@yoctocell.xyz> writes:
> +@cindex miscellaneous build utilities
> +This section documents some miscellaneous utilities that are useful to
> +have.

"that are useful to have" is implied by its presence in the manual, so
it's unnecessary to write it.

Also, you might consider adding a rule to .dir-locals.el:

  (eval . (put list-when 'scheme-indent-function 1))

which would cause it to be indented like this:

  (list-when expr1
    expr2
    expr3)

(I pointed out a couple other minor grammar corrections in the last
review--did you see those?)

--
Sarah

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

[Xinglu Chen]

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On Fri, Sep 17 2021, Sarah Morgensen wrote:

> Hi,
>
> Looking at this again I have a couple more comments.
>
> Xinglu Chen <public@yoctocell.xyz> writes:
>> +@cindex miscellaneous build utilities
>> +This section documents some miscellaneous utilities that are useful to
>> +have.
>
> "that are useful to have" is implied by its presence in the manual, so
> it's unnecessary to write it.

Good point.

> Also, you might consider adding a rule to .dir-locals.el:
>
>   (eval . (put list-when 'scheme-indent-function 1))
>
> which would cause it to be indented like this:
>
>   (list-when expr1
>     expr2
>     expr3)

Yes, that would be a good idea.

> (I pointed out a couple other minor grammar corrections in the last
> review--did you see those?)

Oops, looks like I missed those.  I will send a v2 with the suggestions
applied.  :-)


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[bug#50614] [PATCH core-updates v2] 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.
* .dir-locals.el: Set ‘scheme-indent-function’ to 1 for ‘list-when’.
---
Changes since v1:

* Add an entry for ‘list-when’ in .dir-locals.el.

* Some fixes to the documentation.

 .dir-locals.el        |  1 +
 doc/guix.texi         | 34 ++++++++++++++++++++++++++++++++++
 guix/build/utils.scm  | 17 ++++++++++++++++-
 tests/build-utils.scm |  8 ++++++++
 4 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/.dir-locals.el b/.dir-locals.el
index 919ed1d1c4..554044ee8c 100644
--- a/.dir-locals.el
+++ b/.dir-locals.el
@@ -56,6 +56,7 @@ ((nil
    (eval . (put 'lambda* 'scheme-indent-function 1))
    (eval . (put 'substitute* 'scheme-indent-function 1))
    (eval . (put 'match-record 'scheme-indent-function 2))
+   (eval . (put 'list-when 'scheme-indent-function 1))
 
    ;; 'modify-inputs' and its keywords.
    (eval . (put 'modify-inputs 'scheme-indent-function 1))
diff --git a/doc/guix.texi b/doc/guix.texi
index 9a3e8ae12c..4bea366b8e 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -8801,6 +8801,40 @@
         `("PATH" ":" prefix ,(list coreutils))))))
 @end lisp
 
+@subsection Miscellaneous
+
+@cindex miscellaneous build utilities
+This section documents some miscellaneous utilities.
+
+@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 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 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 @@ (define-module (guix build utils)
 
             make-desktop-entry-file
 
-            locale-category->string))
+            locale-category->string
+
+            list-when))
 
 \f
 ;;;
@@ -1613,6 +1616,18 @@ (define (locale-category->string category)
              LC_NAME LC_NUMERIC LC_PAPER LC_TELEPHONE
              LC_TIME)))
 
+\f
+;;;
+;;; 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 @@ (define-module (test build-utils)
           ("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

[bug#50614] [PATCH core-updates v2] build: utils: Add ‘list-when’ macro.

[Maxime Devos]

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Xinglu Chen schreef op zo 19-09-2021 om 15:50 [+0200]:
> * 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.
> * .dir-locals.el: Set ‘scheme-indent-function’ to 1 for ‘list-when’.
> ---

If you will be doing this in (guix build utils), leading to a world-rebuild,
you light want to define list-unless as well.  E.g., 'boost' has

    (native-inputs
     `(("perl" ,perl)
       ,@(if (%current-target-system)
             '()
             `(("python" ,python-wrapper)))
       ("tcsh" ,tcsh)))

with 'list-when', this could be rewritten to

    (native-inputs
     `(("perl" ,perl)
       ,@(list-when (not (%current-target-system))
           `("python" ,python-wrapper))
       ("tcsh" ,tcsh)))

but '(when (not ...) EXP ...)' is simply '(unless EXP ...)',
so it would be nice to be able to do

    (native-inputs
     `(("perl" ,perl)
       ,@(list-unless (%current-target-system)
           `("python" ,python-wrapper))
       ("tcsh" ,tcsh)))

Greetings,
Maxime.

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[bug#50614] [PATCH core-updates] build: utils: Add ‘optional’ macro.

[zimoun]

Hi,

On Sun, 19 Sep 2021 at 15:50, Xinglu Chen <public@yoctocell.xyz> wrote:

> +@lisp
> +(if @var{test}
> +    (list @var{consequent} @dots{})
> +    '())
> +@end lisp

[...]

> +@lisp
> +(arguments
> + `(#:configure-flags (list "--localstatedir=/var"
> +                           "--sysconfdir=/etc"
> +                           ,@@(list-when (hurd-target?) "--with-courage"))))
> +@end lisp

Personally, I am not convinced it helps the readibility.  But that’s a
matter of taste. :-)

My concern is about the coherence.  First, ’list-unless’ is also
required by a similar pattern, see for instance:

--8<---------------cut here---------------start------------->8---
gnu/packages/guile.scm:162:             ,@(if (target-mingw?) '() `(("bash" ,bash-minimal)))))
--8<---------------cut here---------------end--------------->8---

And second, these 2 patterns ’list-when’ and ’list-unless’ are used in
by many files, see guix/build-system, guix/import, guix/describe,
gnu/packages, gnu/system, gnu/services files.  For the oneline pattern:
“ag --scheme '@\(if' | grep '()'”.  All should be replaced; which
implies a world-rebuild I guess.

Well, all in all, I am not convinced that all this work is worth for a
small debatable readibility improvement. :-)

All the best,
simon