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

[Sarah Morgensen <iskarian@mgsn.dev>]

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