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From: =?utf-8?Q?Ludovic_Court=C3=A8s?= <ludo@gnu.org>
Subject: Parameterized packages
Date: Tue, 14 May 2019 13:54:18 +0200
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To: Guix-devel <Guix-devel@gnu.org>

Hello Guix!

While thinking about <https://issues.guix.info/issue/35671> and looking
for ways to allow users to install just the locales they need right from
=E2=80=98guix package=E2=80=99, I realized that =E2=80=9Cparameterized pack=
ages=E2=80=9D are a
low-hanging fruit (by =E2=80=9Cparameterized packages=E2=80=9D I mean somet=
hing kinda
like Gentoo USE flags, or like a procedure if you will, but with a
command-line interface.)

Namely, one could write:

  (package
    (name "glibc-utf8-locales")
    ;; =E2=80=A6
    (arguments
      `(#:phases =E2=80=A6 ,(assoc-ref (package-properties this-package)
                               'locales)
        =E2=80=A6)))


and then we=E2=80=99d have a package transformation option like, say,
=E2=80=98--with-argument=3Dglibc-utf8-locales:locales=3Dzh_CN.utf8=E2=80=99=
, which would
add the right property, like so:

  (package
    (inherit glibc-utf8-locales)
    (properties `((locales . ("zh_CN.utf8")))))

and tadaam! we have a parameterized package.

There=E2=80=99s a couple of gotchas:

  =E2=80=A2 We=E2=80=99d need to store more info in manifest entries so that
    transformation options are preserved upon =E2=80=98guix upgrade=E2=80=
=99.

  =E2=80=A2 We might need to expose the package parameters somehow, so that=
 if
    one types =E2=80=98--with-argument=3Dfoobar=3Dbaz=E2=80=99, they get a =
correct error
    message saying that =E2=80=9Cfoobar=E2=80=9D is not a known parameter.

  =E2=80=A2 We might want to make the CLI option less verbose too, but how?

Thoughts, comrades?

Thanks,
Ludo=E2=80=99.