From mboxrd@z Thu Jan  1 00:00:00 1970
From: ludo@gnu.org (Ludovic =?UTF-8?Q?Court=C3=A8s?=)
Subject: bug#19816: 'guix environment' always downloads all the outputs of
	dependencies
Date: Sun, 08 Feb 2015 19:24:37 +0100
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To: 19816@debbugs.gnu.org
Cc: Dave Thompson <davet@gnu.org>

>From <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=3D19641#16>:

There=E2=80=99s another problem, though.  When a dependency is a multiple-o=
utput
package, all its outputs are added to the environment, because
=E2=80=98package->transitive-inputs=E2=80=99 discards the information of wh=
ich output is
needed.

So for instance, both the =E2=80=98out=E2=80=99 and the =E2=80=98debug=E2=
=80=99 output of Coreutils end
up being downloaded and added to the environment, even though only =E2=80=
=98out=E2=80=99
is an input.

Now, the problem is that =E2=80=98build-derivations=E2=80=99 can only build=
 *all* the
outputs of the given derivation.  This could be worked around either:

  1. by creating a =E2=80=9Csink=E2=80=9D derivation, for instance with
     =E2=80=98profile-derivation=E2=80=99, that could refer precisely to th=
e output(s)
     needed; not ideal.

  2. by using (build-things (list "/the/output/path")) and resorting to
     =E2=80=98build-derivations=E2=80=99 only if the =E2=80=98build-things=
=E2=80=99 call did nothing
     (when passed a non-.drv store item, =E2=80=98build-things=E2=80=99 tri=
es to
     substitute and does nothing if that fails.)

Ludo=E2=80=99.