From mboxrd@z Thu Jan  1 00:00:00 1970
From: ludo@gnu.org (Ludovic =?utf-8?Q?Court=C3=A8s?=)
Subject: Re: package.scm: (profile-derivation (%store) '())
Date: Fri, 20 Sep 2013 22:21:15 +0200
Message-ID: <877gebdz2c.fsf@gnu.org>
References: <87r4cjjsu2.fsf@karetnikov.org>
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In-Reply-To: <87r4cjjsu2.fsf@karetnikov.org> (Nikita Karetnikov's message of
	"Fri, 20 Sep 2013 21:39:17 +0400")
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To: Nikita Karetnikov <nikita@karetnikov.org>
Cc: guix-devel@gnu.org

Nikita Karetnikov <nikita@karetnikov.org> skribis:

> I=E2=80=99d like to know how =E2=80=98roll-back=E2=80=99 creates an empty=
 generation because
> it=E2=80=99s necessary to do the same for =E2=80=98--delete-generations=
=E2=80=99.
>
> However, I fail to understand how (profile-derivation (%store) =E2=80=98(=
))
> works (or any other function that uses (%store)).  I assume that some
> code should set =E2=80=98%store=E2=80=99 to something else.  Like so:
>
> scheme@(guix scripts package)> %store
> $1 =3D #<<parameter> 934e7f8 proc: #<procedure 934e810 at ice-9/boot-9.sc=
m:3154:17 () | (x)>>
> scheme@(guix scripts package)> (%store)
> $2 =3D #f
> scheme@(guix scripts package)> (%store "FOO")
> $3 =3D #f
> scheme@(guix scripts package)> (%store)
> $4 =3D "FOO"
>
> But I don=E2=80=99t see anything similar in =E2=80=98package.scm=E2=80=99.

=E2=80=98%store=E2=80=99 is a SRFI-39 parameter (info "(guile) Parameters")=
, aka. a
dynamically-scoped variable.

It is initialized with the =E2=80=98parameterize=E2=80=99 form, which sets =
its value for
the dynamic extent of its body.

HTH,
Ludo=E2=80=99.