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From: ludo@gnu.org (Ludovic =?utf-8?Q?Court=C3=A8s?=)
Subject: Re: string-append plus package
Date: Mon, 19 Dec 2016 10:47:26 +0100
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	(Hartmut Goebel's message of "Mon, 19 Dec 2016 01:13:10 +0100")
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To: Hartmut Goebel <h.goebel@crazy-compilers.com>
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Hartmut Goebel <h.goebel@crazy-compilers.com> skribis:

> Am 08.12.2016 um 20:56 schrieb Leo Famulari:
>>> Here is the service-definition I use:
>>>
>>>              (nginx-service #:vhost-list
>>>                (list (nginx-vhost-configuration=20=20=20=20=20
>>>                       (root (string-append nginx "/share/nginx/html"))
>> I believe that file-append is intended for this use case.
>
> Maybe, but I can't get it to work. This minimal system declarision fails
> with "In procedure string-append: Wrong type (expecting string):
> #<<file-append> base: #<package nginx@1.11.6 gnu/packages/web.scm:126
> 2a236c0> suffix: ("/")>"
>
> (use-modules (gnu))
> (use-service-modules networking web)
> (define NGINX (file-append nginx "/"))
> (define TEST (string-append NGINX ""))

The result of =E2=80=98file-append=E2=80=99 is not a string, but =E2=80=98s=
tring-append=E2=80=99 expects
two strings, hence the error.

=E2=80=98file-append=E2=80=99 returns a =E2=80=9Cstring-valued gexp=E2=80=
=9D.  This is to say that, in a
staging context, it will produce a string.  For example:

  (scheme-file "foo" #~(foo bar #$(file-append nginx "/foo/bar")))

leads to a file =E2=80=9Cfoo=E2=80=9D containing:

  (foo bar "/gnu/store/=E2=80=A6-nginx-1.2.3/foo/bar")

HTH!

Ludo=E2=80=99.