What is wrong with these few lines of guile code?

What is wrong with these few lines of guile code?

[Hartmut Goebel]

Hallo,

i replaced the code of gnu/system/install.scm(make-cow-store target) [1]
with the code below. Now when running this code (which is triggered by
"herd start cow-store /mnt" in the installation-os)|I get this error:
|

herd: exception caught while executing 'start' on service 'cow-store':
ERROR: Unbound variable: gexp

What is wrong with this code? (Parentheses are matching, of course :-)

[1]
https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/system/install.scm#n70

(define (make-cow-store target)
  "Return a gexp that makes the store copy-on-write, using TARGET as the
backing store.  This is useful when TARGET is on a hard disk, whereas the
current store is on a RAM disk."

  (define (set-store-permissions directory)
    ;; Set the right perms on DIRECTORY to use it as the store.
    #~(begin
        (chown #$directory 0 30000)             ;use the fixed
'guixbuild' GID
        (chmod #$directory #o1775)))

  #~(begin
      ;; Bind-mount TARGET's /tmp in case we need space to build things.
      (let ((tmpdir (string-append #$target "/tmp")))
        (mkdir-p tmpdir)
        (mount tmpdir "/tmp" "none" MS_BIND))

      (let* ((rw-dir (string-append target #$%backing-directory))
             ;; FIXME: calculate work-dir from backing-directory:
             ;; normpath(backing-directory + "../.overlayfs-workdir")
             (work-dir (string-append target "/tmp/.overlayfs-workdir")))
        (mkdir-p rw-dir)
        (mkdir-p work-dir)
        (mkdir-p "/.rw-store")
        #$(set-store-permissions #~rw-dir)
        #$(set-store-permissions "/.rw-store")

        ;; Mount the overlay, then atomically make it the store.
        (mount "none" "/.rw-store" "overlay"
               (string-append "lowerdir=" #$(%store-prefix)
                              "upperdir=" #~rw-dir ","
                              "workdir=" #~work-dir))
        (mount "/.rw-store" #$(%store-prefix) "" MS_MOVE)
        (rmdir "/.rw-store"))))

-- 
Regards
Hartmut Goebel

| Hartmut Goebel          | h.goebel@crazy-compilers.com               |
| www.crazy-compilers.com | compilers which you thought are impossible |

Re: What is wrong with these few lines of guile code?

[Mike]

On 10/31/2017 09:41 AM, Hartmut Goebel wrote:
> Hallo,
>
> i replaced the code of gnu/system/install.scm(make-cow-store target) [1]
> with the code below. Now when running this code (which is triggered by
> "herd start cow-store /mnt" in the installation-os)|I get this error:
> |
>
> herd: exception caught while executing 'start' on service 'cow-store':
> ERROR: Unbound variable: gexp
>
> What is wrong with this code? (Parentheses are matching, of course :-)
I'm a Scheme novice, but is your use of 'target' instead of '#$target' 
further down the definition body a problem?  Or is that a proper use of 
Guile macros?

-Mike

> [1]
> https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/system/install.scm#n70
>
> (define (make-cow-store target)
>    "Return a gexp that makes the store copy-on-write, using TARGET as the
> backing store.  This is useful when TARGET is on a hard disk, whereas the
> current store is on a RAM disk."
>
>    (define (set-store-permissions directory)
>      ;; Set the right perms on DIRECTORY to use it as the store.
>      #~(begin
>          (chown #$directory 0 30000)             ;use the fixed
> 'guixbuild' GID
>          (chmod #$directory #o1775)))
>
>    #~(begin
>        ;; Bind-mount TARGET's /tmp in case we need space to build things.
>        (let ((tmpdir (string-append #$target "/tmp")))
>          (mkdir-p tmpdir)
>          (mount tmpdir "/tmp" "none" MS_BIND))
>
>        (let* ((rw-dir (string-append target #$%backing-directory))
>               ;; FIXME: calculate work-dir from backing-directory:
>               ;; normpath(backing-directory + "../.overlayfs-workdir")
>               (work-dir (string-append target "/tmp/.overlayfs-workdir")))
>          (mkdir-p rw-dir)
>          (mkdir-p work-dir)
>          (mkdir-p "/.rw-store")
>          #$(set-store-permissions #~rw-dir)
>          #$(set-store-permissions "/.rw-store")
>
>          ;; Mount the overlay, then atomically make it the store.
>          (mount "none" "/.rw-store" "overlay"
>                 (string-append "lowerdir=" #$(%store-prefix)
>                                "upperdir=" #~rw-dir ","
>                                "workdir=" #~work-dir))
>          (mount "/.rw-store" #$(%store-prefix) "" MS_MOVE)
>          (rmdir "/.rw-store"))))
>

Re: What is wrong with these few lines of guile code?

[Christopher Baines]

[-- Attachment #1: Type: text/plain, Size: 1268 bytes --]

On Tue, 31 Oct 2017 14:41:46 +0100
Hartmut Goebel <h.goebel@crazy-compilers.com> wrote:

> Hallo,
> 
> i replaced the code of gnu/system/install.scm(make-cow-store target) [1]
> with the code below. Now when running this code (which is triggered by
> "herd start cow-store /mnt" in the installation-os)|I get this error:
> |
> 
> herd: exception caught while executing 'start' on service 'cow-store':
> ERROR: Unbound variable: gexp
> 
> What is wrong with this code? (Parentheses are matching, of course :-)

...

>         ;; Mount the overlay, then atomically make it the store.
>         (mount "none" "/.rw-store" "overlay"
>                (string-append "lowerdir=" #$(%store-prefix)
>                               "upperdir=" #~rw-dir ","
>                               "workdir=" #~work-dir))

Here, as rw-dir and work-dir are just inside the gexp, you don't need
to ungexp, or gexp them (as is being done here), so I'd remove the #~.

This matches the error you're getting, as #~ is short for (gexp ...) as
I understand it.

>         (mount "/.rw-store" #$(%store-prefix) "" MS_MOVE)
>         (rmdir "/.rw-store"))))
> 


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Re: What is wrong with these few lines of guile code?

[Hartmut Goebel]

Am 31.10.2017 um 18:50 schrieb Christopher Baines:
> This matches the error you're getting, as #~ is short for (gexp ...) as
> I understand it.

Thanks you, this did the trick :-) Although this reminds my on the magic
signs in perl :-(

From the "Zen of Python":

Explicit is better than implicit.
Readability counts.

-- 
Regards
Hartmut Goebel

| Hartmut Goebel          | h.goebel@crazy-compilers.com               |
| www.crazy-compilers.com | compilers which you thought are impossible |

Re: What is wrong with these few lines of guile code?

[Ricardo Wurmus]

Hartmut Goebel <h.goebel@crazy-compilers.com> writes:

> Am 31.10.2017 um 18:50 schrieb Christopher Baines:
>> This matches the error you're getting, as #~ is short for (gexp ...) as
>> I understand it.
>
> Thanks you, this did the trick :-) Although this reminds my on the magic
> signs in perl :-(

It’s a reader macro.  They all start with # and are followed by a
character.  You can install a reader macro to transform the expression
as it is read.  For example, I used a reader macro in earlier versions
of my operating system configuration so that I can avoid wrapping file
contents in string quotes.

Reader macros are frowned upon when they are added for no good reason.

A reader macro is used here, because to Schemers it comes natural to use
syntax like “`” (quasiquote), “'” (quote), “,” (unquote), and “,@”
(unquote-splicing).  Gexps are analogous to quoted S-expressions, so
(gexp …) “quotes” and (ungexp …) unquotes, etc.  The reader extensions
positively affect readability as they strip off one layer of
parenthetical expressions, at the cost of extra syntax.

For more information on the thinking behind Gexps and their relation to
quasiquotation see Ludo’s excellent paper “Code Staging in GNU Guix”,
available at <https://hal.inria.fr/hal-01580582/en>.

--
Ricardo

GPG: BCA6 89B6 3655 3801 C3C6  2150 197A 5888 235F ACAC
https://elephly.net

Re: What is wrong with these few lines of guile code?

[Pjotr Prins]

On Fri, Nov 03, 2017 at 03:34:23PM +0100, Hartmut Goebel wrote:
> Am 31.10.2017 um 18:50 schrieb Christopher Baines:
> > This matches the error you're getting, as #~ is short for (gexp ...) as
> > I understand it.
> 
> Thanks you, this did the trick :-) Although this reminds my on the magic
> signs in perl :-(

It is not magic (unlike Perl). In Guile you can use almost any
character to give something a name. This does not come natural to us
non-Lisp programmers. The #~ is merely an agreed convention to name
certain macros. And macros, essentially, are syntax expansions. No
magic. Though macros can do magic, but that is a different story ;)

Pj.