Re: G-Expressions manual. change user shell in guix config.scm

[rendaw <7e9wc56emjakcm@s.rendaw.me>]

On 5/4/19 12:36 AM, znavko@disroot.org wrote:
> I want to try to use G-expressions to change 'mom' user shell to dash.
> I have this error:
>
> # guix system reconfigure config-znavko.scm
> ...
> building /gnu/store/zhmd8fr5v86wnaf6apcz4281c008fjv5-shepherd-user-homes.scm.drv...
> building /gnu/store/6wnbkfdqy3qmbcjz00d1w5p8mw1rkyaa-shells.drv...
> /builder for `/gnu/store/6wnbkfdqy3qmbcjz00d1w5p8mw1rkyaa-shells.drv' failed with exit code 1
> build of /gnu/store/6wnbkfdqy3qmbcjz00d1w5p8mw1rkyaa-shells.drv failed
> View build log at '/var/log/guix/drvs/6w/nbkfdqy3qmbcjz00d1w5p8mw1rkyaa-shells.drv.bz2'.
> cannot build derivation `/gnu/store/36ah9x5q8nj6y01fs32brcndmkgyvqmv-etc.drv': 1 dependencies couldn't be built
> building /gnu/store/kvdsb795wn1ic8p9kcdsbkd9vw5v4dm2-shepherd.conf.drv...
> cannot build derivation `/gnu/store/cmfly4pn3hs2y38km0l3yn3phkxy84xj-system.drv': 1 dependencies couldn't be built
> guix system: error: build of `/gnu/store/cmfly4pn3hs2y38km0l3yn3phkxy84xj-system.drv' failed
>
> # tail -n1 /var/log/guix/drvs/6w/nbkfdqy3qmbcjz00d1w5p8mw1rkyaa-shells.drv
> ERROR: Wrong type to apply: "/gnu/store/bqmib4vf9mr8dkqx4dqpcqrnb93giwci-dash-0.5.10.2"
> it is here:
> (user-account (name "mom") (group "users")
> (supplementary-groups '("wheel" "netdev" "audio" "video"))
> (home-directory "/home/mom")
> (shell #~(#$dash)))
> I read Guix Refernce Manual 'G-Expressions' section, but there types described quite a little: Scheme Syntax: #~exp  Scheme Syntax: (gexp exp)   
>
> 	Return a G-expression containing exp. exp may contain one or more of the following forms:  #$obj (ungexp obj)  
>
> 	Introduce a reference to obj. obj may have one of the supported types, for example a package or a derivation, in which case the ungexp form is replaced by its output file name—e.g., "/gnu/store/…-coreutils-8.22. 
>
> 	If obj is a list, it is traversed and references to supported objects are substituted similarly. 
>
> 	If obj is another gexp, its contents are inserted and its dependencies are added to those of the containing gexp. 
>
> 	If obj is another kind of object, it is inserted as is.     
> My wrong config is attached.
>
> But I've found on github workable example https://github.com/meiyopeng/guix-config/blob/master/meiyo/systems/default.scm
>
> And rewrite config with file-append Scheme procedure. This works:
>
>  (user-account (name "mom") (group "users")
>  (supplementary-groups '("wheel" "netdev" "audio" "video"))
>  (home-directory "/home/mom")
>  (shell (file-append dash "/bin/dash")))
>
> But I am confused, cause I do not know why config works without #~ and #~ but only file-append function? Is it still G-Expression (shell (file-append dash "/bin/dash"))?

I'm not 100% sure I'm correct, but I think there are a couple issues here.


So first of all, I think you have extra parentheses.  You're saying you
want the gexp of the "form" where the lowered dash is the first element,
but you actually want the gexp of the lowered dash itself (no form).
It's the difference between `("/gnu/store/...-dash/")` and
`"/gnu/store/...-dash/"` (the former might be evaluated as an invalid
function call).

So try:

#~#$dash

instead of

#~(#$dash)


However, `dash` is already an object that lowers to a string, and since
`(shell ...)` needs a (n object that lowers to a) path string the #~#$
is unnecessary - you can just do `(shell dash)`.


That being said, the lowered form of `dash` is the string of the path to
the derivation directory rather than the string of the path to an
executable, so you either need to do `#~(string-append #$dash
"/bin/dash")` to get the executable path string or else `(file-append
dash "/bin/dash")` as a shortcut.  `(file-append ...)` returns an object
that lowers to a string.

I hope this helps slightly - it's a really good question and I think I
learned from it too.

... ah, and it makes sense that `(shell ...)` is evaluated in a
g-expression context (and thus will lower lowerable objects) --
g-expressions are usually evaluated at boot time rather than
build/reconfigure time, and in disk-image at least the user accounts are
created at boot.