Subject: bug#41140: “guix system” suggests wrong module import when using “remove”

From: Ricardo Wurmus <[email-masked]>
Date: Sat, 09 May 2020 00:00:05 +0200

I want to delete a service from %desktop-services, so I write

--8<---------------cut here---------------start------------->8---
…
(services (remove
           (lambda (service)
             (eq? (service-kind service) that-annoying-service-type))
           %desktop-services))
…
--8<---------------cut here---------------end--------------->8---

Then I run “guix system build config.scm” and Guix tells me that it
doesn’t know what “remove” is:

--8<---------------cut here---------------start------------->8---
config.scm:56:18: error: remove: unbound variable
hint: Did you forget `(use-modules (rnrs lists))'?
--8<---------------cut here---------------end--------------->8---

I add the suggested module import to my file and it no longer complains,
but that-annoying-service-type has not actually been removed.  Oh no!

Of course, the Guix manual says this:

--8<---------------cut here---------------start------------->8---
   Again, ‘%desktop-services’ is just a list of service objects.  If you
want to remove services from there, you can do so using the procedures
for list filtering (*note (guile)SRFI-1 Filtering and Partitioning::).
For instance, the following expression returns a list that contains all
the services in ‘%desktop-services’ minus the Avahi service:

     (remove (lambda (service)
               (eq? (service-kind service) avahi-service-type))
             %desktop-services)
--8<---------------cut here---------------end--------------->8---

But maybe I don’t know what SRFI-1 is and I trust that the hint Guix
provides will be the right thing to do.

But now I’m curious and I look at the documentation for “remove” from
(rnrs lists):

--8<---------------cut here---------------start------------->8---
 -- Scheme Procedure: remp proc list
 -- Scheme Procedure: remove obj list
 -- Scheme Procedure: remv obj list
 -- Scheme Procedure: remq obj list
     ‘remove’, ‘remv’, and ‘remq’ are identical to the ‘delete’, ‘delv’,
     and ‘delq’ procedures provided by Guile’s core library, (*note List
     Modification::).  ‘remp’ is identical to the alternate ‘remove’
     procedure provided by SRFI-1; *Note SRFI-1 Deleting::.
--8<---------------cut here---------------end--------------->8---

Oh.

So here are my questions:

* can we prefer (srfi srfi-1) over (rnrs lists) in the suggestions for “remove”?

* can we avoid this by extending modify-services to support “delete”
  much like modify-phases, and suggesting to use that instead of
  “remove”?

--
Ricardo