Sorry, I forgot (modify-services) function again.
Trying first config was unsuccessful, but using examples from here https://wiki.archlinux.org/index.php/Libinput <https://wiki.archlinux.org/index.php/Libinput> I've got my touchpad reacts on tap and double-clicks.

I think the driver option is important for me Driver \"libinput\"
Here's my workable config for touchpad in GuixSD:


  (services (cons*  
                    ;;(service postgresql-service-type)
                    (xfce-desktop-service)
                    (modify-services      
                      ;;(remove (lambda (service)
                      ;;  (eq? (service-kind service)
                      ;;    wpa-supplicant-service-type))
                      ;;  (remove (lambda (service)
                      ;;    (eq? (service-kind service)
                      ;;      static-networking-service-type))
                          (remove (lambda (service)
                            (eq? (service-kind service)
                              ntp-service-type))
                            (remove (lambda (service)
                              (eq? (service-kind service)
                                avahi-service-type))
                              (modify-services %desktop-services

                                (slim-service-type config =>
                                              (slim-configuration
                                               (inherit config)
                                               (startx
                                                (xorg-start-command
                                                 #:configuration-file
                                                 (xorg-configuration-file
                                                  #:extra-config
                                                  '(
"Section \"InputClass\"
        Identifier \"touchpad\"
        Driver \"libinput\"
        MatchIsTouchpad \"on\"
        Option \"Tapping\" \"on\"
EndSection"
                                 ))))))
)


                            )  ;end of remove avahi
                          )  ;end of remove2 ntp
                        ;;)  ;end of remove3 networking
                      ;)  ;end of remove4 wpa-supplicant
                      (elogind-service-type
                        c => (elogind-configuration (handle-lid-switch 'ignore)))
                    );;end of modify desktop-services
  ));;end of services


See attachment with full config.scm

Thanks, Pierr Neidhardt for that link here https://lists.gnu.org/archive/html/help-guix/2018-10/msg00140.html <https://lists.gnu.org/archive/html/help-guix/2018-10/msg00140.html> !

Feb 10, 2019, 11:36 AM by znavko@tutanota.com:

>
> Hello! I am not able to determine what is the problem while I'm trying to reconfigure system.
> I only add code for touchpad tap from 63 to 78 lines in config.scm (see attachment).
>
> # guix system reconfigure /etc/config.scm
> /etc/config.scm:47:20: error: config: unbound variable
> hint: Did you forget a `use-modules' form?
>
> Please, what I need to use in use-modules section?
>
> =========part f config.scm=========
> (use-modules (gnu) (gnu system nss)
>              (gnu system locale) ;;for locale-definition
>              (gnu services desktop)
>              ;;(gnu services dns) ;;for dnsmasq
>              (srfi srfi-1) ;;for remove function
>              (gnu services networking) ;;for remove ntp
>              (gnu services avahi) ;;for remove avahi
>              (gnu services xorg)
>              ;;(gnu services databases);;for postgres
>              (gnu packages admin) ;;for wpa_supplicant
>
> )
> (use-service-modules desktop)
> (use-package-modules certs gnome)
> ....
>
>   (services (cons*  
>                     ;;(service postgresql-service-type)
>                     (xfce-desktop-service)
>                     (modify-services      
>                       ;;(remove (lambda (service)
>                       ;;  (eq? (service-kind service)
>                       ;;    wpa-supplicant-service-type))
>                       ;;  (remove (lambda (service)
>                       ;;    (eq? (service-kind service)
>                       ;;      static-networking-service-type))
>                           (remove (lambda (service)
>                             (eq? (service-kind service)
>                               ntp-service-type))
>                             (remove (lambda (service)
>                               (eq? (service-kind service)
>                                 avahi-service-type))
>                               %desktop-services
>
>                                 (slim-service-type config =>
>                                               (slim-configuration
>                                                (inherit config)
>                                                (startx
>                                                 (xorg-start-command
>                                                  #:configuration-file
>                                                  (xorg-configuration-file
>                                                   #:extra-config
>                                                   '(
> "Section \"InputClass\"
>         Identifier \"touchpad-all\"
>         MatchIsTouchpad \"on\"
>         Option \"Tapping\" \"on\"
> EndSection"
>                                  ))))))
>
>
>                             )  ;end of remove avahi
>                           )  ;end of remove2 ntp
>                         ;;)  ;end of remove3 networking
>                       ;)  ;end of remove4 wpa-supplicant
>                       (elogind-service-type
>                         c => (elogind-configuration (handle-lid-switch 'ignore)))
>                     );;end of modify desktop-services
>   ));;end of services
> ====================
>