services: configuration: Use *unspecified* instead of 'disabled.

Use *unspecified* as a marker for field values that have not been set.

Rationale: 'disabled may easily clash with user values for boolean fields, is
confusing (i.e. its meaning is *not* boolean false, but unspecified) and it
also passes silently through the symbol? predicate of a field of type symbol.

* gnu/services/configuration.scm (configuration-missing-default-value):
Renamed from configuration-no-default-value.
(define-maybe-helper): Use *unspecified* instead of 'disabled, and make
the default value optional.
* gnu/home/services/desktop.scm (home-redshift-configuration):
Change (maybe-xyz 'disabled) to maybe-xyz.
* gnu/services/authentication.scm (nslcd-configuration): Likewise.
* gnu/services/cgit.scm (repository-cgit-configuration): Likewise.
* gnu/services/file-sharing.scm (serialize-maybe-string)
(serialize-maybe-file-object): Use 'unspecified?' instead of (eq? val
'disabled).
* gnu/services/messaging.scm (raw-content?): Likewise.
(ssl-configuration): Change (maybe-xyz 'disabled) to maybe-xyz.
(prosody-configuration): Likewise.
* gnu/services/file-sharing.scm (transmission-daemon-configuration):
Likewise.
* gnu/services/messaging.scm (define-all-configurations):
Use *unspecified* instead of 'disabled'.
* gnu/services/networking.scm (opendht-configuration): Likewise.
* gnu/services/pm.scm (tlp-configuration): Likewise.
* gnu/services/telephony.scm (jami-account): Likewise.
(jami-configuration): Likewise.
* gnu/services/vpn.scm (openvpn-client-configuration): Likewise.
* tests/services/configuration.scm ("maybe type, no default")
("maybe type, with default"): New tests.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

11 files changed, 148 insertions, 138 deletions

diff --git a/gnu/home/services/desktop.scm b/gnu/home/services/desktop.scm
index cbb9cf76da..b0f4d969b0 100644
--- a/gnu/home/services/desktop.scm
+++ b/gnu/home/services/desktop.scm
@@ -91,26 +91,26 @@ queried for location information.")
    "Nighttime color temperature (kelvins).")
 
   (daytime-brightness
-   (maybe-inexact-number 'disabled)
+   maybe-inexact-number
    "Daytime screen brightness, between 0.1 and 1.0.")
   (nighttime-brightness
-   (maybe-inexact-number 'disabled)
+   maybe-inexact-number
    "Nighttime screen brightness, between 0.1 and 1.0.")
 
   (latitude
-   (maybe-inexact-number 'disabled)
+   maybe-inexact-number
    "Latitude, when @code{location-provider} is @code{'manual}.")
   (longitude
-   (maybe-inexact-number 'disabled)
+   maybe-inexact-number
    "Longitude, when @code{location-provider} is @code{'manual}.")
 
   (dawn-time
-   (maybe-string 'disabled)
+   maybe-string
    "Custom time for the transition from night to day in the
 morning---@code{\"HH:MM\"} format.  When specified, solar elevation is not
 used to determine the daytime/nighttime period.")
   (dusk-time
-   (maybe-string 'disabled)
+   maybe-string
    "Likewise, custom time for the transition from day to night in the
 evening.")
 
diff --git a/gnu/services/authentication.scm b/gnu/services/authentication.scm
index cb0ef6d85a..f7becdfafb 100644
--- a/gnu/services/authentication.scm
+++ b/gnu/services/authentication.scm
@@ -218,7 +218,7 @@
 
   ;; Runtime options
   (threads
-   (maybe-number 'disabled)
+   maybe-number
    "The number of threads to start that can handle requests and perform LDAP
 queries.  Each thread opens a separate connection to the LDAP server.  The
 default is to start 5 threads.")
@@ -243,45 +243,45 @@ messages with the specified log level or higher are logged.")
    "The list of LDAP server URIs.  Normally, only the first server will be
 used with the following servers as fall-back.")
   (ldap-version
-   (maybe-string 'disabled)
+   maybe-string
    "The version of the LDAP protocol to use.  The default is to use the
 maximum version supported by the LDAP library.")
   (binddn
-   (maybe-string 'disabled)
+   maybe-string
    "Specifies the distinguished name with which to bind to the directory
 server for lookups.  The default is to bind anonymously.")
   (bindpw
-   (maybe-string 'disabled)
+   maybe-string
    "Specifies the credentials with which to bind.  This option is only
 applicable when used with binddn.")
   (rootpwmoddn
-   (maybe-string 'disabled)
+   maybe-string
    "Specifies the distinguished name to use when the root user tries to modify
 a user's password using the PAM module.")
   (rootpwmodpw
-   (maybe-string 'disabled)
+   maybe-string
    "Specifies the credentials with which to bind if the root user tries to
 change a user's password.  This option is only applicable when used with
 rootpwmoddn")
 
   ;; SASL authentication options
   (sasl-mech
-   (maybe-string 'disabled)
+   maybe-string
    "Specifies the SASL mechanism to be used when performing SASL
 authentication.")
   (sasl-realm
-   (maybe-string 'disabled)
+   maybe-string
    "Specifies the SASL realm to be used when performing SASL authentication.")
   (sasl-authcid
-   (maybe-string 'disabled)
+   maybe-string
    "Specifies the authentication identity to be used when performing SASL
 authentication.")
   (sasl-authzid
-   (maybe-string 'disabled)
+   maybe-string
    "Specifies the authorization identity to be used when performing SASL
 authentication.")
   (sasl-canonicalize?
-   (maybe-boolean 'disabled)
+   maybe-boolean
    "Determines whether the LDAP server host name should be canonicalised.  If
 this is enabled the LDAP library will do a reverse host name lookup.  By
 default, it is left up to the LDAP library whether this check is performed or
@@ -289,7 +289,7 @@ not.")
 
   ;; Kerberos authentication options
   (krb5-ccname
-   (maybe-string 'disabled)
+   maybe-string
    "Set the name for the GSS-API Kerberos credentials cache.")
 
   ;; Search / mapping options
@@ -302,11 +302,11 @@ not.")
 default scope is subtree; base scope is almost never useful for name service
 lookups; children scope is not supported on all servers.")
   (deref
-   (maybe-deref-option 'disabled)
+   maybe-deref-option
    "Specifies the policy for dereferencing aliases.  The default policy is to
 never dereference aliases.")
   (referrals
-   (maybe-boolean 'disabled)
+   maybe-boolean
    "Specifies whether automatic referral chasing should be enabled.  The
 default behaviour is to chase referrals.")
   (maps
@@ -322,132 +322,132 @@ applies and an LDAP search filter expression.")
 
   ;; Timing / reconnect options
   (bind-timelimit
-   (maybe-number 'disabled)
+   maybe-number
    "Specifies the time limit in seconds to use when connecting to the
 directory server.  The default value is 10 seconds.")
   (timelimit
-   (maybe-number 'disabled)
+   maybe-number
    "Specifies the time limit (in seconds) to wait for a response from the LDAP
 server.  A value of zero, which is the default, is to wait indefinitely for
 searches to be completed.")
   (idle-timelimit
-   (maybe-number 'disabled)
+   maybe-number
    "Specifies the period if inactivity (in seconds) after which the con‐
 nection to the LDAP server will be closed.  The default is not to time out
 connections.")
   (reconnect-sleeptime
-   (maybe-number 'disabled)
+   maybe-number
    "Specifies the number of seconds to sleep when connecting to all LDAP
 servers fails.  By default one second is waited between the first failure and
 the first retry.")
   (reconnect-retrytime
-   (maybe-number 'disabled)
+   maybe-number
    "Specifies the time after which the LDAP server is considered to be
 permanently unavailable.  Once this time is reached retries will be done only
 once per this time period.  The default value is 10 seconds.")
 
   ;; TLS options
   (ssl
-   (maybe-ssl-option 'disabled)
+   maybe-ssl-option
    "Specifies whether to use SSL/TLS or not (the default is not to).  If
 'start-tls is specified then StartTLS is used rather than raw LDAP over SSL.")
   (tls-reqcert
-   (maybe-tls-reqcert-option 'disabled)
+   maybe-tls-reqcert-option
    "Specifies what checks to perform on a server-supplied certificate.
 The meaning of the values is described in the ldap.conf(5) manual page.")
   (tls-cacertdir
-   (maybe-string 'disabled)
+   maybe-string
    "Specifies the directory containing X.509 certificates for peer authen‐
 tication.  This parameter is ignored when using GnuTLS.")
   (tls-cacertfile
-   (maybe-string 'disabled)
+   maybe-string
    "Specifies the path to the X.509 certificate for peer authentication.")
   (tls-randfile
-   (maybe-string 'disabled)
+   maybe-string
    "Specifies the path to an entropy source.  This parameter is ignored when
 using GnuTLS.")
   (tls-ciphers
-   (maybe-string 'disabled)
+   maybe-string
    "Specifies the ciphers to use for TLS as a string.")
   (tls-cert
-   (maybe-string 'disabled)
+   maybe-string
    "Specifies the path to the file containing the local certificate for client
 TLS authentication.")
   (tls-key
-   (maybe-string 'disabled)
+   maybe-string
    "Specifies the path to the file containing the private key for client TLS
 authentication.")
 
   ;; Other options
   (pagesize
-   (maybe-number 'disabled)
+   maybe-number
    "Set this to a number greater than 0 to request paged results from the LDAP
 server in accordance with RFC2696.  The default (0) is to not request paged
 results.")
   (nss-initgroups-ignoreusers
-   (maybe-ignore-users-option 'disabled)
+   maybe-ignore-users-option
    "This option prevents group membership lookups through LDAP for the
 specified users.  Alternatively, the value 'all-local may be used.  With that
 value nslcd builds a full list of non-LDAP users on startup.")
   (nss-min-uid
-   (maybe-number 'disabled)
+   maybe-number
    "This option ensures that LDAP users with a numeric user id lower than the
 specified value are ignored.")
   (nss-uid-offset
-   (maybe-number 'disabled)
+   maybe-number
    "This option specifies an offset that is added to all LDAP numeric user
 ids.  This can be used to avoid user id collisions with local users.")
   (nss-gid-offset
-   (maybe-number 'disabled)
+   maybe-number
    "This option specifies an offset that is added to all LDAP numeric group
 ids.  This can be used to avoid user id collisions with local groups.")
   (nss-nested-groups
-   (maybe-boolean 'disabled)
+   maybe-boolean
    "If this option is set, the member attribute of a group may point to
 another group.  Members of nested groups are also returned in the higher level
 group and parent groups are returned when finding groups for a specific user.
 The default is not to perform extra searches for nested groups.")
   (nss-getgrent-skipmembers
-   (maybe-boolean 'disabled)
+   maybe-boolean
    "If this option is set, the group member list is not retrieved when looking
 up groups.  Lookups for finding which groups a user belongs to will remain
 functional so the user will likely still get the correct groups assigned on
 login.")
   (nss-disable-enumeration
-   (maybe-boolean 'disabled)
+   maybe-boolean
    "If this option is set, functions which cause all user/group entries to be
 loaded from the directory will not succeed in doing so.  This can dramatically
 reduce LDAP server load in situations where there are a great number of users
 and/or groups.  This option is not recommended for most configurations.")
   (validnames
-   (maybe-string 'disabled)
+   maybe-string
    "This option can be used to specify how user and group names are verified
 within the system.  This pattern is used to check all user and group names
 that are requested and returned from LDAP.")
   (ignorecase
-   (maybe-boolean 'disabled)
+   maybe-boolean
    "This specifies whether or not to perform searches using case-insensitive
 matching.  Enabling this could open up the system to authorization bypass
 vulnerabilities and introduce nscd cache poisoning vulnerabilities which allow
 denial of service.")
   (pam-authc-ppolicy
-   (maybe-boolean 'disabled)
+   maybe-boolean
    "This option specifies whether password policy controls are requested and
 handled from the LDAP server when performing user authentication.")
   (pam-authc-search
-   (maybe-string 'disabled)
+   maybe-string
    "By default nslcd performs an LDAP search with the user's credentials after
 BIND (authentication) to ensure that the BIND operation was successful.  The
 default search is a simple check to see if the user's DN exists.  A search
 filter can be specified that will be used instead.  It should return at least
 one entry.")
   (pam-authz-search
-   (maybe-string 'disabled)
+   maybe-string
    "This option allows flexible fine tuning of the authorisation check that
 should be performed.  The search filter specified is executed and if any
 entries match, access is granted, otherwise access is denied.")
   (pam-password-prohibit-message
-   (maybe-string 'disabled)
+   maybe-string
    "If this option is set password modification using pam_ldap will be denied
 and the specified message will be presented to the user instead.  The message
 can be used to direct the user to an alternative means of changing their
diff --git a/gnu/services/cgit.scm b/gnu/services/cgit.scm
index bfc89a40a4..c2c003983a 100644
--- a/gnu/services/cgit.scm
+++ b/gnu/services/cgit.scm
@@ -241,27 +241,27 @@ is no suitable HEAD.")
    (repo-file-object "")
    "Override the default @code{email-filter}.")
   (enable-commit-graph?
-   (maybe-repo-boolean 'disabled)
+   maybe-repo-boolean
    "A flag which can be used to disable the global setting
 @code{enable-commit-graph?}.")
   (enable-log-filecount?
-   (maybe-repo-boolean 'disabled)
+   maybe-repo-boolean
    "A flag which can be used to disable the global setting
 @code{enable-log-filecount?}.")
   (enable-log-linecount?
-   (maybe-repo-boolean 'disabled)
+   maybe-repo-boolean
    "A flag which can be used to disable the global setting
 @code{enable-log-linecount?}.")
   (enable-remote-branches?
-   (maybe-repo-boolean 'disabled)
+   maybe-repo-boolean
    "Flag which, when set to @code{#t}, will make cgit display remote
 branches in the summary and refs views.")
   (enable-subject-links?
-   (maybe-repo-boolean 'disabled)
+   maybe-repo-boolean
    "A flag which can be used to override the global setting
 @code{enable-subject-links?}.")
   (enable-html-serving?
-   (maybe-repo-boolean 'disabled)
+   maybe-repo-boolean
    "A flag which can be used to override the global setting
 @code{enable-html-serving?}.")
   (hide?
diff --git a/gnu/services/configuration.scm b/gnu/services/configuration.scm
index bdca33ed68..f6b20fb82b 100644
--- a/gnu/services/configuration.scm
+++ b/gnu/services/configuration.scm
@@ -93,7 +93,7 @@
 (define (configuration-missing-field kind field)
   (configuration-error
    (format #f "~a configuration missing required field ~a" kind field)))
-(define (configuration-no-default-value kind field)
+(define (configuration-missing-default-value kind field)
   (configuration-error
    (format #f "The field `~a' of the `~a' configuration record \
 does not have a default value" field kind)))
@@ -142,7 +142,8 @@ does not have a default value" field kind)))
                                     (id #'stem #'serialize-maybe- #'stem))))
        #`(begin
            (define (maybe-stem? val)
-             (or (eq? val 'disabled) (stem? val)))
+             (or (unspecified? val)
+                 (stem? val)))
            #,@(if serialize?
                   (list #'(define (serialize-maybe-stem field-name val)
                             (if (stem? val)
@@ -170,10 +171,10 @@ does not have a default value" field kind)))
      (values #'(field-type def)))
     ((field-type)
      (identifier? #'field-type)
-     (values #'(field-type 'disabled)))
+     (values #'(field-type *unspecified*)))
     (field-type
      (identifier? #'field-type)
-     (values #'(field-type 'disabled)))))
+     (values #'(field-type *unspecified*)))))
 
 (define (define-configuration-helper serialize? serializer-prefix syn)
   (syntax-case syn ()
@@ -219,9 +220,7 @@ does not have a default value" field kind)))
                                           source-properties->location))
                           (innate))
                #,@(map (lambda (name getter def)
-                         (if (eq? (syntax->datum def) (quote 'undefined))
-                             #`(#,name #,getter)
-                             #`(#,name #,getter (default #,def))))
+                         #`(#,name #,getter (default #,def)))
                        #'(field ...)
                        #'(field-getter ...)
                        #'(field-default ...)))
@@ -236,8 +235,8 @@ does not have a default value" field kind)))
                         (lambda ()
                           (display '#,(id #'stem #'% #'stem))
                           (if (eq? (syntax->datum field-default)
-                                   'undefined)
-                              (configuration-no-default-value
+                                   '*unspecified*)
+                              (configuration-missing-default-value
                                '#,(id #'stem #'% #'stem) 'field)
                               field-default)))
                       (documentation doc))
diff --git a/gnu/services/file-sharing.scm b/gnu/services/file-sharing.scm
index e3d681b08f..e32d1f145d 100644
--- a/gnu/services/file-sharing.scm
+++ b/gnu/services/file-sharing.scm
@@ -115,8 +115,7 @@ type generated and used by Transmission clients, suitable for passing to the
 (set! serialize-maybe-string
   (lambda (field-name val)
     (serialize-string field-name
-                      (if (and (symbol? val)
-                               (eq? val 'disabled))
+                      (if (unspecified? val)
                           ""
                           val))))
 
@@ -181,8 +180,7 @@ type generated and used by Transmission clients, suitable for passing to the
 (define-maybe file-object)
 (set! serialize-maybe-file-object
   (lambda (field-name val)
-    (if (and (symbol? val)
-             (eq? val 'disabled))
+    (if (unspecified? val)
         (serialize-string field-name "")
         (serialize-file-object field-name val))))
 
@@ -281,7 +279,7 @@ torrent is being downloaded, then moved to @code{download-dir} once the
 torrent is complete.  Otherwise, files for all torrents (including those still
 being downloaded) will be placed in @code{download-dir}.")
   (incomplete-dir
-   (maybe-string 'disabled)
+   maybe-string
    "The directory in which files from incompletely downloaded torrents will be
 held when @code{incomplete-dir-enabled?} is @code{#t}.")
   (umask
@@ -305,7 +303,7 @@ for new @file{.torrent} files and the torrents they describe added
 automatically (and the original files removed, if
 @code{trash-original-torrent-files?} is @code{#t}).")
   (watch-dir
-   (maybe-string 'disabled)
+   maybe-string
    "The directory to be watched for @file{.torrent} files indicating new
 torrents to be added, when @code{watch-dir-enabled} is @code{#t}.")
   (trash-original-torrent-files?
@@ -401,11 +399,11 @@ upstream gateway automatically using @acronym{UPnP} and @acronym{NAT-PMP}.")
 @code{prefer-unencrypted-connections}, @code{prefer-encrypted-connections} or
 @code{require-encrypted-connections}.")
   (peer-congestion-algorithm
-   (maybe-string 'disabled)
+   maybe-string
    "The TCP congestion-control algorithm to use for peer connections,
 specified using a string recognized by the operating system in calls to
-@code{setsockopt} (or set to @code{disabled}, in which case the
-operating-system default is used).
+@code{setsockopt} (or leave it unset, in which case the operating-system
+default is used).
 
 Note that on GNU/Linux systems, the kernel must be configured to allow
 processes to use a congestion-control algorithm not in the default set;
@@ -465,7 +463,7 @@ torrent before it is regenerated.")
    "When @code{#t}, the daemon will ignore peers mentioned in the blocklist it
 has most recently downloaded from @code{blocklist-url}.")
   (blocklist-url
-   (maybe-string 'disabled)
+   maybe-string
    "The URL of a peer blocklist (in @acronym{P2P}-plaintext or eMule
 @file{.dat} format) to be periodically downloaded and applied when
 @code{blocklist-enabled?} is @code{#t}.")
@@ -564,11 +562,11 @@ which allows remote control of the daemon via its Web interface, the
 the side effect of disabling host-name whitelisting (see
 @code{rpc-host-whitelist-enabled?}.")
   (rpc-username
-   (maybe-string 'disabled)
+   maybe-string
    "The username required by clients to access the @acronym{RPC} interface
 when @code{rpc-authentication-required?} is @code{#t}.")
   (rpc-password
-   (maybe-transmission-password-hash 'disabled)
+   maybe-transmission-password-hash
    "The password required by clients to access the @acronym{RPC} interface
 when @code{rpc-authentication-required?} is @code{#t}.  This must be specified
 using a password hash in the format recognized by Transmission clients, either
@@ -613,7 +611,7 @@ they are added in ``paused'' state.")
 @code{script-torrent-done-filename} will be invoked each time a torrent
 completes.")
   (script-torrent-done-filename
-   (maybe-file-object 'disabled)
+   maybe-file-object
    "A file name or file-like object specifying a script to run each time a
 torrent completes, when @code{script-torrent-done-enabled?} is @code{#t}.")
   (scrape-paused-torrents-enabled?
diff --git a/gnu/services/messaging.scm b/gnu/services/messaging.scm
index 48eff27b49..651f90adb2 100644
--- a/gnu/services/messaging.scm
+++ b/gnu/services/messaging.scm
@@ -76,6 +76,7 @@
       (lambda (field target)
         (and (memq (syntax->datum target) `(common ,arg)) field)))
     (syntax-case stx ()
+      ;; TODO Also handle (field-type) form, without a default.
       ((_ stem (field (field-type def) doc target) ...)
        (with-syntax (((new-field-type ...)
                       (map (lambda (field-type target)
@@ -89,7 +90,7 @@
                      ((new-def ...)
                       (map (lambda (def target)
                              (if (eq? 'common (syntax->datum target))
-                                 #''disabled def))
+                                 #'*unspecified* def))
                            #'(def ...) #'(target ...)))
                      ((new-doc ...)
                       (map (lambda (doc target)
@@ -199,7 +200,7 @@
 (define-maybe file-object-list)
 
 (define (raw-content? val)
-  (not (eq? val 'disabled)))
+  (not (unspecified? val)))
 (define (serialize-raw-content field-name val)
   val)
 (define-maybe raw-content)
@@ -227,15 +228,15 @@ just joined the room."))
 
 (define-configuration ssl-configuration
   (protocol
-   (maybe-string 'disabled)
+   maybe-string
    "This determines what handshake to use.")
 
   (key
-   (maybe-file-name 'disabled)
+   maybe-file-name
    "Path to your private key file.")
 
   (certificate
-   (maybe-file-name 'disabled)
+   maybe-file-name
    "Path to your certificate file.")
 
   (capath
@@ -244,48 +245,48 @@ just joined the room."))
 trust when verifying the certificates of remote servers.")
 
   (cafile
-   (maybe-file-object 'disabled)
+   maybe-file-object
    "Path to a file containing root certificates that you wish Prosody to trust.
 Similar to @code{capath} but with all certificates concatenated together.")
 
   (verify
-   (maybe-string-list 'disabled)
+   maybe-string-list
    "A list of verification options (these mostly map to OpenSSL's
 @code{set_verify()} flags).")
 
   (options
-   (maybe-string-list 'disabled)
+   maybe-string-list
    "A list of general options relating to SSL/TLS.  These map to OpenSSL's
 @code{set_options()}.  For a full list of options available in LuaSec, see the
 LuaSec source.")
 
   (depth
-   (maybe-non-negative-integer 'disabled)
+   maybe-non-negative-integer
    "How long a chain of certificate authorities to check when looking for a
 trusted root certificate.")
 
   (ciphers
-   (maybe-string 'disabled)
+   maybe-string
    "An OpenSSL cipher string.  This selects what ciphers Prosody will offer to
 clients, and in what order.")
 
   (dhparam
-   (maybe-file-name 'disabled)
+   maybe-file-name
    "A path to a file containing parameters for Diffie-Hellman key exchange.  You
 can create such a file with:
 @code{openssl dhparam -out /etc/prosody/certs/dh-2048.pem 2048}")
 
   (curve
-   (maybe-string 'disabled)
+   maybe-string
    "Curve for Elliptic curve Diffie-Hellman. Prosody's default is
 @samp{\"secp384r1\"}.")
 
   (verifyext
-   (maybe-string-list 'disabled)
+   maybe-string-list
    "A list of \"extra\" verification options.")
 
   (password
-   (maybe-string 'disabled)
+   maybe-string
    "Password for encrypted private keys."))
 (define (serialize-ssl-configuration field-name val)
   #~(format #f "ssl = {\n~a};\n"
@@ -473,12 +474,12 @@ by the Prosody service.  See @url{https://prosody.im/doc/logging}."
      global)
 
     (http-max-content-size
-     (maybe-non-negative-integer 'disabled)
+     (maybe-non-negative-integer *unspecified*)
      "Maximum allowed size of the HTTP body (in bytes)."
      common)
 
     (http-external-url
-     (maybe-string 'disabled)
+     (maybe-string *unspecified*)
      "Some modules expose their own URL in various ways.  This URL is built
 from the protocol, host and port used.  If Prosody sits behind a proxy, the
 public URL will be @code{http-external-url} instead.  See
@@ -555,7 +556,7 @@ support.  To add an external component, you simply fill the hostname field.  See
      int-component)
 
     (mod-muc
-     (maybe-mod-muc-configuration 'disabled)
+     (maybe-mod-muc-configuration *unspecified*)
      "Multi-user chat (MUC) is Prosody's module for allowing you to create
 hosted chatrooms/conferences for XMPP users.
 
@@ -572,7 +573,7 @@ See also @url{https://prosody.im/doc/modules/mod_muc}."
      ext-component)
 
     (raw-content
-     (maybe-raw-content 'disabled)
+     (maybe-raw-content *unspecified*)
      "Raw content that will be added to the configuration file."
      common)))
 
diff --git a/gnu/services/networking.scm b/gnu/services/networking.scm
index 90b9317510..a9560db66b 100644
--- a/gnu/services/networking.scm
+++ b/gnu/services/networking.scm
@@ -771,17 +771,17 @@ logging is disabled.")
    "The node host name that is used to make the first connection to the
 network.  A specific port value can be provided by appending the @code{:PORT}
 suffix.  By default, it uses the Jami bootstrap nodes, but any host can be
-specified here.  It's also possible to disable bootstrapping by setting this
-to the @code{'disabled} symbol.")
+specified here.  It's also possible to disable bootstrapping by explicitly
+setting this field to the @code{*unspecified*} value.")
   (port
    (maybe-number 4222)
-   "The UDP port to bind to.  When set to @code{'disabled}, an available port
-is automatically selected.")
+   "The UDP port to bind to.  When set to @code{*unspecified*}, an available
+port is automatically selected.")
   (proxy-server-port
-   (maybe-number 'disabled)
+   maybe-number
    "Spawn a proxy server listening on the specified port.")
   (proxy-server-port-tls
-   (maybe-number 'disabled)
+   maybe-number
    "Spawn a proxy server listening to TLS connections on the specified
 port."))
 
diff --git a/gnu/services/pm.scm b/gnu/services/pm.scm
index e48236dbca..3daf484cc1 100644
--- a/gnu/services/pm.scm
+++ b/gnu/services/pm.scm
@@ -114,55 +114,55 @@ before syncing on AC.")
    "Same as @code{max-lost-work-secs-on-ac} but on BAT mode.")
 
   (cpu-scaling-governor-on-ac
-   (maybe-space-separated-string-list 'disabled)
+   maybe-space-separated-string-list
    "CPU frequency scaling governor on AC mode.  With intel_pstate
 driver, alternatives are powersave and performance.  With acpi-cpufreq driver,
 alternatives are ondemand, powersave, performance and conservative.")
 
   (cpu-scaling-governor-on-bat
-   (maybe-space-separated-string-list 'disabled)
+   maybe-space-separated-string-list
    "Same as @code{cpu-scaling-governor-on-ac} but on BAT mode.")
 
   (cpu-scaling-min-freq-on-ac
-   (maybe-non-negative-integer 'disabled)
+   maybe-non-negative-integer
    "Set the min available frequency for the scaling governor on AC.")
 
   (cpu-scaling-max-freq-on-ac
-   (maybe-non-negative-integer 'disabled)
+   maybe-non-negative-integer
    "Set the max available frequency for the scaling governor on AC.")
 
   (cpu-scaling-min-freq-on-bat
-   (maybe-non-negative-integer 'disabled)
+   maybe-non-negative-integer
    "Set the min available frequency for the scaling governor on BAT.")
 
   (cpu-scaling-max-freq-on-bat
-   (maybe-non-negative-integer 'disabled)
+   maybe-non-negative-integer
    "Set the max available frequency for the scaling governor on BAT.")
 
   (cpu-min-perf-on-ac
-   (maybe-non-negative-integer 'disabled)
+   maybe-non-negative-integer
    "Limit the min P-state to control the power dissipation of the CPU,
 in AC mode.  Values are stated as a percentage of the available performance.")
 
   (cpu-max-perf-on-ac
-   (maybe-non-negative-integer 'disabled)
+   maybe-non-negative-integer
    "Limit the max P-state to control the power dissipation of the CPU,
 in AC mode.  Values are stated as a percentage of the available performance.")
 
   (cpu-min-perf-on-bat
-   (maybe-non-negative-integer 'disabled)
+   maybe-non-negative-integer
    "Same as @code{cpu-min-perf-on-ac} on BAT mode.")
 
   (cpu-max-perf-on-bat
-   (maybe-non-negative-integer 'disabled)
+   maybe-non-negative-integer
    "Same as @code{cpu-max-perf-on-ac} on BAT mode.")
 
   (cpu-boost-on-ac?
-   (maybe-boolean 'disabled)
+   maybe-boolean
    "Enable CPU turbo boost feature on AC mode.")
 
   (cpu-boost-on-bat?
-   (maybe-boolean 'disabled)
+   maybe-boolean
    "Same as @code{cpu-boost-on-ac?} on BAT mode.")
 
   (sched-powersave-on-ac?
@@ -179,7 +179,7 @@ used under light load conditions.")
    "Enable Linux kernel NMI watchdog.")
 
   (phc-controls
-   (maybe-string 'disabled)
+   maybe-string
    "For Linux kernels with PHC patch applied, change CPU voltages.
 An example value would be @samp{\"F:V F:V F:V F:V\"}.")
 
@@ -205,16 +205,16 @@ performance, normal, powersave.")
    "Same as @code{disk-apm-bat} but on BAT mode.")
 
   (disk-spindown-timeout-on-ac
-   (maybe-space-separated-string-list 'disabled)
+   maybe-space-separated-string-list
    "Hard disk spin down timeout.  One value has to be specified for
 each declared hard disk.")
 
   (disk-spindown-timeout-on-bat
-   (maybe-space-separated-string-list 'disabled)
+   maybe-space-separated-string-list
    "Same as @code{disk-spindown-timeout-on-ac} but on BAT mode.")
 
   (disk-iosched
-   (maybe-space-separated-string-list 'disabled)
+   maybe-space-separated-string-list
    "Select IO scheduler for disk devices.  One value has to be specified
 for each declared hard disk.  Example alternatives are cfq, deadline and noop.")
 
@@ -228,16 +228,16 @@ min_power, medium_power, max_performance.")
    "Same as @code{sata-linkpwr-ac} but on BAT mode.")
 
   (sata-linkpwr-blacklist
-   (maybe-string 'disabled)
+   maybe-string
    "Exclude specified SATA host devices for link power management.")
 
   (ahci-runtime-pm-on-ac?
-   (maybe-on-off-boolean 'disabled)
+   maybe-on-off-boolean
    "Enable Runtime Power Management for AHCI controller and disks
 on AC mode.")
 
   (ahci-runtime-pm-on-bat?
-   (maybe-on-off-boolean 'disabled)
+   maybe-on-off-boolean
    "Same as @code{ahci-runtime-pm-on-ac} on BAT mode.")
 
   (ahci-runtime-pm-timeout
@@ -254,19 +254,19 @@ default, performance, powersave.")
    "Same as @code{pcie-aspm-ac} but on BAT mode.")
 
   (start-charge-thresh-bat0
-   (maybe-non-negative-integer 'disabled)
+   maybe-non-negative-integer
    "Percentage when battery 0 should begin charging.")
 
   (stop-charge-thresh-bat0
-   (maybe-non-negative-integer 'disabled)
+   maybe-non-negative-integer
    "Percentage when battery 0 should stop charging.")
 
   (start-charge-thresh-bat1
-   (maybe-non-negative-integer 'disabled)
+   maybe-non-negative-integer
    "Percentage when battery 1 should begin charging.")
 
   (stop-charge-thresh-bat1
-   (maybe-non-negative-integer 'disabled)
+   maybe-non-negative-integer
    "Percentage when battery 1 should stop charging.")
 
   (radeon-power-profile-on-ac
@@ -346,7 +346,7 @@ on and auto.")
 blacklisted ones.")
 
   (runtime-pm-blacklist
-   (maybe-space-separated-string-list 'disabled)
+   maybe-space-separated-string-list
    "Exclude specified PCI(e) device addresses from Runtime Power Management.")
 
   (runtime-pm-driver-blacklist
@@ -359,7 +359,7 @@ Runtime Power Management.")
    "Enable USB autosuspend feature.")
 
   (usb-blacklist
-   (maybe-string 'disabled)
+   maybe-string
    "Exclude specified devices from USB autosuspend.")
 
   (usb-blacklist-wwan?
@@ -367,12 +367,12 @@ Runtime Power Management.")
    "Exclude WWAN devices from USB autosuspend.")
 
   (usb-whitelist
-   (maybe-string 'disabled)
+   maybe-string
    "Include specified devices into USB autosuspend, even if they are
 already excluded by the driver or via @code{usb-blacklist-wwan?}.")
 
   (usb-autosuspend-disable-on-shutdown?
-   (maybe-boolean 'disabled)
+   maybe-boolean
    "Enable USB autosuspend before shutdown.")
 
   (restore-device-state-on-startup?
diff --git a/gnu/services/telephony.scm b/gnu/services/telephony.scm
index 0a11e283a0..e8bfbc88c5 100644
--- a/gnu/services/telephony.scm
+++ b/gnu/services/telephony.scm
@@ -157,7 +157,7 @@ the @samp{root} user (i.e., not in the store), to guard against leaking the
 secret key material of the Jami account it contains."
    empty-serializer)
   (allowed-contacts
-   (maybe-account-fingerprint-list 'disabled)
+   maybe-account-fingerprint-list
    "The list of allowed contacts for the account, entered as their 40
 characters long fingerprint.  Messages or calls from accounts not in that list
 will be rejected.  When unspecified, the configuration of the account archive
@@ -166,7 +166,7 @@ allowance, which typically defaults to allow any contact to communicate with
 the account."
    empty-serializer)
   (moderators
-   (maybe-account-fingerprint-list 'disabled)
+   maybe-account-fingerprint-list
    "The list of contacts that should have moderation privileges (to ban, mute,
 etc. other users) in rendezvous conferences, entered as their 40 characters
 long fingerprint.  When unspecified, the configuration of the account archive
@@ -175,24 +175,24 @@ anyone to moderate."
    empty-serializer)
   ;; The serializable fields below are to be set with set-account-details.
   (rendezvous-point?
-   (maybe-boolean 'disabled)
+   maybe-boolean
    "Whether the account should operate in the rendezvous mode.  In this mode,
 all the incoming audio/video calls are mixed into a conference.  When left
 unspecified, the value from the account archive prevails.")
   (peer-discovery?
-   (maybe-boolean 'disabled)
+   maybe-boolean
    "Whether peer discovery should be enabled.  Peer discovery is used to
 discover other OpenDHT nodes on the local network, which can be useful to
 maintain communication between devices on such network even when the
 connection to the the Internet has been lost.  When left unspecified, the
 value from the account archive prevails.")
   (bootstrap-hostnames
-   (maybe-string-list 'disabled)
+   maybe-string-list
    "A list of hostnames or IPs pointing to OpenDHT nodes, that should be used
 to initially join the OpenDHT network.  When left unspecified, the value from
 the account archive prevails.")
   (name-server-uri
-   (maybe-string 'disabled)
+   maybe-string
    "The URI of the name server to use, that can be used to retrieve the
 account fingerprint for a registered username."))
 
@@ -214,7 +214,7 @@ SET-ACCOUNT-DETAILS."
                                    name ((configuration-field-getter field)
                                          jami-account-object)))
                            ;; The define-maybe default serializer produces an
-                           ;; empty string for the 'disabled value.
+                           ;; empty string for unspecified values.
                            (value* (if (string-null? value)
                                        #f
                                        value)))
@@ -247,7 +247,7 @@ SET-ACCOUNT-DETAILS."
    (boolean #f)
    "Whether to force automatic answer to incoming calls.")
   (accounts
-   (maybe-jami-account-list 'disabled)
+   maybe-jami-account-list
    "A list of Jami accounts to be (re-)provisioned every time the Jami daemon
 service starts.  When providing this field, the account directories under
 @file{/var/lib/jami/} are recreated every time the service starts, ensuring a
@@ -307,7 +307,7 @@ CONFIG, a <jami-configuration> object."
          (dbus (jami-configuration-dbus config))
          (dbus-daemon (file-append dbus "/bin/dbus-daemon"))
          (accounts (jami-configuration-accounts config))
-         (declarative-mode? (not (eq? 'disabled accounts))))
+         (declarative-mode? (not (unspecified? accounts))))
 
     (with-extensions (list guile-packrat ;used by guile-ac-d-bus
                            guile-ac-d-bus
@@ -649,7 +649,7 @@ argument, either a registered username or the fingerprint of the account.")
                                           account-details)
                            (let ((username (archive-name->username
                                             archive)))
-                             (when (not (eq? 'disabled allowed-contacts))
+                             (when (not (unspecified? allowed-contacts))
                                ;; Reject calls from unknown contacts.
                                (set-account-details
                                 '(("DHT.PublicInCalls" . "false")) username)
@@ -659,7 +659,7 @@ argument, either a registered username or the fingerprint of the account.")
                                ;; Add allowed ones.
                                (for-each (cut add-contact <> username)
                                          allowed-contacts))
-                             (when (not (eq? 'disabled moderators))
+                             (when (not (unspecified? moderators))
                                ;; Disable the 'AllModerators' property.
                                (set-all-moderators #f username)
                                ;; Remove all moderators.
diff --git a/gnu/services/vpn.scm b/gnu/services/vpn.scm
index a3dc96c1a2..8be632d55f 100644
--- a/gnu/services/vpn.scm
+++ b/gnu/services/vpn.scm
@@ -363,7 +363,7 @@ poll/epoll/select prior to the write operation.")
 channel to protect against DoS attacks.")
 
    (auth-user-pass
-     (maybe-string 'disabled)
+    maybe-string
      "Authenticate with server using username/password.  The option is a file
 containing username/password on 2 lines.  Do not use a file-like object as it
 would be added to the store and readable by any user.")
diff --git a/tests/services/configuration.scm b/tests/services/configuration.scm
index 0debf8095b..334a1e409b 100644
--- a/tests/services/configuration.scm
+++ b/tests/services/configuration.scm
@@ -136,3 +136,15 @@
 
 (test-assert "maybe value without serialization no procedure bound"
   (not (defined? 'serialize-maybe-string)))
+
+(test-assert "maybe type, no default"
+  (unspecified?
+   (config-with-maybe-string/no-serialization-name
+    (config-with-maybe-string/no-serialization))))
+
+(test-assert "maybe type, with default"
+  (equal?
+   "foo"
+   (config-with-maybe-string/no-serialization-name
+    (config-with-maybe-string/no-serialization
+     (name "foo")))))