Add infrastructure for contacts and calendars

This also adds the modules

  - programs.vdirsyncer,
  - programs.khal, and
  - services.vdirsyncer

that integrate with the new infrastructure.

Co-authored-by: Andrew Scott <3648487+ayyjayess@users.noreply.github.com>
Co-authored-by: Sebastian Zivota <sebastian.zivota@mailbox.org>

modules/accounts/calendar.nix

@@ -0,0 +1,177 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.accounts.calendar;
+
+  localModule = name: types.submodule {
+    options = {
+      path = mkOption {
+        type = types.str;
+        default = "${cfg.basePath}/${name}";
+        description = "The path of the storage.";
+      };
+
+      type = mkOption {
+        type = types.enum [ "filesystem" "singlefile" ];
+        description = "The type of the storage.";
+      };
+
+      fileExt = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        description = "The file extension to use.";
+      };
+
+      encoding = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        description = ''
+          File encoding for items, both content and file name.
+          Defaults to UTF-8.
+        '';
+      };
+    };
+  };
+
+  remoteModule = types.submodule {
+    options = {
+      type = mkOption {
+        type = types.enum [ "caldav" "http" "google_calendar" ];
+        description = "The type of the storage.";
+      };
+
+      url = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        description = "The url of the storage.";
+      };
+
+      userName = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        description = "User name for authentication.";
+      };
+
+      userNameCommand = mkOption {
+        type = types.nullOr (types.listOf types.str);
+        default = null;
+        example = [ "~/get-username.sh" ];
+        description = ''
+          A command that prints the user name to standard
+          output.
+        '';
+      };
+
+      passwordCommand = mkOption {
+        type = types.nullOr (types.listOf types.str);
+        default = null;
+        example = [ "pass" "caldav" ];
+        description = ''
+          A command that prints the password to standard
+          output.
+        '';
+      };
+    };
+  };
+
+  calendarOpts = { name, config, ... }: {
+    options = {
+      name = mkOption {
+        type = types.str;
+        readOnly = true;
+        description = ''
+          Unique identifier of the calendar. This is set to the
+          attribute name of the calendar configuration.
+        '';
+      };
+
+
+      primary = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether this is the primary account. Only one account may be
+          set as primary.
+        '';
+      };
+
+      primaryCollection = mkOption {
+        type = types.str;
+        description = ''
+          The primary collection of the account. Required when an account has
+          multiple collections.
+        '';
+      };
+
+      local = mkOption {
+        type = types.nullOr (localModule name);
+
+        default = null;
+        description = ''
+          Local configuration for the calendar.
+        '';
+      };
+
+      remote = mkOption {
+        type = types.nullOr remoteModule;
+
+        default = null;
+        description = ''
+          Remote configuration for the calendar.
+        '';
+      };
+    };
+
+    config = mkMerge [
+      {
+        name = name;
+        khal.type = mkOptionDefault null;
+      }
+    ];
+  };
+
+in
+
+{
+  options.accounts.calendar = {
+    basePath = mkOption {
+      type = types.str;
+      default = "${config.home.homeDirectory}/.calendars/";
+      defaultText = "$HOME/.calendars";
+      description = ''
+        The base directory in which to save calendars.
+      '';
+    };
+
+    accounts = mkOption {
+      type = types.attrsOf (types.submodule [
+        calendarOpts
+        (import ../programs/vdirsyncer-accounts.nix)
+        (import ../programs/khal-accounts.nix)
+        (import ../programs/khal-calendar-accounts.nix)
+      ]);
+      default = {};
+      description = "List of calendars.";
+    };
+  };
+  config = mkIf (cfg.accounts != {}) {
+    assertions =
+      let
+        primaries =
+          catAttrs "name"
+          (filter (a: a.primary)
+          (attrValues cfg.accounts));
+      in
+        [{
+          assertion = length primaries <= 1;
+          message =
+            "Must have at most one primary calendar accounts but found "
+            + toString (length primaries)
+            + ", namely "
+            + concatStringsSep ", " primaries;
+        }];
+  };
+}

modules/accounts/contacts.nix

@@ -0,0 +1,140 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.accounts.contact;
+
+  localModule = name: types.submodule {
+    options = {
+      path = mkOption {
+        type = types.str;
+        default = "${cfg.basePath}/${name}";
+        description = "The path of the storage.";
+      };
+
+      type = mkOption {
+        type = types.enum [ "filesystem" "singlefile" ];
+        description = "The type of the storage.";
+      };
+
+      fileExt = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        description = "The file extension to use.";
+      };
+
+      encoding = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        description = ''
+          File encoding for items, both content and file name.
+          Defaults to UTF-8.
+        '';
+      };
+    };
+  };
+
+  remoteModule = types.submodule {
+    options = {
+      type = mkOption {
+        type = types.enum [ "carddav" "http" "google_contacts" ];
+        description = "The type of the storage.";
+      };
+
+      url = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        description = "The url of the storage.";
+      };
+
+      userName = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        description = "User name for authentication.";
+      };
+
+      userNameCommand = mkOption {
+        type = types.nullOr (types.listOf types.str);
+        default = null;
+        example = [ "~/get-username.sh" ];
+        description = ''
+          A command that prints the user name to standard
+          output.
+        '';
+      };
+
+      passwordCommand = mkOption {
+        type = types.nullOr (types.listOf types.str);
+        default = null;
+        example = [ "pass" "caldav" ];
+        description = ''
+          A command that prints the password to standard
+          output.
+        '';
+      };
+    };
+  };
+
+  contactOpts = { name, config, ... }: {
+    options = {
+      name = mkOption {
+        type = types.str;
+        readOnly = true;
+        description = ''
+          Unique identifier of the contact account. This is set to the
+          attribute name of the contact configuration.
+        '';
+      };
+
+      local = mkOption {
+        type = types.nullOr (localModule name);
+        default = null;
+        description = ''
+          Local configuration for the contacts.
+        '';
+      };
+
+      remote = mkOption {
+        type = types.nullOr remoteModule;
+        default = null;
+        description = ''
+          Remote configuration for the contacts.
+        '';
+      };
+    };
+
+    config = mkMerge [
+      {
+        name = name;
+      }
+    ];
+  };
+
+in
+
+{
+  options.accounts.contact = {
+    basePath = mkOption {
+      type = types.str;
+      default = "${config.home.homeDirectory}/.contacts/";
+      defaultText = "$HOME/.contacts";
+      description = ''
+        The base directory in which to save contacts.
+      '';
+    };
+
+    accounts = mkOption {
+      type = types.attrsOf (types.submodule [
+        contactOpts
+        (import ../programs/vdirsyncer-accounts.nix)
+        (import ../programs/khal-accounts.nix)
+      ]);
+      default = {};
+      description = "List of contacts.";
+    };
+  };
+  config = mkIf (cfg.accounts != {}) {
+  };
+}

modules/misc/news.nix

@@ -1481,6 +1481,30 @@ in
           A new module is available: 'programs.lf'
         '';
       }
+
+      {
+        time = "2020-04-26T13:32:17+00:00";
+        message = ''
+          A number of new modules are available:
+
+            - 'accounts.calendar',
+            - 'accounts.contact',
+            - 'programs.khal',
+            - 'programs.vdirsyncer', and
+            - 'services.vdirsyncer' (Linux only).
+
+          The two first modules offer a number of options for
+          configuring calendar and contact accounts. This includes,
+          for example, information about carddav and caldav servers.
+
+          The khal and vdirsyncer modules make use of this new account
+          infrastructure.
+
+          Note, these module are still somewhat experimental and their
+          structure should not be seen as final, some modifications
+          may be necessary as new modules are added.
+        '';
+      }
     ];
   };
 }

modules/modules.nix

@@ -22,6 +22,8 @@ let
 
   allModules = [
     (loadModule ./accounts/email.nix { })
+    (loadModule ./accounts/calendar.nix { })
+    (loadModule ./accounts/contacts.nix { })
     (loadModule ./files.nix { })
     (loadModule ./home-environment.nix { })
     (loadModule ./manual.nix { })
@@ -74,6 +76,7 @@ let
     (loadModule ./programs/jq.nix { })
     (loadModule ./programs/kakoune.nix { })
     (loadModule ./programs/keychain.nix { })
+    (loadModule ./programs/khal.nix { })
     (loadModule ./programs/kitty.nix { })
     (loadModule ./programs/lesspipe.nix { })
     (loadModule ./programs/lf.nix { })
@@ -107,6 +110,7 @@ let
     (loadModule ./programs/texlive.nix { })
     (loadModule ./programs/tmux.nix { })
     (loadModule ./programs/urxvt.nix { })
+    (loadModule ./programs/vdirsyncer.nix { })
     (loadModule ./programs/vim.nix { })
     (loadModule ./programs/vscode.nix { })
     (loadModule ./programs/vscode/haskell.nix { })
@@ -161,6 +165,7 @@ let
     (loadModule ./services/udiskie.nix { })
     (loadModule ./services/unclutter.nix { })
     (loadModule ./services/unison.nix { condition = hostPlatform.isLinux; })
+    (loadModule ./services/vdirsyncer.nix { condition = hostPlatform.isLinux; })
     (loadModule ./services/window-managers/awesome.nix { })
     (loadModule ./services/window-managers/bspwm/default.nix { condition = hostPlatform.isLinux; })
     (loadModule ./services/window-managers/i3-sway/i3.nix { })

modules/programs/khal-accounts.nix

@@ -0,0 +1,26 @@
+{ config, lib, ... }:
+
+with lib;
+
+{
+  options.khal = {
+    enable = lib.mkEnableOption "khal access";
+
+    readOnly = mkOption {
+      type = types.bool;
+      description = ''
+        Keep khal from making any changes to this account.
+      '';
+      default = false;
+    };
+
+    glob = mkOption {
+      type = types.str;
+      default = "*";
+      description = ''
+        The glob expansion to be searched for events or birthdays when type
+        is set to discover.
+      '';
+    };
+  };
+}

modules/programs/khal-calendar-accounts.nix

@@ -0,0 +1,13 @@
+{ config, lib, ... }:
+
+with lib;
+
+{
+  options.khal = {
+    type = mkOption {
+      type = types.nullOr (types.enum [ "calendar" "discover"]);
+      description = ''
+      '';
+    };
+  };
+}

modules/programs/khal.nix

@@ -0,0 +1,64 @@
+# khal config loader is sensitive to leading space !
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.programs.khal;
+
+  khalCalendarAccounts = filterAttrs (_: a: a.khal.enable)
+    (config.accounts.calendar.accounts);
+
+  khalContactAccounts = mapAttrs (_: v: v // {type = "birthdays";})
+  (filterAttrs (_: a: a.khal.enable)
+  (config.accounts.contact.accounts));
+
+  khalAccounts = khalCalendarAccounts // khalContactAccounts;
+
+  primaryAccount = findSingle (a: a.primary) null null
+    (mapAttrsToList (n: v: v // {name= n;}) khalAccounts);
+in
+
+{
+  options.programs.khal = {
+    enable = mkEnableOption "khal, a CLI calendar application";
+  };
+
+  config = mkIf cfg.enable {
+    home.packages =  [ pkgs.khal ];
+
+    xdg.configFile."khal/config".text = concatStringsSep "\n" (
+    [
+      "[calendars]"
+    ]
+    ++ (mapAttrsToList (name: value: concatStringsSep "\n"
+      ([
+        ''[[${name}]]''
+        ''path = ${value.local.path + "/" + (optionalString (value.khal.type == "discover") value.khal.glob)}''
+      ]
+      ++ optional (value.khal.readOnly) "readonly = True"
+      ++ optional (!isNull value.khal.type) "type = ${value.khal.type}"
+      ++ ["\n"]
+      )
+      ) khalAccounts)
+    ++
+    [
+    (generators.toINI {} {
+      default = optionalAttrs (!isNull primaryAccount) {
+	default_calendar = if isNull primaryAccount.primaryCollection then primaryAccount.name else primaryAccount.primaryCollection;
+      };
+
+      locale = {
+	timeformat = "%H:%M";
+	dateformat = "%Y-%m-%d";
+	longdateformat = "%Y-%m-%d";
+	datetimeformat = "%Y-%m-%d %H:%M";
+	longdatetimeformat = "%Y-%m-%d %H:%M";
+	weeknumbers = "right";
+      };
+    })
+    ]
+    );
+  };
+}

modules/programs/vdirsyncer-accounts.nix

@@ -0,0 +1,193 @@
+{ lib, ... }:
+
+with lib;
+
+let
+
+  collection = types.either types.str (types.listOf types.str);
+
+in
+
+{
+  options.vdirsyncer = {
+    enable = mkEnableOption "synchronization using vdirsyncer";
+
+    collections = mkOption {
+      type = types.nullOr (types.listOf collection);
+      default = null;
+      description = ''
+        The collections to synchronize between the storages.
+      '';
+    };
+
+    conflictResolution = mkOption {
+      type = types.nullOr (types.either (types.enum ["remote wins" "local wins"]) (types.listOf types.str));
+      default = null;
+      description = ''
+        What to do in case of a conflict between the storages.
+        Either <literal>"remote wins"</literal>
+        or <literal>"local wins"</literal>
+        or a list that contains a command to run.
+        By default, an error message is printed.
+      '';
+    };
+
+    partialSync = mkOption {
+      type = types.nullOr (types.enum [ "revert" "error" "ignore" ]);
+      default = null;
+      description = ''
+        What should happen if synchronization in one direction
+        is impossible due to one storage being read-only.
+        Defaults to <literal>"revert"</literal>.</para>
+        <para>See
+        <link xlink:href="https://vdirsyncer.pimutils.org/en/stable/config.html#pair-section"/>
+        for more information.
+      '';
+    };
+
+    metadata = mkOption {
+      type = types.listOf types.str;
+      default = [];
+      example = [ "color" "displayname" ];
+      description = ''
+        Metadata keys that should be synchronized
+        when vdirsyncer metasync is executed.
+      '';
+    };
+
+    timeRange = mkOption {
+      type = types.nullOr (types.submodule {
+        options = {
+          start = mkOption {
+            type = types.str;
+            description = "Start of time range to show.";
+          };
+
+          end = mkOption {
+            type = types.str;
+            description = "End of time range to show.";
+          };
+        };
+      });
+
+      default = null;
+      description = ''
+        A time range to synchronize. start and end
+        can be any Python expression that returns
+        a <literal>datetime.datetime</literal> object.
+      '';
+      example = {
+        start = "datetime.now() - timedelta(days=365)";
+        end = "datetime.now() + timedelta(days=365)";
+      };
+    };
+
+    itemTypes = mkOption {
+      type = types.nullOr (types.listOf types.str);
+      default = null;
+      description = ''
+        Kinds of items to show. The default is to
+        show everything. This depends on particular
+        features of the server, the results are not
+        validated.
+      '';
+    };
+
+    verify = mkOption {
+      type = types.nullOr types.bool;
+      default = null;
+      description = "Verify SSL certificate.";
+    };
+
+    verifyFingerprint = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = ''
+        Optional. SHA1 or MD5 fingerprint of the expected server certificate.</para>
+
+        <para>See
+        <link xlink:href="https://vdirsyncer.pimutils.org/en/stable/ssl-tutorial.html#ssl-tutorial"/>
+        for more information.
+      '';
+    };
+
+    auth = mkOption {
+      type = types.nullOr (types.enum ["basic" "digest" "guess"]);
+      default = null;
+      description = ''
+        Authentication settings. The default is <literal>"basic"</literal>.
+      '';
+    };
+
+    authCert = mkOption {
+      type = types.nullOr (types.either types.str (types.listOf types.str));
+      default = null;
+      description = ''
+      Either a path to a certificate with a client certificate
+      and the key or a list of paths to the files with them.
+     '';
+    };
+
+    userAgent = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = ''
+        The user agent to report to the server.
+        Defaults to <literal>"vdirsyncer"</literal>.
+      '';
+    };
+
+    postHook = mkOption {
+      type = types.lines;
+      default = "";
+      description = ''
+        Command to call for each item creation and modification.
+        The command will be called with the path of the new/updated
+        file.
+      '';
+    };
+
+    ## Options for google storages
+
+    tokenFile = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = ''
+        A file path where access tokens
+        are stored.
+      '';
+    };
+
+    clientIdCommand = mkOption {
+      type = types.nullOr (types.listOf types.str);
+      default = null;
+      example = [ "pass" "client_id" ];
+      description = ''
+        A command that prints the OAuth credentials to standard
+        output.
+
+        OAuth credentials, obtained from the Google API Manager.</para>
+
+        <para> See
+        <link xlink:href="https://vdirsyncer.pimutils.org/en/stable/config.html#google"/>
+        for more information.
+      '';
+    };
+
+    clientSecretCommand = mkOption {
+      type = types.nullOr (types.listOf types.str);
+      default = null;
+      example = [ "pass" "client_secret" ];
+      description = ''
+        A command that prints the OAuth credentials to standard
+        output.
+
+        OAuth credentials, obtained from the Google API Manager.</para>
+
+        <para> See
+        <link xlink:href="https://vdirsyncer.pimutils.org/en/stable/config.html#google"/>
+        for more information.
+      '';
+    };
+  };
+}

modules/programs/vdirsyncer.nix

@@ -0,0 +1,245 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.programs.vdirsyncer;
+
+  vdirsyncerCalendarAccounts =
+    filterAttrs (_: v: v.vdirsyncer.enable)
+    (mapAttrs' (n: v: nameValuePair ("calendar_" + n) v)  config.accounts.calendar.accounts);
+
+  vdirsyncerContactAccounts =
+    filterAttrs (_: v: v.vdirsyncer.enable)
+    (mapAttrs' (n: v: nameValuePair ("contacts_" + n) v)  config.accounts.contact.accounts);
+
+  vdirsyncerAccounts = vdirsyncerCalendarAccounts // vdirsyncerContactAccounts;
+
+  wrap = s: ''"${s}"'';
+
+  listString = l: ''[${concatStringsSep ", " l}]'';
+
+  boolString = b: if b then "true" else "false";
+
+  localStorage = a:
+  filterAttrs (_: v: v != null)
+  ((getAttrs [ "type" "fileExt" "encoding" ] a.local) // {
+    path = a.local.path;
+    postHook =
+      pkgs.writeShellScriptBin "post-hook" a.vdirsyncer.postHook
+      + "/bin/post-hook";
+  });
+
+  remoteStorage = a:
+  filterAttrs (_: v: v != null)
+  ((getAttrs [
+    "type"
+    "url"
+    "userNameCommand"
+    "passwordCommand"
+  ] a.remote) //
+  (if a.vdirsyncer == null
+   then {}
+   else getAttrs [
+    "itemTypes"
+    "verify"
+    "verifyFingerprint"
+    "auth"
+    "authCert"
+    "userAgent"
+    "tokenFile"
+    "clientIdCommand"
+    "clientSecretCommand"
+    "timeRange"
+   ] a.vdirsyncer));
+
+  pair = a:
+  with a.vdirsyncer;
+  filterAttrs (k: v: k == "collections" || (v != null && v != []))
+  (getAttrs [ "collections" "conflictResolution" "metadata" "partialSync" ] a.vdirsyncer);
+
+  pairs = mapAttrs (_: v: pair v) vdirsyncerAccounts;
+  localStorages = mapAttrs (_: v: localStorage v) vdirsyncerAccounts;
+  remoteStorages = mapAttrs (_: v: remoteStorage v) vdirsyncerAccounts;
+
+  optionString = n: v:
+  if (n == "type") then ''type = "${v}"''
+  else if (n == "path") then ''path = "${v}"''
+  else if (n == "fileExt") then ''fileext = "${v}"''
+  else if (n == "encoding") then ''encoding = "${v}"''
+  else if (n == "postHook") then ''post_hook = "${v}"''
+  else if (n == "url") then ''url = "${v}"''
+  else if (n == "timeRange") then ''
+    start_date = "${v.start}"
+    end_date = "${v.end}"''
+  else if (n == "itemTypes") then ''
+    item_types = ${listString (map wrap v)}''
+  else if (n == "userName") then ''username = "${v}"''
+  else if (n == "userNameCommand") then ''
+    username.fetch = ${listString (map wrap (["command"] ++ v))}''
+  else if (n == "password") then ''password = "${v}"''
+  else if (n == "passwordCommand") then ''
+    password.fetch = ${listString (map wrap (["command"] ++ v))}''
+  else if (n == "passwordPrompt") then ''
+    password.fetch = ["prompt", "${v}"]''
+  else if (n == "verify") then ''
+    verify = ${if v then "true" else "false"}''
+  else if (n == "verifyFingerprint") then ''
+    verify_fingerprint = "${v}"''
+  else if (n == "auth") then ''auth = "${v}"''
+  else if (n == "authCert" && isString(v)) then ''
+    auth_cert = "${v}"''
+  else if (n == "authCert") then ''
+    auth_cert = ${listString (map wrap v)}''
+  else if (n == "userAgent") then ''useragent = "${v}"''
+  else if (n == "tokenFile") then ''token_file = "${v}"''
+  else if (n == "clientId") then ''client_id = "${v}"''
+  else if (n == "clientIdCommand") then ''
+    client_id.fetch = ${listString (map wrap (["command"] ++ v))}''
+  else if (n == "clientSecret") then ''client_secret = "${v}"''
+  else if (n == "clientSecretCommand") then ''
+    client_secret.fetch = ${listString (map wrap (["command"] ++ v))}''
+  else if (n == "metadata") then ''metadata = ${listString (map wrap v)}''
+  else if (n == "partialSync") then ''partial_sync = "${v}"''
+  else if (n == "collections") then
+    let
+           contents = map (c: if (isString c)
+                          then ''"${c}"''
+                          else mkList (map wrapString c)) v;
+    in ''collections = ${if ((isNull v) || v == []) then "null" else listString contents}''
+  else if (n == "conflictResolution") then
+    if v == "remote wins"
+      then ''conflict_resolution = "a wins"''
+      else if v == "local wins"
+      then ''conflict_resolution = "b wins"''
+      else ''conflict_resolution = ${mkList (map wrapString (["command"] ++ v))}''
+  else throw "Unrecognized option: ${n}";
+
+  attrsString = a: concatStringsSep "\n" (mapAttrsToList optionString a);
+
+  pairString = n: v: ''
+    [pair ${n}]
+    a = "${n}_remote"
+    b = "${n}_local"
+    ${attrsString v}
+  '';
+
+  configFile = pkgs.writeText "config" ''
+    [general]
+    status_path = "${cfg.statusPath}"
+
+    ### Pairs
+
+    ${concatStringsSep "\n" (mapAttrsToList pairString pairs)}
+
+    ### Local storages
+
+    ${concatStringsSep "\n\n" (mapAttrsToList (n: v: "[storage ${n}_local]" + "\n" + attrsString v) localStorages)}
+
+    ### Remote storages
+
+    ${concatStringsSep "\n\n" (mapAttrsToList (n: v: "[storage ${n}_remote]" + "\n" + attrsString v) remoteStorages)}
+  '';
+in
+
+{
+  options = {
+    programs.vdirsyncer = {
+      enable = mkEnableOption "vdirsyncer";
+
+      package = mkOption {
+        type = types.package;
+        default = pkgs.vdirsyncer;
+        defaultText = "pkgs.vdirsyncer";
+        description = ''
+          vdirsyncer package to use.
+        '';
+      };
+
+      statusPath = mkOption {
+        type = types.str;
+        default = "${config.xdg.dataHome}/vdirsyncer/status";
+        defaultText = "$XDG_DATA_HOME/vdirsyncer/status";
+        description = ''
+          A directory where vdirsyncer will store some additional data for the next sync.
+          </para>
+
+          <para>For more information, see
+          <link xlink:href="https://vdirsyncer.pimutils.org/en/stable/config.html#general-section"/>
+        '';
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    assertions = let
+
+      requiredOptions = t:
+      if (t == "caldav" || t == "carddav" || t == "http") then [ "url" ]
+      else if (t == "filesystem") then [ "path" "fileExt" ]
+      else if (t == "singlefile") then [ "path" ]
+      else if (t == "google_calendar" || t == "google_contacts") then
+      [ "tokenFile" "clientId" "clientSecret"]
+      else throw "Unrecognized storage type: ${t}";
+
+      allowedOptions = let
+        remoteOptions = [
+          "userName"
+          "userNameCommand"
+          "password"
+          "passwordCommand"
+          "passwordPrompt"
+          "verify"
+          "verifyFingerprint"
+          "auth"
+          "authCert"
+          "userAgent"
+        ];
+      in t:
+      if (t == "caldav")
+        then [ "timeRange" "itemTypes" ] ++ remoteOptions
+      else if (t == "carddav" || t == "http")
+        then remoteOptions
+      else if (t == "filesystem")
+        then [ "fileExt" "encoding" "postHook" ]
+      else if (t == "singlefile")
+        then [ "encoding" ]
+      else if (t == "google_calendar") then
+        [ "timeRange" "itemTypes" "clientIdCommand" "clientSecretCommand" ]
+      else if (t == "google_contacts") then [ "clientIdCommand" "clientSecretCommand" ]
+      else throw "Unrecognized storage type: ${t}";
+
+      assertStorage = n: v:
+      let
+        allowed = allowedOptions v.type ++ (requiredOptions v.type);
+      in
+        mapAttrsToList (
+        a: v': [
+          {
+            assertion = (elem a allowed);
+            message = ''
+              Storage ${n} is of type ${v.type}. Option
+              ${a} is not allowed for this type.
+            '';
+          }
+        ] ++
+        (let required = filter (a: !hasAttr "${a}Command" v) (requiredOptions v.type);
+         in map (a: [{
+                assertion = hasAttr a v;
+                message = ''
+                  Storage ${n} is of type ${v.type}, but required
+                  option ${a} is not set.
+                '';
+              }]) required)
+      ) (removeAttrs v ["type" "_module"]);
+
+      storageAssertions = flatten (mapAttrsToList assertStorage localStorages)
+                          ++ flatten (mapAttrsToList assertStorage remoteStorages);
+
+
+    in storageAssertions;
+    home.packages = [ cfg.package ];
+    xdg.configFile."vdirsyncer/config".source = configFile;
+  };
+}

modules/services/vdirsyncer.nix

@@ -0,0 +1,90 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.services.vdirsyncer;
+
+  vdirsyncerOptions =
+    [ ] ++ optional (cfg.verbosity != null) "--verbosity ${cfg.verbosity}"
+      ++ optional (cfg.configFile != null) "--config ${cfg.configFile}";
+
+in
+
+{
+  meta.maintainers = [ maintainers.pjones ];
+
+  options.services.vdirsyncer = {
+    enable = mkEnableOption "vdirsyncer";
+
+    package = mkOption {
+      type = types.package;
+      default = pkgs.vdirsyncer;
+      defaultText = "pkgs.vdirsyncer";
+      example = literalExample "pkgs.vdirsyncer";
+      description = "The package to use for the vdirsyncer binary.";
+    };
+
+    frequency = mkOption {
+      type = types.str;
+      default = "*:0/5";
+      description = ''
+        How often to run vdirsyncer.  This value is passed to the systemd
+        timer configuration as the onCalendar option.  See
+        <citerefentry>
+          <refentrytitle>systemd.time</refentrytitle>
+          <manvolnum>7</manvolnum>
+        </citerefentry>
+        for more information about the format.
+      '';
+    };
+
+    verbosity = mkOption {
+      type = types.nullOr (types.enum [ "CRITICAL" "ERROR" "WARNING" "INFO" "DEBUG"]);
+      default = null;
+      description = ''
+        Whether vdirsyncer should produce verbose output.
+      '';
+    };
+
+    configFile = mkOption {
+      type = types.nullOr types.path;
+      default = null;
+      description = ''
+        Optional configuration file to link to use instead of
+        the default file (<filename>$XDG_CONFIG_HOME/vdirsyncer/config</filename>).
+      '';
+    };
+  };
+
+  config = mkIf cfg.enable {
+    systemd.user.services.vdirsyncer = {
+      Unit = {
+        Description = "vdirsyncer calendar&contacts synchronization";
+        PartOf = [ "network-online.target" ];
+      };
+
+      Service = {
+        Type = "oneshot";
+        # TODO `vdirsyncer discover`
+        ExecStart = "${cfg.package}/bin/vdirsyncer sync ${concatStringsSep " " vdirsyncerOptions}";
+      };
+    };
+
+    systemd.user.timers.vdirsyncer = {
+      Unit = {
+        Description = "vdirsyncer calendar&contacts synchronization";
+      };
+
+      Timer = {
+        OnCalendar = cfg.frequency;
+        Unit = "vdirsyncer.service";
+      };
+
+      Install = {
+        WantedBy = [ "timers.target" ];
+      };
+    };
+  };
+}