add configuration option

nixos-module.nix

@@ -3,6 +3,52 @@
 let
   yammat = import ./. { inherit pkgs; };
   cfg = config.services.yammat;
+  defaultConfig = ''
+    # Values formatted like "_env:ENV_VAR_NAME:default_value" can be overridden by the specified environment variable.
+    # See https://github.com/yesodweb/yesod/wiki/Configuration#overriding-configuration-values-with-environment-variables
+
+    static-dir:     "_env:STATIC_DIR:static"
+    host:           "_env:HOST:*4" # any IPv4 host
+    port:           "_env:PORT:3000"
+    approot:        "_env:APPROOT:http://localhost:3000"
+    ip-from-header: "_env:IP_FROM_HEADER:false"
+
+    # Optional values with the following production defaults.
+    # In development, they default to the inverse.
+    #
+    # development: false
+    # detailed-logging: false
+    # should-log-all: false
+    # reload-templates: false
+    # mutable-static: false
+    # skip-combining: false
+
+    database:
+      user:     "_env:PGUSER:yammat"
+      password: "_env:PGPASS:yammat"
+      host:     "_env:PGHOST:localhost"
+      port:     "_env:PGPORT:5432"
+      database: "_env:PGDATABASE:yammat"
+      poolsize: "_env:PGPOOLSIZE:10"
+
+    email:
+    - "nek0@momen"
+    currency: "€"
+    cash_charge: 50
+
+    copyright: "Powered by YAMMAT"
+    copyright_link: "https://github.com/nek0/yammat"
+
+    block_users: false
+
+    sendmail-location: "/usr/sbin/sendmail"
+    from-mail: "matemat@matemat.hq.c3d2.de"
+
+    # optional administrative credentials.
+    # credentials:
+    #   login: "admin"
+    #   password: "password"
+  '';
 in
 {
   options.services.yammat = with lib; {
@@ -18,6 +64,14 @@ in
       default = "yammat";
       description = "System group to run Yammat";
     };
+
+    config = mkOption {
+      type = types.lines
+      example = ''
+        ${defaultConfig}
+      '';
+      description = "Configuration for Yammat";
+    };
   };
 
   config = lib.mkIf cfg.enable {
@@ -55,5 +109,10 @@ in
         ExecStart = "${yammat}/bin/yammat";
       };
     };
+
+    environment.etc."yammat.yml" = {
+      enable = true;
+      text = cfg.config;
+    };
   };
 }