Simplify $NIX_PATH evaluation.

configuration.nix

@@ -1,10 +1,16 @@
 { lib, ... }:
 
+with builtins;
 with lib;
 
 let
+  fn = import ./fn.nix { inherit lib; };
   # hostName can be set with -I hostName=$HOSTNAME, defaults to the contents of /secret/hostName
-  hostName = (findFirst (elem: elem.prefix == "hostName") { path = (fileContents /secret/hostName); } builtins.nixPath).path;
+  hostName = let
+    hostVal = (tryEval <hostName>).value;
+  in fn.ifelse (hostVal != false)
+    hostVal
+    (fileContents /secret/hostName);
   machinePath = (builtins.toPath ( ./machines + ("/" + hostName)));
   machineConf = machinePath + "/configuration.nix";
   machineOpts = machinePath + "/options.nix";

options/machine.nix

@@ -68,7 +68,8 @@ in {
     };
     secretPath = mkOption {
       type = types.str;
-      default = (findFirst (elem: elem.prefix == "secretPath") { path = "/secret"; } builtins.nixPath).path;
+      default = let sPathVal = (tryEval <secretPath>).value;
+                in fn.ifelse (sPathVal != false) sPathVal "/secret";
       description = ''
         Path to you systems secret folder containing files with sensitive information.
       '';