introduce lib.rad-dev.mapAttrsToString, fix mc-router

Signed-off-by: ahuston-0 <aliceghuston@gmail.com>

lib/default.nix

@@ -58,5 +58,20 @@
     # fileList :: Path -> String -> [Path]
     fileList = dir: map (file: dir + "/${file}") (ls dir);
 
+    # reduce an attribute set to a string
+    #
+    # example:
+    # given attrset {host1 = "palatine-hill"; host2 = "jeeves";}
+    # and func (host: hostname: host + " is " + hostname + ", " )
+    # mapAttrsToString would return 'host1 is palatine-hill, host2 is jeeves, '
+    #
+    # args:
+    # func: an function to apply to attrSet to turn each entry into one string
+    # attrSet: an attribute set to reduce
+    #
+    # type:
+    # mapAttrsToString :: AttrSet -> (String -> Any -> String) -> String
+    mapAttrsToString =
+      func: attrSet: (lib.foldl' (cur: next: cur + next) "" (lib.mapAttrsToList func attrSet));
   };
 }