Use null for erased values to aid serialization

port/Node.newt

@@ -8,12 +8,12 @@ pfunc readFile uses (fs MkIORes Left Right) : (fn : String) -> IO (Either String
   let result
   try {
     let content = fs.readFileSync(fn, 'utf8')
-    result = Right(undefined, undefined, content)
+    result = Right(null, null, content)
   } catch (e) {
     let err = ""+e
-    result = Left(undefined, undefined, e)
+    result = Left(null, null, e)
   }
-  return MkIORes(undefined, result, w)
+  return MkIORes(null, result, w)
 }`
 
 -- I wonder if I should automatically `uses` the constructors in the types
@@ -21,12 +21,12 @@ pfunc writeFile uses (fs MkIORes MkUnit) : String → String → IO (Either Stri
   let result
   try {
     fs.writeFileSync(fn, content, 'utf8')
-    result = Right(undefined, undefined, MkUnit)
+    result = Right(null, null, MkUnit)
   } catch (e) {
       let err = ""+e
-      result = Left(undefined, undefined, e)
+      result = Left(null, null, e)
   }
-  return MkIORes(undefined, result, w)
+  return MkIORes(null, result, w)
 }`
 
 -- maybe System.exit or something, like the original putStrLn msg >> exitFailure