Make DOMObject a little more type safe.

src/Haste/Reactive/DOM.hs

@@ -1,5 +1,4 @@
-{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, UndecidableInstances #-}
-{-# OPTIONS_GHC -fno-warn-unused-binds #-}
+{-# LANGUAGE GADTs, FlexibleInstances, MultiParamTypeClasses #-}
 -- | DOM events and utilities for the Haste reactive library.
 module Haste.Reactive.DOM (valueOf, valueAt, DOMObject, domObj) where
 import FRP.Fursuit
@@ -8,22 +7,20 @@ import Data.String
 
 -- | Represents a DOM object; a DOM object consists of an object ID,
 --   corresponding to the object's ID attribute in the HTML, and an attribute.
-data DOMObject = D {
-    d_object :: String,
-    d_attr   :: String
-  }
+data DOMObject a where
+  D :: (Showable a, Readable a) => ElemID -> PropID -> DOMObject a
 
 -- | Create a 'DOMObject' from a string describing the object. For example,
 --   domObj "myobject.value" corresponds to the value attribute of the object
 --   with the ID "myobject".
-domObj :: String -> DOMObject
+domObj :: (Showable a, Readable a) => String -> DOMObject a
 domObj str =
   case span (/= '.') str of
     ([], _)     -> error "domObj: No object ID given!"
     (_, [])     -> error "domObj: No object attribute given!"
     (obj, attr) -> D obj (tail attr)
 
-instance IsString DOMObject where
+instance (Showable a, Readable a) => IsString (DOMObject a) where
   fromString = domObj
 
 -- | The value property of the given element, updated whenever an onchange
@@ -49,5 +46,5 @@ valueAt e evt = do
      then error $ "Not found: " ++ e
      else return sig
 
-instance Showable a => Sink DOMObject a where
+instance Sink (DOMObject a) a where
   (D obj attr) << val = sink (setProp obj attr . toStr) val