Add withElement and updateAttr functions to the DOM library.

See the comments for each function in lib/dom.ls for details.

examples/scroller/scroller.ls

@@ -4,18 +4,9 @@ import io;
 import std;
 import dom;
 
-update :: DOMElement -> Int -> IO ();
-update e t = do {
-    txt <- getAttr e "innerHTML";
-    setAttr e "innerHTML" (tail txt ++ [head txt]);
-    setTimeout (update e t) t;
+update t e = do {
+    updateAttr e "innerHTML" (\txt -> tail txt ++ [head txt]);
+    setTimeout (update t e) t;
   };
 
-scroll :: String -> Int -> IO ();
-scroll id t = do {
-    elem <- getElementById id;
-    case elem of
-      (Just e) -> update e t;
-      _        -> error ("No such element: " ++ id);
-      ;
-  };
+scroll id t = withElement id (update t);

lib/dom.ls

@@ -1,7 +1,7 @@
 -- This is basically a wrapper for the more useful DOM manipulation functions.
 export getElementById, parentNode, getAttr, setAttr, firstChild, lastChild,
        nextSibling, prevSibling, childNodes, siblings, domElemValid,
-       DOMElement;
+       DOMElement, withElement, updateAttr;
 import io;
 import std;
 
@@ -19,6 +19,17 @@ getElementById id = do {
       if valid then return (Just x) else return Nothing;
   };
 
+-- Performs an IO action using the element with the given ID.
+-- If the element doesn't exist, Nothing is returned.
+withElement :: String -> (DOMElement -> IO a) -> IO (Maybe a);
+withElement id m = do {
+    e <- getElementById id;
+    case e of
+      (Just e) -> do {x <- m e; return (Just x);};
+      _        -> return Nothing;
+      ;
+  };
+
 -- Reads any DOM node text attribute.
 getAttr :: DOMElement -> String -> IO String;
 getAttr e attr = _jsfun "(function(e,s) {return e[s];})" 2 e attr;
@@ -30,6 +41,15 @@ setAttr e attr s = do {
     return ();
   };
 
+-- Applies the given function to the value of the given attribute of the given
+-- element and updates that attribute with the result. If the element doesn't
+-- exist, the function does nothing.
+updateAttr :: DOMElement -> String -> (String -> String) -> IO ();
+updateAttr e a f = do {
+    s <- getAttr e a;
+    setAttr e a (f s);
+  };
+
 -- Wrapper for <element>.parentNode
 parentNode :: DOMElement -> IO DOMElement;
 parentNode e = do {