Proposal for lazyif functionallity in elm-virtualdom

lazyIf-virtualdom.diff

diff --git a/src/Native/VirtualDom.js b/src/Native/VirtualDom.js
index efb9ce7..45ec52e 100644
--- a/src/Native/VirtualDom.js
+++ b/src/Native/VirtualDom.js
@@ -1407,13 +1407,18 @@ Elm.Native.VirtualDom.make = function(elm)
 		return newNode;
 	}
 
+	function predicate(a, b)
+	{
+		return a === b;
+	}
+
 	function lazyRef(fn, a)
 	{
 		function thunk()
 		{
 			return fn(a);
 		}
-		return new Thunk(fn, [a], thunk);
+		return new Thunk(fn, [a], predicate, thunk);
 	}
 
 	function lazyRef2(fn, a, b)
@@ -1422,7 +1427,7 @@ Elm.Native.VirtualDom.make = function(elm)
 		{
 			return A2(fn, a, b);
 		}
-		return new Thunk(fn, [a,b], thunk);
+		return new Thunk(fn, [a,b], predicate, thunk);
 	}
 
 	function lazyRef3(fn, a, b, c)
@@ -1431,10 +1436,20 @@ Elm.Native.VirtualDom.make = function(elm)
 		{
 			return A3(fn, a, b, c);
 		}
-		return new Thunk(fn, [a,b,c], thunk);
+		return new Thunk(fn, [a,b,c], predicate, thunk);
+	}
+
+	function lazyPredicateRef(fn, p, a)
+	{
+		function thunk()
+		{
+			return fn(a);
+		}
+
+		return new Thunk(fn, [a], p, thunk)
 	}
 
-	function Thunk(fn, args, thunk)
+	function Thunk(fn, args, predicate, thunk)
 	{
 		/* public (used by VirtualDom.js) */
 		this.vnode = null;
@@ -1444,6 +1459,7 @@ Elm.Native.VirtualDom.make = function(elm)
 		this.fn = fn;
 		this.args = args;
 		this.thunk = thunk;
+		this.predicate = predicate;
 	}
 
 	Thunk.prototype.type = "Thunk";
@@ -1462,7 +1478,7 @@ Elm.Native.VirtualDom.make = function(elm)
 
 		for (var i = cargs.length; i--; )
 		{
-			if (cargs[i] !== pargs[i])
+			if (!previous.predicate(cargs[i], pargs[i]))
 			{
 				return true;
 			}
@@ -1495,6 +1511,8 @@ Elm.Native.VirtualDom.make = function(elm)
 		lazy2: F3(lazyRef2),
 		lazy3: F4(lazyRef3),
 
+		lazyIf: F3(lazyPredicateRef),
+
 		toElement: F3(toElement),
 		fromElement: fromElement,
 
diff --git a/src/VirtualDom.elm b/src/VirtualDom.elm
index e685fac..62a52ea 100644
--- a/src/VirtualDom.elm
+++ b/src/VirtualDom.elm
@@ -5,6 +5,7 @@ module VirtualDom
     , Property, property, attribute
     , on, onWithOptions, Options, defaultOptions
     , lazy, lazy2, lazy3
+    , lazyIf, lazyIf2, lazyIf3
     ) where
 
 {-| API to the core diffing algorithm. Can serve as a foundation for libraries
@@ -221,3 +222,18 @@ lazy2 =
 lazy3 : (a -> b -> c -> Node) -> a -> b -> c -> Node
 lazy3 =
     Native.VirtualDom.lazy3
+
+
+lazyIf : (a -> Node) -> (a -> a -> Bool) -> a -> Node
+lazyIf =
+    Native.VirtualDom.lazyIf
+
+
+lazyIf2 : ((a, b) -> Node) -> ((a, b) -> (a, b) -> Bool) -> (a, b) -> Node
+lazyIf2 =
+    Native.VirtualDom.lazyIf2
+
+
+lazyIf3 : ((a, b, c) -> Node) -> ((a, b, c) -> (a, b, c) -> Bool) -> (a, b, c) -> Node
+lazyIf3 =
+    Native.VirtualDom.lazyIf3
diff --git a/src/wrapper.js b/src/wrapper.js
index 47de45c..886306b 100644
--- a/src/wrapper.js
+++ b/src/wrapper.js
@@ -200,13 +200,18 @@ Elm.Native.VirtualDom.make = function(elm)
 		return newNode;
 	}
 
+	function predicate(a, b)
+	{
+		return a === b;
+	}
+
 	function lazyRef(fn, a)
 	{
 		function thunk()
 		{
 			return fn(a);
 		}
-		return new Thunk(fn, [a], thunk);
+		return new Thunk(fn, [a], predicate, thunk);
 	}
 
 	function lazyRef2(fn, a, b)
@@ -215,7 +220,7 @@ Elm.Native.VirtualDom.make = function(elm)
 		{
 			return A2(fn, a, b);
 		}
-		return new Thunk(fn, [a,b], thunk);
+		return new Thunk(fn, [a,b], predicate, thunk);
 	}
 
 	function lazyRef3(fn, a, b, c)
@@ -224,10 +229,20 @@ Elm.Native.VirtualDom.make = function(elm)
 		{
 			return A3(fn, a, b, c);
 		}
-		return new Thunk(fn, [a,b,c], thunk);
+		return new Thunk(fn, [a,b,c], predicate, thunk);
+	}
+
+	function lazyPredicateRef(fn, p, a)
+	{
+		function thunk()
+		{
+			return fn(a);
+		}
+
+		return new Thunk(fn, [a], p, thunk)
 	}
 
-	function Thunk(fn, args, thunk)
+	function Thunk(fn, args, predicate, thunk)
 	{
 		/* public (used by VirtualDom.js) */
 		this.vnode = null;
@@ -237,6 +252,7 @@ Elm.Native.VirtualDom.make = function(elm)
 		this.fn = fn;
 		this.args = args;
 		this.thunk = thunk;
+		this.predicate = predicate;
 	}
 
 	Thunk.prototype.type = "Thunk";
@@ -255,7 +271,7 @@ Elm.Native.VirtualDom.make = function(elm)
 
 		for (var i = cargs.length; i--; )
 		{
-			if (cargs[i] !== pargs[i])
+			if (!previous.predicate(cargs[i], pargs[i]))
 			{
 				return true;
 			}
@@ -288,6 +304,8 @@ Elm.Native.VirtualDom.make = function(elm)
 		lazy2: F3(lazyRef2),
 		lazy3: F4(lazyRef3),
 
+		lazyIf: F3(lazyPredicateRef),
+
 		toElement: F3(toElement),
 		fromElement: fromElement,