$sum and $val also supports bind.

src/helpers.coffee

@@ -22,12 +22,41 @@ $watch = (collection, {
   if: cond
 
   bind
+
+  # Initial value.
+  init
 }, fn) ->
   # The default value is simply the value of the item.
   val ?= -> @val
 
+  watcher = {
+    # Method allowing the cleanup when the helper is no longer used.
+    cleanUp: -> # TODO: noop for now.
+
+    # Keys of items using the current helper.
+    consumers: Object.create null
+
+    generator: ->
+      {key} = this
+
+      # Register this item has a consumer.
+      watcher.consumers[@key] = true
+
+      # Returns the value for this item if any or the common value.
+      values = watcher.values
+      if key of values
+        values["$#{key}"]
+      else
+        values.common
+
+    # Current values.
+    values: Object.create null
+  }
+  watcher.values.common = init
+
   process = (event, items) ->
-    values = []
+    # Values are grouped by namespace.
+    valuesByNamespace = Object.create null
 
     $_.each items, (item) ->
       return unless not cond? or cond.call item
@@ -40,10 +69,29 @@ $watch = (collection, {
       else
         'common'
 
-      values.push [event, value, namespace]
+      (valuesByNamespace[namespace] ?= []).push value
 
-    # If something has been processed, call `fn`.
-    fn values unless $_.isEmpty values
+    # For each namespace.
+    for namespace, values of valuesByNamespace
+
+      # Updates the value.
+      value = watcher.values[namespace]
+      ctx = {
+        value: if value is undefined then init else value
+      }
+      changed = if event is 'enter'
+        fn.call ctx, values, []
+      else
+        fn.call ctx, [], values
+
+      # Notifies watchers unless it is known the value has not
+      # changed.
+      unless changed is false
+        watcher.values[namespace] = ctx.value
+        if namespace is 'common'
+          collection.touch watcher.consumers
+        else
+          collection.touch (namespace.substr 1)
 
   processOne = (event, item) ->
     process event, [item]
@@ -70,6 +118,9 @@ $watch = (collection, {
 
     collection.on 'any', updateMultiple
 
+  # Returns the watcher object.
+  watcher
+
 #=====================================================================
 
 # Creates a set of value from various items.
@@ -77,104 +128,67 @@ $set = (options) ->
   # Contrary to other helpers, the default value is the key.
   options.val ?= -> @key
 
-  # Keys of items using this value.
-  users = Object.create null
+  options.init = []
 
-  #
-  sets = {
-    common: []
-  }
+  watcher = $watch this, options, (entered, exited) ->
+    changed = false
 
-  $watch this, options, (values) =>
-    # Marks changed namespaces to avoid unnecessary updates.
-    changed = {}
+    for value in entered
+      if @value.indexOf value is -1
+        @value.push value
+        changed = true
 
-    $_.each values, ([event, value, namespace]) ->
-      set = (sets[namespace] ?= [])
-      if event is 'enter'
-        return unless set.indexOf value is -1
-        set.push value
-        changed[namespace] = true
-      else
-        changed[namespace] = true if $removeValue set, value
+    for value in exited
+      changed = true if $removeValue @value, value
 
-    if changed.common
-      @touch users
-      delete changed.common
-    for key in changed
-      @touch (key.substr 1) # Remove the leading “$”.
+    changed
 
-  # This function both allows users to register to this set and gives
-  # them the current value.
-  ->
-    # Registers this item as a consumer.
-    users[@key] = true
-
-    # Returns its dedicated value or the common one.
-    sets["$#{@key}"] ? sets.common
+  watcher.generator
 
 #---------------------------------------------------------------------
 
 $sum = (options) ->
-  # Keys of items using this value.
-  users = Object.create null
+  options.init ?= 0
 
-  # The current sum.
-  if options.init
-    sum = options.init
-    delete options.init
-  else
-    sum = 0
+  watcher = $watch this, options, (entered, exited) ->
+    prev = @value
 
-  $watch this, options, (values) =>
-    prev = sum
+    @value += value for value in entered
+    @value -= value for value in exited
 
-    $_.each values, ([event, value]) ->
-      sum += if event is 'enter' then value else -value
+    @value isnt prev
 
-    @touch users if sum isnt prev
-
-  # This function both allows users to register to this sum and gives
-  # them the current value.
-  ->
-    users[@key] = true
-    sum
+  watcher.generator
 
 #---------------------------------------------------------------------
 
 # Uses a value from another item.
 #
-# Important note: This helper is badly specified when binding to
-# multiple items.
+# Important note: Behavior is not specified when binding to multiple
+# items.
 $val = (options) ->
-  # Keys of items using this value.
-  users = Object.create null
-
   # The default value.
   def = options.default
   delete options.default
 
+  options.init ?= def
+
   # Should the last value be kept instead of returning to the default
   # value when no items are available!
   keepLast = !!options.keepLast
   delete options.keepLast
 
-  # The current value.
-  value = def
+  watcher = $watch this, options, (entered, exited) ->
+    prev = @value
 
-  $watch this, options, (values) =>
-    prev = value
+    if not $_.isEmpty entered
+      @value = entered[0]
+    else
+      @value = def unless keepLast
 
-    for [event, value] in values
-      break if event is 'enter'
+    @value isnt prev
 
-      value = def unless keepLast
-
-    @touch users if value isnt prev
-
-  ->
-    users[@key] = true
-    value
+  watcher.generator
 
 #=====================================================================