diff --git a/timer.lua b/timer.lua
index fd1b163..7c6bcaf 100644
--- a/timer.lua
+++ b/timer.lua
@@ -27,29 +27,36 @@ THE SOFTWARE.
 local Timer = {}
 Timer.__index = Timer
 
+local function _nothing_() end
+
 local function new()
 	return setmetatable({functions = {}}, Timer)
 end
 
 function Timer:update(dt)
 	local to_remove = {}
-	for func, delay in pairs(self.functions) do
+	for handle, delay in pairs(self.functions) do
 		delay = delay - dt
 		if delay <= 0 then
-			to_remove[#to_remove+1] = func
+			to_remove[#to_remove+1] = handle
 		end
-		self.functions[func] = delay
+		self.functions[handle] = delay
+		handle.func(dt, delay)
 	end
-	for _,func in ipairs(to_remove) do
-		self.functions[func] = nil
-		func(func)
+	for _,handle in ipairs(to_remove) do
+		self.functions[handle] = nil
+		handle.after(handle.after)
 	end
 end
 
+function Timer:do_for(delay, func, after)
+	local handle = {func = func, after = after}
+	self.functions[handle] = delay
+	return handle
+end
+
 function Timer:add(delay, func)
-	assert(not self.functions[func], "Function already scheduled to run.")
-	self.functions[func] = delay
-	return func
+	return self:do_for(delay, _nothing_, func)
 end
 
 function Timer:addPeriodic(delay, func, count)
@@ -64,30 +71,14 @@ function Timer:addPeriodic(delay, func, count)
 	end)
 end
 
-function Timer:cancel(func)
-	self.functions[func] = nil
+function Timer:cancel(handle)
+	self.functions[handle] = nil
 end
 
 function Timer:clear()
 	self.functions = {}
 end
 
-local function Interpolator(length, func)
-	local t = 0
-	return function(dt, ...)
-		t = t + dt
-		return t <= length and func((t-dt)/length, ...) or nil
-	end
-end
-
-local function Oscillator(length, func)
-	local t = 0
-	return function(dt, ...)
-		t = (t + dt) % length
-		return func(t/length, ...)
-	end
-end
-
 -- default timer
 local default = new()
 
@@ -95,10 +86,9 @@ local default = new()
 return setmetatable({
 	new          = new,
 	update       = function(...) return default:update(...) end,
+	do_for       = function(...) return default:do_for(...) end,
 	add          = function(...) return default:add(...) end,
 	addPeriodic  = function(...) return default:addPeriodic(...) end,
 	cancel       = function(...) return default:cancel(...) end,
 	clear        = function(...) return default:clear(...) end,
-	Interpolator = Interpolator,
-	Oscillator   = Oscillator
 }, {__call = new})