Pragmatic Lua - Error Handling, OOP, Closure and Coroutine

30 Mar 2013

This is a follow-up post of my previous Lua introduction. If you are new to Lua, you might want to read Pragmatic Lua Basics in 30 Minutes first.

Error Handling

Lua does not have try-catch like syntax for error handling. In fact, error handling in Lua is very minimal. It is usually done via error() and pcall(). In most other languages, you can write something like this (I use Lua syntax)

function i_might_throw_exception()
	if something_is_wrong then
		throw Exception("Something is wrong")
	end
	...
	return something
end

try
	local result = i_might_throw_exception()
catch Exception e
	print(e)
end

Well, in real Lua, you have to write following

function i_might_throw_exception()
	if something_is_wrong then
		error("Something is wrong")
	end
	...
	return something
end

local success, result = pcall(i_might_throw_exception)

if not success then
	print(result) -- print out error message
end

pcall will run the function in protected mode. The first return value is always a boolean, indicating whether the function call is successful (i.e. no errors). The return value of the original function will be returned following this boolean flag. If something is wrong, the second return value will be error message.

Object Oriented Programming

Lua does not have built-in object type. Neither does it have class. OOP is a paradigm, you don’t need object in the language to do OOP. In Lua, OOP is usually done via table. In other languages, you can think object as a structure that holds values and has methods. Since function is first-class citizen, a table in Lua can hold values and functions, which is essentially the object.

Some common tasks of OOP in other languages are

class Vehicle
	public speed
	
	function constructor(initial_speed)
		this.speed = initial_speed
	end

	function speed_up(by)
		this.speed = this.speed + by
	end

	function stop()
		this.speed = 0
	end
end

class Car extends Vehicle
	function speed_up(by)
		if this.speed + by > 100 then
			this.speed = 100
		else
			this.speed = this.speed + by
		end
	end
end

local v = new Vehicle(50)
v.speed_up(60)
print(v.speed) -- 110
v.stop()
print(v.speed) -- 0

local c = new Car(50)
c.speed_up(60)
print(c.speed) -- 100
c.stop()
print(c.speed) -- 0

In real Lua, you can write

Vehicle = {}

function Vehicle.new(self, initial_speed)
	local obj = { speed = initial_speed }
  	setmetatable(obj, { __index = Vehicle })
	return obj
end

function Vehicle.speed_up(self, by)
	self.speed = this.speed + by
end

-- this is a syntax sugar, and is equivalent of writing function Vehicle.stop(self)
function Vehicle:stop()
	self.speed = 0
end

function Car:new(initial_speed)
	local obj = { speed = initial_speed }
  	setmetatable(obj, { __index = Vehicle })
	return obj
end

function Car:speed_up(by)
	if self.speed + by > 100 then
		self.speed = 100
	else
		self.speed = self.speed + by
	end
end

local v = Vehicle.new(50)
v.speed_up(v, 60) -- or you can write v:speed_up(60)
print(v.speed) -- 110
v:stop()
print(v.speed) -- 0

local c = Car.new(50)
c:speed_up(60)
print(c.speed) -- 100
c:stop()
print(c.speed) -- 0

Two points to note:

• Constructtion of a new object and inheritance are achieved via setmetatable. metatable is a normal table with special keys that are hooked up to events. In this case, __index is triggered whenever the caller tries to look up some key that does not exist in the table. This works exactly like prototype-based inheritance in JavaScript.
• The syntax Table:method(a, b, c) is a syntax sugar. When defining a method, it is equivalent to Table.method(self, a, b, c), when you call the method, it is equivalent to table.method(table, a, b, c). Essentially, you are passing the table as a parameter in function.

Closure

Closure in Lua is very similar to JavaScript, but the concept of closure is not straightforward. This StackOverflow answer is particularly useful in understanding the concept.

function speed_up()
	local speed = 50
	return function()
		speed = speed + 10
	end
end

su = speed_up
print(su()) -- 60
print(su()) -- 70

In the above example, when you call su(), you have access to local speed, even though you are out of scope already.

Coroutine

Lua has built-in coroutine support. Coroutine itself is quite easy to understand. It is a block of code that can be paused and resumed. However, to actually make use of coroutine to achieve concurrency is difficult. I will only introduce Coroutine basics in Lua. If you come from Ruby world, fiber in Ruby (added in 1.9) is quite similar to coroutine. In fact, they borrow yield and resume from Lua.

local co = coroutine.create(function ()
	coroutine.yield(1)
	coroutine.yield(2)
	coroutine.yield(3)
end)

When you create a coroutine, it will not automatically start. In fact, it will pause at yield, you have to resume it.

coroutine.resume(co) -- true, 1
coroutine.resume(co) -- true, 2
coroutine.resume(co) -- true, 3

coroutine.resume() first returns a boolean flag indicating whether there is any error - the coroutine body is running in protected mode. The values you yield in coroutine body is gonna be returned following the boolean flag. When the coroutine yields all values, it will be dead and when you try to resume a dead coroutine, it will return error.

coroutine.resume(co) -- false, cannot resume dead coroutine

Coroutine works like a thread, however, the pausing and resuming is done in the code, not in OS level. And you cannot force a coroutine to pause at any time - coroutine only pauses at the point you have specified. Switching between coroutines costs considerably small and synchronization is explicitly achieved in your code. However, coroutine is not real multi-threading in the sense that any blocking code will block the whole program, while in multi-threading it only blocks that thread. That’s why to achieve currency, you have to make your code non-blocking.

That’s the end of my introduction to Lua. Again, like I said in my previous post, I am by no means a “Lua expert”. So please correct me for any mistakes and join further discussions on HN.