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add Lua classes tutorial

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%% title = "implementing classes in Lua"
% id = "01JKKQHG5DHSAN8FNC27WM0RE5"
- while reading Lua, you may have stumbled upon something that looks like this:
```lua
-- Declare a base `Entity` class...
local Entity = Object:inherit()
function Entity:tick() end
function Entity:draw() end
-- and an inheriting `Player` class.
local Player = Entity:inherit()
```
% id = "01JKKQHG5DDM42QXJ6RQJ6MYGG"
- this is the way people generally do object-oriented programming in the language.
% id = "01JKKQHG5DAM1JSKSHDPPT5DZ3"
- for someone coming from a language like Java, where classes are a syntactic construct---`class Cat extends Animal`---it can feel weird to see them declared this way---as local variables, using regular functions to implement inheritance.
% id = "01JKKQHG5DJEJ6EM5NM24EWZW0"
- but worry not!
this tutorial will hopefully clear up any confusion you might have, using beginner-friendly language, and simple examples.
% id = "01JKKQHG5D3M7RY6D9EXMXEB3P"
- ### metatables
% id = "01JKKQHG5DX4PEJ1GW27S257T3"
- before we start, we need to talk about *metatables*.
these are Lua's way of allowing users to _overload operators_.
% id = "01JKKQHG5DBRZA5EAF7D6C11KA"
- operators include arithmetic: `+`, `-`, `*`, `/`, but also things like indexing tables `a[b]`, creating new indices in tables `a[b] = c`, or function calls `a(b, c, d)`.
% id = "01JKKQHG5DWNC80HZAXXQ9JR7Z"
- we call it operator _overloading_, because we _overload_ the default meaning of the operator with our own, custom definition.
% id = "01JKKQHG5D62EMCSEPTQAN82XH"
- we can set the metatable of a table using [`setmetatable(t, metatable)`](https://www.lua.org/manual/5.4/manual.html#pdf-setmetatable).
% id = "01JKKQHG5D86T78ESRTQBEJ5T4"
- the `metatable` is another table, that contains fields for overriding these operators.
% id = "01JKKQHG5DMCH6P2X8C23EHKW0"
- the most important field of metatables we'll be focusing on today is `__index`, which defines a _fallback_ for the `a[b]` operator---and by extension, also `a.b`, which is syntactic sugar for `a["b"]`.
% id = "01JKKQHG5DCNFHQ72NGAE4T12D"
- #### `__index`
% id = "01JKKQHG5DXTZDGY9CHH456388"
- the `__index` field is used when an otherwise `nil` field is accessed in a table.
consider this:
```lua
local t = { a = 1 }
print(t.b) --> nil
```
% id = "01JKKQHG5DR51DKCN70ZJAGBM1"
- in this case, `t` does not have a metatable with `__index`, so `nil` is returned.
to change this behaviour, we override `__index` by telling Lua a function to run whenever the key doesn't exist.
```lua
local fallback = { b = 2 }
setmetatable(t, {
-- The first argument is the table that's indexed, and the second argument is the index,
-- i.e. the arguments map to `the_table[index]`.
__index = function (the_table, index)
return fallback[index]
end,
})
print(t.b) --> 2
```
% id = "01JKKQHG5DG2EGZTBEZ38YK8KW"
- however, there is a more compact and faster way of doing this.
`__index` is special, because in addition to being able to set it to a function, we can also set it to a table:
```lua
setmetatable(t, {
__index = fallback,
})
print(t.b) --> 2
```
this avoids the need to allocate a local function, which can be costly if you run it many times in a game loop!
% id = "01JKKQHG5DTZG3T5NQYWSQYGQF"
- ### method call syntax
% id = "01JKKQHG5DYJBYRMSK7MJWZT5F"
- there's one thing we need to get out of the way before we move on, and that is Lua's _method call syntax_ `a:method(b)`.
this syntax is equivalent to:
```lua
a.method(a, b)
```
% id = "01JKKQHG5D35F2P45MA33QNAER"
- basically, the thing before `:` is passed as the first argument to the thing before `:`'s `method` function.
% id = "01JKKQHG5DC2MDYZ9M07ZZ8J9D"
- Lua also has a syntax sugar for declaring functions on tables:
```lua
local t = {}
function t.do_stuff()
print("hi")
end
```
% id = "01JKKQHG5DTF67PWWTWPFMYT6Q"
- to complement the `:` call syntax, there's also the `:` function declaration syntax.
```lua
function t:do_thing()
self.aaa = 1
end
-- desugars to
function t.do_thing(self)
self.aaa = 1
end
```
as this example shows, this syntax simply inserts a parameter named `self` before all other parameters.
% id = "01JKKQHG5D2QEHBJ32NF0VCFEX"
- the call and declaration syntaxes are not tied together in any way, so the dot and colon syntax could be mixed however one wants, but it's probably better not to.
% id = "01JKKQHG5DNY2EPFN195KYJVEW"
- bear in mind that your function declarations also serve the purpose of documentation, and using the `:` syntax in declarations makes it clearer you're supposed to call the functions with the `:` syntax.
% id = "01JKKQHG5DY4JZDZTB5B7W9D8N"
- with that knowledge, we can move on to creating classes.
% id = "01JKKQHG5DCTRRFQQ7NNNBM8XS"
- ### classes
% id = "01JKKQHG5D4M3K6D4ECF2M2QFD"
- we can use `__index` fallback tables to model classes quite easily.
% id = "01JKKQHG5D8QVHGTDGEP9RYBJG"
- let's create a class `Cat` with two methods `meow` and `feed`:
```lua
local Cat = {}
function Cat:meow()
print("meow")
end
function Cat:feed()
self.food = self.food + 1
end
```
% id = "01JKKQHG5DWP99Q74Q62RVS176"
- we also need a method for creating cats, which I'll call `new`:
```lua
function Cat:new()
local cat = {}
cat.food = 10
return cat
end
```
% id = "01JKKQHG5DDVYTWXZYS711DVWT"
- we can now use the API like this:
```lua
local kitty = Cat:new()
Cat.meow(kitty)
Cat.feed(kitty)
print(kitty.food) --> 11
```
but, note how we have to namespace the `Cat` functions specifically, and we cannot use the `:` method call operator yet.
the table returned by `Cat:new()` does not have the methods `meow` and `feed` for that to work.
% id = "01JKKQHG5D0KT6YAR9MWAC00N0"
- so to provide it with these methods, we can use our handy `__index` metamethod:
```lua
function Cat:new()
local cat = {}
cat.food = 10
-- setmetatable returns its first argument. How convenient!
return setmetatable(cat, { __index = Cat })
end
```
% id = "01JKKQHG5DQZRNTCWK2JACFJWT"
- _now_ we'll be able to create cats that can meow on their own:
```lua
kitty = Cat:new()
kitty:meow()
kitty:feed()
print(kitty.food) --> 11
```
% id = "01JKKQHG5DK3S08JBG24970YV6"
- however, creating an extra metatable every single time we create a cat is pretty inefficient!
we can exploit the fact that Lua doesn't really care about metatable fields it doesn't know about, and make `Cat` itself into a metatable:
```lua
Cat.__index = Cat
function Cat:new()
local cat = {}
cat.food = 10
return setmetatable(cat, Cat)
end
```
% id = "01JKKQHG5DYG4C3Y5H4WEYQ025"
- but note how we've declared `Cat:new` with the special method syntax.
we call the method like `Cat:new()`, which desugars to `Cat.new(Cat)`, which means that the implicit `self` parameter _is_ already the `Cat` table!
thus, we can simplify the call to `setmetatable`, to remove the redundant reference to `Cat`:
```lua
return setmetatable(cat, self)
```
% id = "01JKKQHG5D1ZTAT0JPNPWDE2WC"
- with all these improvements, here's how the code looks so far.
```lua
local Cat = {}
Cat.__index = Cat
function Cat:new()
local cat = {}
cat.food = 10
return setmetatable(cat, self)
end
function Cat:meow()
print("meow!")
end
function Cat:feed()
self.food = self.food + 1
end
```
% id = "01JKKQHG5DHXF00NJPPVXBZ98K"
- ### inheritance
% id = "01JKKQHG5DB3JWK6YCG1N47JZ1"
- given this fairly simple way of creating classes, we can now expand this idea to inheritance.
% id = "01JKKQHG5D7N0A33S109YF2NV6"
- conceptually, inheriting from a class is pretty simple: what we want to do, is to have all of the parent class's methods available on the child class.
I think you might see where this is going now: all we need to do to create a subclass, is to create a new class, whose metatable's `__index` points to the parent class.
% id = "01JKKQHG5DSB0DQPPWQZNPWY1M"
- let's rewrite our example with the kitty to generalise animals under a single class:
```
Animal
- food: integer
: speak()
: feed()
Cat : Animal
: speak()
```
% id = "01JKKQHG5D4PZMTFRFV4T6HSA1"
- so, starting with the base `Animal` class...
```lua
local Animal = {}
Animal.__index = Animal
-- We don't create a `new` method, because we don't want people creating "generic" animals.
-- This makes our class _abstract_.
-- speak is a function that must be overridden by all subclasses, so we make it error by default.
function Animal:speak() error("not implemented") end
function Animal:feed()
self.food = self.food + 1
end
```
% id = "01JKKQHG5D0ENDQ5QQTBAA79GQ"
- we can define a `Cat` class as a subclass of `Animal`:
```lua
local Cat = {}
-- We still need to override __index, so that the metatable we set in our own constructor
-- has our overridden `speak()` method.
Cat.__index = Cat
-- To be able to call `Animal` methods from `Cat`, we set it as its metatable.
-- Remember that `Animal.__index == Animal`.
setmetatable(Cat, Animal)
function Cat:new()
-- Ultra-shorthand way of initializing a class instance!
-- No need to declare any temporary locals, we can pass the table into `setmetatable`
-- right away.
return setmetatable({
food = 1,
}, self)
end
-- Don't forget to override speak(), otherwise calling it will error out!
function Cat:speak()
print("meow")
end
```
% id = "01JKKQHG5DCHD37G3WKEBJA5Z6"
- note now that declaring `speak` _does not modify `Animal`_.
for that, we would need to set the _`__newindex`_ metatable field on the `Animal`, not just `__index`.
% id = "01JKKQHG5D08QQ8XSXBRMCDV26"
- now we can create instances of the `Cat`, and it will inherit the `feed` method from `Animal`:
```lua
local kitty = Cat:new()
kitty:speak()
kitty:feed()
print(kitty.food) --> 2
```
% id = "01JKKQHG5DYZW7W7Q9H75HN8BM"
- ### generalising
% id = "01JKKQHG5DTATR750GQ2S7EAPF"
- with all this, we are now ready to pack this subclassing functionality into a nicer package.
speaking of packages, let's create a module `class.lua`:
```lua
local Class = {}
Class.__index = Class
return Class
```
% id = "01JKKQHG5DFTHS8P07T3KET5F8"
- now, let's create a method for inheriting from the class.
```lua
-- insert above `return Class`
function Class:inherit()
local Subclass = {}
Subclass.__index = Subclass
-- Note how `self` in this instance is the parent class, as we call the method like `SomeClass:inherit()`.
setmetatable(Subclass, self)
return Subclass
end
```
% id = "01JKKQHG5DTAK0EA4PR6JX2GV8"
- this is going to let us cleanly inherit from classes, without needing to copy and paste all the `__index` and `setmetatable` boilerplate:
```lua
local Class = require "class"
local Sub = Class:inherit()
```
% id = "01JKKQHG5DKY7ZFWA42D9NJ49C"
- the other boilerplaty bit was initialisation, so let's take care of that:
```lua
-- insert below the `end` of `function Class:inherit()`
-- By default, let's make the base `Class` impossible to instantiate.
-- This should catch bugs if a subclass forgets to override `initialize`.
function Class:initialize()
error("this class cannot be initialized")
end
-- `...` is Lua's notation for collecting a variable number of arguments
function Class:new(...)
local instance = {}
-- `self` is the class we're instantiating, as this method is called like `MyClass:new()`
setmetatable(instance, self)
-- We pass the instance to the class's `initialize()` method, along with all the arguments
-- we received in `new()`.
self.initialize(instance, ...)
return instance
end
```
% id = "01JKKQHG5D8VD9E0SXCY337D3W"
- having that, we can now rewrite our `Animal` example to use our super-simple class library.
```lua
local Class = require "class"
---
local Animal = Class:inherit()
-- We'll provide a convenience function for implementers, for initialising the food value,
-- as well as any other base fields that may come up.
function Animal:_initialize()
self.food = 1
end
function Animal:speak()
error("unimplemented")
end
function Animal:feed()
self.food = self.food + 1
end
---
local Cat = Animal:inherit()
-- Don't forget that our initialize() method errors by default, so it has to be overridden.
function Cat:initialize()
self:_initialize()
end
function Cat:speak()
print("meow")
end
```
% id = "01JKKQHG5DFS98T9X8A8SPYXCT"
- having a nice class library like this makes things a lot more convenient.
no longer do we have to mess with raw metatables!
all we need to do is call `inherit()` or `new()`, and the magic is done for us.
```lua
local kitty = Cat:new()
kitty:speak()
kitty:feed()
print(kitty.food)
```
% id = "01JKKQHG5DSR2Z0XGW6W2H97NH"
- ### wrapping up
% id = "01JKKQHG5DANR0QF460EHKAWMP"
- if you followed this tutorial from beginning to end, you now have a simple library for object-oriented programming in Lua, which supports creating classes, and inheriting from them.
% id = "01JKKQHG5DMCXV1VT37Z8M7P5G"
- to further your understanding, you may want to think about the following:
% id = "01JKKQHG5DRF6RVFG4P9XYDXZZ"
- how would you call the superclass's implementation of a method?
can you think of ways to make it convenient and easy to remember?
% id = "01JKKQHG5DQW7G1HDWYGX4WEYC"
- our class library implements a Ruby-style `Object:new(args)` function for constructing new instances of our class.
Python however, uses the syntax `Object(args)` for constructing instances of objects.
can you think of a way to make our class library use the Python-style syntax?
% id = "01JKKQHG5D7QN029A3F1B6SYC2"
- define a 2D vector class using our class library.
can you think of a way to make use of Lua's native `+`, `-`, `*`, `/` math operators instead of named methods `:add()`, `:sub()`, `:mul()`, `:div()`?
% id = "01JKKQHG5DJP0C1V8K2BQBD6G2"
- try implementing an `object:instanceof(Class)` function, which checks that an object instance inherits from a given class.
% id = "01JKKQHG5DN41D9BME6JXJ3R86"
- Lua is a minimalistic, multi-paradigm language.
can you think of any benefits and drawbacks towards doing object-oriented programming in Lua?
% id = "01JKKQHG5DNRKVQJAVN4KP608R"
- what are some problems for which this style of programming would lend itself as particularly good?
% id = "01JKKQHG5D6TY9QSCQGEQWMZGM"
- and similarly, what are some areas in which this style might not work so well?
% id = "01JKKQHG5DFNRGD6D17RHB3S22"
- ### further reading
% id = "01JKKQHG5DDB411SKNFA8M7K2B"
- you may wanna check these out for additional reference.
% id = "01JKKQHG5DRWGGE7GE3GMCB7E8"
- [the Lua documentation on metatables](https://www.lua.org/manual/5.4/manual.html#2.4)---there's lots of other operators you can overload!
% id = "01JKKQHG5D3PQZY439H61FN27V"
- [rxi's `classic`](https://github.com/rxi/classic/blob/master/classic.lua) module---it's an example of a good, but small class library that has all the features you'd ever need.

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content/treehouse/new.tree
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@ -26,7 +26,26 @@ if you've been wondering what I've been up to, you've come to the right place.
if you want to read any of the posts, follow the links.
it's like that by design.
% tags = ["programming", "lua"]
id = "01JKKQZRSG5ZRNH530D75E2660"
- ### [implementing classes in Lua][page:programming/lua/classes]
% id = "01JKKQZRSGXT99FJ7V3BGKE2D5"
- one of my absolutely favourite parts of Lua is how tiny, but capable it is.
did you know you could implement object-oriented programming without needing any additional syntactic support?
% id = "01JKKQZRSGMN81PXJWGP5Y17WY"
- this is a remaster of an [old tutorial I published as a Gist](https://gist.github.com/liquidev/3f37f94efdacd14a654a4bdc37c8008f) to explain how object-oriented programming works in Lua to someone on the [LÖVE](https://love2d.org/) Discord server.
% id = "01JKKQZRSGMW486TGSXQVM8FV3"
- thus there's a high likelihood you've never read it.
however, I think it's a pretty nice tutorial, so I'm republishing it here outside the shackles of GitHub.
% id = "01JKKQZRSGNA5NRXR5H6WPZKWA"
- and the programming trickery in it might just open your third eye a bit, so you should read it even if you're not into Lua!
% id = "01JK5SN2ZBDZTFZ27J3KNT4SQV"
tags = ["music"]
- ### [Floating Points - Tilt Shift / Ablaze][page:music/tilt-shift-ablaze]
% id = "01JK5SNYKRK08F5DJM4JGBK4C4"
@ -38,6 +57,7 @@ if you've been wondering what I've been up to, you've come to the right place.
so that I can remember, and so the world can see, too...
% id = "01JHXVRT2HR6TXC2V9JG2XTZVB"
tags = ["music"]
- ### [The Flashbulb - Flacks / aBliss][page:music/flacks]
% id = "01JHXVRT2H2CTGBEDYWCMDTTS3"

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static/syntax/lua.json
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@ -19,7 +19,7 @@
},
{ "regex": "0[xX]\\.[0-9a-fA-F]+([pP][-+]?[0-9]+)?", "is": "literal" },
{
"regex": "[0-9][0-9_]+(\\.[0-9_]*([eE][-+]?[0-9_]+)?)?",
"regex": "[0-9]+(\\.[0-9]*([eE][-+]?[0-9]+)?)?",
"is": "literal"
},
{
@ -30,7 +30,7 @@
}
},
{ "regex": "\\.\\.\\.", "is": "punct" },
{ "regex": "[+=/*^%#<>~.-]+", "is": "operator" },
{ "regex": "[+=/*^%#<>~.:-]+", "is": "operator" },
{
"regex": "([a-zA-Z_][a-zA-Z0-9_]*)\\(",
"is": {