how to get the class type in lua / translation from python

Question

I am trying to find something analogous to the class in lua. In python, I would do:

a = {}
type(a)
>>>> dict

So I have the object vocab in lua. When I print the object, I get:

print(vocab)
>>> {
3 : 20
o : 72
m : 70
d : 61
( : 9
}

how can I get lua to spit the object, something analogous to the type() in python? - which will give you the class of the object

Answer 1

There are 8 types in Lua : nil, boolean, number, string, function, thread, table and userdata . You can find out which of these basic types your object belongs to using built-in type() function:

type('Hello world')                    == 'string'
type(3.14)                             == 'number'
type(true)                             == 'boolean'
type(nil)                              == 'nil'
type(print)                            == 'function'
type(coroutine.create(function() end)) == 'thread'
type({})                               == 'table'
type(torch.Tensor())                   == 'userdata'

Note that the type of torch.Tensor is userdata. That makes sense since torch library is written in C.

The type userdata is provided to allow arbitrary C data to be stored in Lua variables. A userdata value is a pointer to a block of raw memory. Userdata has no predefined operations in Lua, except assignment and identity test.

The metatable for the userdata is put in the registry, and the __index field points to the table of methods so that the object:method() syntax will work.

So, dealing with a userdata object, we do not know what it is but have a list of its methods and can invoke them.

It would be great if custom objects had some mechanism (a method or something) to see their custom types. And guess what? Torch objects have one:

t = torch.Tensor()
type(t)       == 'userdata' # Because the class was written in C
torch.type(t) == 'torch.DoubleTensor'
# or
t:type()      == 'torch.DoubleTensor'

Speaking of Torch . It has its own object system emulator, and you are free to create some torch classes yourself and check their types the same way. For Lua , however, such classes/objects are nothing more than ordinary tables.

local A = torch.class('ClassA')
function A:__init(val)
    self.val = val
end

local B, parent = torch.class('ClassB', 'ClassA')
function B:__init(val)
    parent.__init(self, val)
end

b = ClassB(5)
type(b)       == 'table' # Because the class was written in Lua
torch.type(b) == 'ClassB'
b:type() # exception; Custom Torch classes have no :type() method by defauld

Answer 2

It is said that Lua defines "mechanisms, not policies". Classes would be a "policy". You can implement the effect of classes however you like; Lua provides many mechanisms that could be used to do so. But Lua has no single way of declaring such a construct.

Lua's type standard method will only return the broad category of a Lua value: number, string, table, etc. And since Lua has only one kind of data structure (table), every kind of class (not generated from C) would be of the "table" type.

This has the side effect of making it difficult in Lua to use someone else's policies. For example, if Torch has a way to distinguish its own "classes" from regular tables, that would work. But it wouldn't be able to distinguish some other class implementation from a regular table.

So there's no general way to distinguish a table from some form of "class".