Operators can only be defined one way

[mkster]

Describe the bug
Its seems like operators can only be defined one way? I want number * Vec3 to be Vec3. Am I missing something?

To Reproduce

    --- @class Vec3
    --- @operator mul(number): Vec3
    local Vec3 = {}

    local t1 = Vec3() * 2 ---Vec3
    local t2 = 2 * Vec3() --Integer!

[sumneko]

Dose not support class as the second parameter for now.

[sewbacca]

I would suggest that bi-operators accept two arguments, instead of one. One argument assumes that the first one allways will be the class itself, which is not the case A bi-operator function can only assume, that one of the arguments is of the type of the class, it is defined in.
In addition to that, the implied first parameter, is inconsistent. Check out this example:

---@class namespace.ClassName
---@operator unm(): namespace.ClassName

I would assume, that since add and mul, etc. assume that the first argument is of type namespace.ClassName, the __unm operator does assume it too, but the warning is confusing. However when I use the unary minus operator, it correctly assigns the type:

---@type namespace.ClassName
local b
local a = -b -- a is of type namespace.ClassName

What I would like to see, is explicit argument descriptors, for enabling different overloads:
Maybe the first parameter could be implied, if it is omitted. However one argument type allways has to be in the class it is defined in:

---@class A # operator add returns lastly added type
---@operator add(A): A
---@operator add(number): number
---@operator add(number, A): A
---@operator unm(A): A
---@operator unm(): A

Is there any reason @sumneko, why operator overloads cannot be implied from functions, defined in the class?

---@class A
local A = {}

---@generic T
---@param b T
---@return T
function A.__add(a, b)
    return b
end

[sumneko]

The current standard comes from #599 .
I can supports these at the same time:

---@class A
---@operator add: A --> eqauls to `add(A, any): A`
---@operator add(): A --> eqauls to `add(A, any): A`
---@operator add(number): A --> eqauls to `add(A, number): A`
---@operator add(number, A): A --> used for `1 + A`, but it may make people interested if it can be `add(number, B): A` in `class A`

Is there any reason @sumneko, why operator overloads cannot be implied from functions, defined in the class?

Strictly speaking, if I do not analyze the setmetatable, I cannot assume that the method of __add is related to additional operations.

[mkster]

Yeah I think this is needed:

---@operator add(number, A): A

With something like this there can also be this case

---@class A
---@operator add(A, B): A 

---@class B
---@operator add(A, B): B

local res = A + B -- A or B?

In that case res should be A because in lua metamethods the left table is checked before the right table for the presence of an arithmetic metaevent.

[sewbacca]

---@operator add(number, A): A --> used for 1 + A, but it may make people interested if it can be add(number, B): A in `class

Then probably a warning, that this annotation can never be used (or will be ignored), should be thrown.

Strictly speaking, if I do not analyze the setmetatable, I cannot assume that the method of __add is related to additional operations.

Is it possible to anlayze setmetatable() without performance issues?

[sumneko]

Is it possible to anlayze setmetatable() without performance issues?

Yes, just need analyze it once, then I can cache the result.

Unless I assume that the semantics of Class:__add is to add additional operations to Class.

---@class A
local mt = {}
mt.__index = mt
mt.__add = function () end

local t = setmetatable({}, mt)

In this code, I can infer local t is class A by __index, and I can infer t has "additional operation", but I cannot infer class A has "additional operation"

[CelticMinstrel]

The current standard comes from #599 . I can supports these at the same time:

---@class A
---@operator add: A --> eqauls to `add(A, any): A`
---@operator add(): A --> eqauls to `add(A, any): A`
---@operator add(number): A --> eqauls to `add(A, number): A`
---@operator add(number, A): A --> used for `1 + A`, but it may make people interested if it can be `add(number, B): A` in `class A`

These meanings seem quite strange to me. The most common use-case for operators, I believe, would be defining them to be symmetric. Thus, I would expect something more along the following lines:

---@class A
---@operator add:A --> add(A, A):A
---@operator add():A --> add(A, any):A and add(any, A):A
---@operator add(number):A --> add(A, number):A and add(number, A):A

Of course, there are times when it shouldn't be symmetric, so having a way to declare asymmetric operators (probably by specifying both parameters) is also important.

[tomlau10]

Hi, I come from here #2777 (comment), and it contains a preliminary implementation idea for symmetric operator. Maybe someone would be interested to finish it and open a PR 😄 .

Btw I never thought of an asymmetric __add... what could be a possible use case for that 😕 ?

---

update: I see @SeanTheBuilder1 just implemented this symmetric operator logic in #2779 🎉

[sewbacca]

I do believe __add does happen less often but custom fractional types or var byte numbers or imaginary numbers should be able to use asymmetric operators to add together correctly.

These are still commutative though.
For multiplication we have scalars, vectors, matricies and quaternions. Here order is crucial for the output type.

If imaginaries handled both fractional and var byte numbers for addition but not the other way around, then an incorrect ordering could throw an error though.

[CelticMinstrel]

Btw I never thought of an asymmetric __add... what could be a possible use case for that 😕 ?

I don't know of a good use-case for an asymmetric __add specifically, but there are definitely cases where you can't just swap the arguments. For example, __mul on a matrix type would need different handling for A * B and B * A.

I don't know of any use-case where swapping the arguments has a completely different meaning and returns a different type though.

[tomlau10]

on a matrix type would need different handling for A * B and B * A.

You're right, I understand that in matrix multiplication AB != BA as it is non commutative.

But when implementing it in programming wise, I guess there will only be a Matrix class and then each matrix object will have a row & column properties? i.e. both objects are in the same Matrix class. 😕

In this (object type) perspective, __mul on a matrix type seems to be symmetric. Because either AB or BA will still give a Matrix type. The row / column are just properties of the class object and unrelated to type inferencing?

[CelticMinstrel]

But when implementing it in programming wise, I guess there will only be a Matrix class and then each matrix object will have a row & column properties? i.e. both objects are in the same Matrix class. 😕

Well, it depends on the approach, but yes, if I were implementing a matrix in Lua, I'd probably do it as one type that has implements __mul and checks the dimensions of its arguments. It is probably possible to jump through some extra hoops and make something like a "matrix class factory" that takes a pair of dimensions and spits out a class representing a matrix of exactly those dimensions, but I don't think I'd do things that way in Lua.

As I said before, I can't think of an actual example in Lua where I'd want to overload an operator in a way that the arguments are of different types and can't be swapped.

But speaking of swapping, Lua doesn't actually swap the arguments (except for comparisons). Instead, it doesn't guarantee that the first argument is of the type owning the metatable. So if a custom overloaded operator can't be swapped, it just needs to raise an error if the first argument doesn't have the same metatable.