Meta-Functions for math operators

[MysticDude]

I was working on a Vec2 lib, it would be so much nicer if we had meta-functions for math.

Example:

Code: Select all

a := Vec2(150, 80)
b := Vec2(30, 490)

c := a + b      ; c := Vec2(a.x + b.x, a.y + b.y)

d := a + 5      ; d := Vec2(a.x + 5, a.y + 5)

e := 5 - b      ; e := Vec2(5 - b.x, 5 - b.y)

class Vec2 {
    __New(x := 0, y := 0) {
        This.x := x
        This.y := y
    }

    __Add(a, b) {
        if IsNumber(a)
            return Vec2(a + b.x, a + b.y)
        if IsNumber(b)
            return Vec2(a.x + b, a.y + b)
        if !(a is Vec2)
            throw ValueError('Math error', -1, a)
        if !(b is Vec2)
            throw ValueError('Math error', -1, b)
        return Vec2(a.x + b.x, a.y + b.y)
    }

    __Sub(a, b) {
        if IsNumber(a)
            return Vec2(a - b.x, a - b.y)
        if IsNumber(b)
            return Vec2(a.x - b, a.y - b)
        if !(a is Vec2)
            throw ValueError('Math error', -1, a)
        if !(b is Vec2)
            throw ValueError('Math error', -1, b)
        return Vec2(a.x - b.x, a.y - b.y)
    }
}

I think LUA sets a good example of how it's done:

https://www.lua.org/pil/13.1.html
http://phrogz.net/lua/LearningLua_MetatableEvents.html

[lexikos]

This is more typically called "operator overloading".

Operator overloading has been on my TODO list for over 10 years. I think the expression evaluator needs to be reworked before it will be feasible.

[MysticDude]

Ah nice, glad it's already on your radar

In KuroilLight's example the method only accepts a single param (the other operand) - presumably there would be a way to determine if the object being invoked (this) was on the left or right? Or is it more typical for only the first operand to have its method invoked?

The wiki page for operator overloading actually notes the fact that Lua allows both.

Lua allows operator overloading as syntactic sugar for method calls with the added feature that if the first operand doesn't define that operator, the method for the second operand will be used.

[lexikos]

I have used operator overloading only in strongly typed languages such as C# or C++, and have never had to consider the order of the operands in practice. I have read how Lua handles it, but don't know if it provides an answer to your question, and haven't given it much thought.

[MysticDude]

With two params the object itself could appear as either one of them, depending on how it was invoked. In the scenario with vectors, it allows any operation between vectors and numbers. Like 5 - v = (5 - v.x, 5 - v.y) or v > 5 which I would treat as 'is the vectors magnitude greater than 5?'

The downside is both params always have to be validated, whereas with a single param, 'this' is obviously already known amd I guess the other operand must be the same type anyway? There's probably a noticeable performance impact in the former.