A parameter pack implementation of `AnimatablePair`. Started off as a weird idea but I think this version works. Now included in my ShapeUp SPM. https://github.com/ryanlintott/ShapeUp

AnimatablePack.swift

//
//  AnimatablePack.swift
//  ShapeUp
//
//  Created by Ryan Lintott on 2023-08-02.
//

import Foundation
import SwiftUI

@available(iOS 17, macOS 14, *)
fileprivate extension VectorArithmetic {
    func addingMagnitudeSquared(to value: inout Double) {
        value += magnitudeSquared
    }
}

/**
 A parameter pack implementation of `AnimatablePair`
 
 Conforming to Animatable with AnimatablePair:

 ```swift
 struct MyShape: Animatable {
     var animatableData: AnimatablePair<CGFloat, AnimatablePair<RelatableValue, Double>> {
         get { AnimatablePair(insetAmount, AnimatablePair(cornerRadius, rotation)) }
         set {
             insetAmount = newValue.first
             cornerRadius = newValue.second.first
             rotation = newValue.second.second
         }
     }
 }
 ```
 
 Conforming to Animatable with AnimatablePack:
 ```swift
 struct MyShape: Animatable {
     var animatableData: AnimatablePack<CGFloat, RelatableValue, Double> {
         get { AnimatablePack(insetAmount, cornerRadius, rotation) }
         set { (insetAmount, cornerRadius, rotation) = newValue() }
     }
 }
 ```
 */
@available(iOS 17, macOS 14, *)
public struct AnimatablePack<each Item: VectorArithmetic>: VectorArithmetic {
    public var item: (repeat each Item)
    
    public init(_ item: repeat each Item) {
        self.item = (repeat each item)
    }
    
    public func callAsFunction() -> (repeat each Item) {
        item
    }
}


@available(iOS 17, macOS 14, *)
public extension AnimatablePack {
    static var zero: Self {
        .init(repeat (each Item).zero)
    }
    
    static func + (lhs: Self, rhs: Self) -> Self {
        .init(repeat (each lhs.item) + (each rhs.item))
    }
    
    static func - (lhs: Self, rhs: Self) -> Self {
        .init(repeat (each lhs.item) - (each rhs.item))
    }
    
    mutating func scale(by rhs: Double) {
        item = (repeat (each item).scaled(by: rhs))
    }
    
    static func == (lhs: Self, rhs: Self) -> Bool {
        (lhs - rhs).magnitudeSquared == .zero
    }
    
    var magnitudeSquared: Double {
        var value = 0.0
        _ = (repeat (each item).addingMagnitudeSquared(to: &value))
        return value
    }
}