How to enable preview in child view, where child view takes a function as a parameter from the parent?

struct ParentView: View {
    var KMOD: Double = 0.0425

    func calculateSetAI(reps: Int) -> Double {
        return 1 / ((Double(reps) * KMOD) - KMOD + 1)
    }
    
    var body: some View {
        ChildView(calculateSetAI: calculateSetAI)
    }
}

struct ChildView: View {
    // some state variables

    var calculateSetAI: (Int) -> Double
    
    // a newFunction() that calls calculateSetAI
    
    var body: some View {
        Text("Output of newFunction: \(newFunction)")
    }
}

Attempt 1 ERROR: Missing argument for parameter

struct ChildView_Previews: PreviewProvider {
    static var previews: some View {
        ChildView()
    }
}

Attempt 2 ERROR: Cannot convert value of type '(Swift.Int) -> Swift.Double' to expected argument type '(Swift.Int) -> Swift.Double'

struct ChildView_Previews: PreviewProvider {
    var KMOD: Double = 0.0425
    func calculateSetAI(reps: Int) -> Double {
        return 1 / ((Double(reps) * KMOD) - KMOD + 1)
    }
    static var previews: some View {
        ChildView(calculateSetAI: calculateSetAI)
    }
}

Attempt 3 warning (and then crashes preview): Cast from '(RICalcView_Previews) -> (Int) -> Double' to unrelated type '(Int) -> Double' always fails

struct RICalcView_Previews: PreviewProvider {
    var KMOD: Double = 0.0425
    func calculateSetAI(reps: Int) -> Double {
        return 1 / ((Double(reps) * KMOD) - KMOD + 1)
    }
    static var previews: some View {
        RICalcView(calculateSetAI: calculateSetAI as! (Int) -> Double)
    }
}

The previews property of a PreviewProvider is static. That means it doesn't have a (hidden) self parameter.

In attempts 2 and 3, you declared calculateSetAI as an instance method, meaning it has to be called on an instance of RICalcView_Previews, which would be passed to it as the (hidden) self parameter. But there is no self value available in the context where you're mentioning it.

One solution is to add static to the declaration of calculateSetAI in attempt 2. You'll also need to add static to KMOD:

struct ChildView_Previews: PreviewProvider {
    static var KMOD: Double { 0.0425 }
    static func calculateSetAI(reps: Int) -> Double {
        return 1 / ((Double(reps) * KMOD) - KMOD + 1)
    }
    static var previews: some View {
        ChildView(calculateSetAI: calculateSetAI)
    }
}

Or you could just use an inline closure:

struct ChildView_Previews: PreviewProvider {
    static var KMOD: Double = 0.0425

    static var previews: some View {
        ChildView {
            return 1 / ((Double(reps) * KMOD) - KMOD + 1)
        }
    }
}

Or you could create an instance of ParentView and use its calculateSetAI method:

struct ChildView_Previews: PreviewProvider {
    static var previews: some View {
        let parent = ParentView()
        ChildView(calculateSetAI: parent.calculateSetAI)
    }
}