Cannot use mutating getter on immutable value: 'self' is immutable error

Question

I'm trying to reuse an older piece of Swift code, but getting an error 'Cannot use mutating getter on immutable value: 'self' is immutable error'. Xcode wanted to add 'mutating' before the func, and offered to do so through a 'fix'. So the error is gone there but still remains at the 'Text' statements.

import SwiftUI

struct ContentView: View {

     typealias PointTuple = (day: Double, mW: Double)
    let points: [PointTuple] = [(0.0, 31.98), (1.0, 31.89), (2.0, 31.77), (4.0, 31.58), (6.0, 31.46)]

    lazy var meanDays = points.reduce(0) { $0 + $1.0 } / Double(points.count)
    lazy var meanMW   = points.reduce(0) { $0 + $1.1 } / Double(points.count)

    lazy var a = points.reduce(0) { $0 + ($1.day - meanDays) * ($1.mW - meanMW) }
    lazy var b = points.reduce(0) { $0 + pow($1.day - meanDays, 2) }

    lazy var m = a / b
    lazy var c = meanMW - m * meanDays        
    lazy var x : Double = bG(day: 3.0)
    lazy var y : Double = bG(day: 5.0)
    lazy var z : Double = bG(day: 7.0)

    mutating func bG(day: Double) -> Double {
        return m * day + c
    }

    var body: some View {
        VStack {
            Text("\(x)")
            Text("\(y)")
            Text("\(z)")
        }
    }
}

#if DEBUG
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}
#endif

Answer 1

Because when you call x inside the struct, it's not clear whether the contentView itself is mutable or not. A value type gets mutable only when it is defined as a var.

So it would help if you wrapped it with an immutable value before using it inside a builder function inside the struct.

like this:

func xValue() -> Double {
    var mutatableSelf = self
    return mutatableSelf.x
}

var body: some View {
    VStack {
        Text("\(xValue())")
    }
}

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💡Note: Lazy property's value will be associated on the first call and this mutates the object. So they are considered as mutating