SwiftUI decimal (Double) formatter as-if in a calculator.

Calculator.swift

import Foundation
import SwiftUI

struct Calculator: View {
    func fmtDouble(dbl: Double) -> String? {
        let formatter =  NumberFormatter()
        formatter.numberStyle = .currencyAccounting
        formatter.minimumFractionDigits = 3
        formatter.formatWidth = 70

        return formatter.string(from: dbl as NSNumber)
    }

    /*
     let formatter = NumberFormatter()
     formatter.numberStyle = .decimal
     formatter.maximumFractionDigits = 2

     let number = NSNumber(value: value)
     let formattedValue = formatter.string(from: number)!
     */
    var body: some View {
        VStack(alignment: .trailing) {
            Image(systemName: "globe")
                .imageScale(.large)
                .foregroundColor(.accentColor)
            Text("Calculator")
            Divider()
            Text(fmtDouble(dbl: 12.34)!)
            Text(fmtDouble(dbl: Double(-3451.789))!)
            Text(Double(-3451.789).debugDescription)
        }
        .padding()
        .fixedSize(horizontal: true, vertical: true)
    }
}

struct Calculator_Previews: PreviewProvider {
    static var previews: some View {
        Calculator()
    }
}

see also: https://www.swiftbysundell.com/articles/formatting-numbers-in-swift/