// for testing/demonstrating the Option monad

using System;

public class option_test
{
  public static Option<double> some(double x) { // syntactic specialization
    return Option<double>.Make(x);
  }
  public static Option<double> NaN = Option<double>.Nothing;
  public static Option<double> safediv(double a, double b) {
     if (b!=0) return some(a/b); else return NaN;
  }//safediv

  // find largest value in an array of doubles, array can be empty
  public static Option<double> smallest(double[] A) {
    if (A.Length==0) return NaN;
    int ai = 0;
    for(int i=1;i<A.Length;i++)
      if (A[i]>A[ai]) ai=i;
    return some(A[ai]);
  }//smallest

  // adopt the Convert.ToDouble method to return None instead of
  // throwing an exception, given an array of strings and an index
  public static Option<double> ToDouble(string[] args, int i) {
    if (args==null || i<0 || i>=args.Length) return Option<double>.Nothing;
    Option<double> answer;
    try {
      answer = Option<double>.Make(Convert.ToDouble(args[i]));
    } catch (Exception e) {
      Console.Error.WriteLine("ERROR: "+e.Message+" Returing None Option");
      answer = Option<double>.Nothing;
    }
    return answer;
  }//ToDouble, moving from exceptions to monads
  
  public static void Main(string[] args) {
    // get value from command-line args, or set to 2.0 as default:
    double divisor = ToDouble(args,0).unwrap_or(2.0); 
    var Result = safediv(4,divisor);
    Console.WriteLine("division result: "+Result);
    Result 
     .map( v => v + 1 )      //non-destructive construction of new Option
     .map_mut( v => v * -1)  //possible destructive change to existing Option
     .and_then(v => {  // can also call .map(..).flatten()
       Console.WriteLine("val is now "+v);
       return safediv(100,(v+3))
              .pair(safediv(200,(v-3)),
	            (x,y)=>x+y);  // return sum of two safedivs
       })
     .match(
        some: v => {Console.WriteLine("final result "+v);},
        none: () => {Console.WriteLine("no result");}
     );
  }//main for testing
}//public class