mcs/tests/gtest-linq-20.cs

   1 using System;
   2
   3 class Maybe<T>
   4 {
   5         public readonly static Maybe<T> Nothing = new Maybe<T> ();
   6         public T Value { get; private set; }
   7         public bool HasValue { get; private set; }
   8         Maybe ()
   9         {
  10                 HasValue = false;
  11         }
  12         public Maybe (T value)
  13         {
  14                 Value = value;
  15                 HasValue = true;
  16         }
  17
  18         public override string ToString ()
  19         {
  20                 if (HasValue)
  21                         return Value.ToString ();
  22                 return string.Empty;
  23         }
  24
  25         public Maybe<U> SelectMany<U> (Func<T, Maybe<U>> k)
  26         {
  27                 if (!HasValue)
  28                         return Maybe<U>.Nothing;
  29                 return k (Value);
  30         }
  31
  32         public Maybe<V> SelectMany<U, V> (
  33                 Func<T, Maybe<U>> selector,
  34                 Func<T, U, V> resultSelector)
  35         {
  36                 if (!HasValue)
  37                         return Maybe<V>.Nothing;
  38                 Maybe<U> n = selector (Value);
  39                 if (!n.HasValue)
  40                         return Maybe<V>.Nothing;
  41                 return resultSelector (Value, n.Value).ToMaybe ();
  42         }
  43 }
  44
  45 static class MaybeExtensions
  46 {
  47         public static Maybe<T> ToMaybe<T> (this T value)
  48         {
  49                 return new Maybe<T> (value);
  50         }
  51 }
  52
  53 class Test
  54 {
  55         public static void Main ()
  56         {
  57                 Console.WriteLine (
  58                         from x in 1.ToMaybe ()
  59                         from y in 2.ToMaybe ()
  60                         from z in 3.ToMaybe ()
  61                         select x + y + z);
  62         }
  63 }