Question

我正在尝试用C＃编写一个简单的Maybe monad。我希望能够使用LINQ查询语法。这是我到目前为止所提出的：

using System;
using System.Collections.Generic; 

abstract class Maybe<A> {
  public abstract Maybe<B> SelectMany<B>(Func<A, Maybe<B>> f);
  public abstract Maybe<B> Select<B>(Func<A, B> f);
}

class Just<A> : Maybe<A> {
  private readonly A a;

  public Just(A a) {
    this.a = a;
  }

  override public Maybe<B> SelectMany<B>(Func<A, Maybe<B>> f) {
    return f(a);
  }

  override public Maybe<B> Select<B>(Func<A, B> f) {
    return new Just<B>(f(a));
  }

  override public string ToString() {
    return "Just " + a;
  }
}

class Nothing<A> : Maybe<A> {
  override public Maybe<B> SelectMany<B>(Func<A, Maybe<B>> f) {
    return new Nothing<B>();
  }

  override public Maybe<B> Select<B>(Func<A, B> f) {
    return new Nothing<B>();
  }

  override public string ToString() {
    return "Nothing";
  }
}

static class Program {
  public static void Main(string[] args) {
    Maybe<int> m = new Just<int>(12);
    Maybe<int> n = new Nothing<int>();
    Maybe<int> result = from m0 in m
                        from n0 in n
                        select m0 + n0;
    Console.WriteLine(result);
  }
}

这是错误消息：

prog.cs(48,25): error CS1501: No overload for method `SelectMany' takes `2' arguments
prog.cs(5,28): (Location of the symbol related to previous error)
Compilation failed: 1 error(s), 0 warnings

任何人都可以指导我如何在Maybe实施中使用查询语法吗？感谢。

Answer 1

SelectMany 必须应声明为静态类中的扩展名，例如：

public static class Maybe {

   public static Maybe<B> SelectMany<B>(this Maybe<A> maybe, Func<A, Maybe<B>> f) {
       return f(a);
   }

   ...
}

修改：

你仍然需要一块。这应该工作：

public static Maybe<V> SelectMany<T, U, V>(this Maybe<T> m, Func<T, Maybe<U>> k, Func<T, U, V> s) { return m.SelectMany(x => k(x).SelectMany(y => new Just<V>(s(x, y)))); }

你需要这个，因为：

from m0 in m from n0 in n select m0 + n0

将翻译成：

m.SelectMany(m0 => n, (m, n0) => m0 + n0);

相反，例如：

var aa = new List<List<string>>(); var bb = from a in aa from b in a select b;

翻译成

aa.SelectMany(a => a);

使用LINQ查询语法和自定义Maybe monad实现

1 个答案: