MaybeMonad 中的类型推断

Question

我正在从这个 site 中尝试一个示例，但是运行进入以下推理问题。

这个例子可能是关于 Monads 的，并且从连续的空检查中保存并以线性方式编写代码。

The type arguments for method Maybe<Customer>.Bind<TO>(Func<Customer, Maybe<TO>>) cannot be inferred from the usage. Try specifying the type arguments explicitly.

Maybe.cs

using System;

namespace Monads
{
    public class Maybe<T> where T : class
    {
        private readonly T value;

        public Maybe(T someValue) 
        {
            if(someValue == null) 
                throw new ArgumentNullException(nameof(someValue));
            this.value = someValue;
        }

        private Maybe(){}

        public Maybe<TO> Bind<TO>(Func<T, Maybe<TO>> func) where TO : class
        {
            return value != null ? func(value) : Maybe<TO>.None();
        }

        public static Maybe<T> None() => new Maybe<T>();
    }
}

IMonadicRepository

namespace Monads
{
    public interface IMonadicRepository
    {
       Maybe<Customer> GetCustomer(int id);
       Maybe<Address> GetAddress(int id);
    }
}

Repository.cs

namespace Monads {
    public class Repository : IMonadicRepository
    {
        public Maybe<Customer> GetCustomer(int id) {
            return new Maybe<Customer>(new Customer(id));
        }

        public Maybe<Address> GetAddress(int id)
        {
            return new Maybe<Address>(new Address(id));
        }
    }
}

Customer.cs

namespace Monads
{
    public class Customer 
    {
        public Customer(int id) {
        }

        public Address Address {get; set;}
    }
}

Address.cs

namespace Monads 
{
    public class Address 
    {
        public Address(int id) {
        }
    }
}

用法 - Programs.cs

namespace Monads
{
    class Program
    {
        static void Main(string[] args)
        {
            IMonadicRepository repository = new Repository();
            repository.GetCustomer(1)
                .Bind(customer => customer.Address); // error
        }
    }
}

错误：无法从用法中推断方法 'Maybe.Bind(Func>)' 的类型参数。尝试明确指定类型参数。

Answer 1

你的问题是在 .Bind(customer => customer.Address) 行中，customer.Address 的类型是 Address。然而，函数 Bind 需要一个 returns 和 Maybe<Address> 而不是 Address.

的 lambda

我不确定这是否是正确的方法，但一种选择是使用 post:

中定义的 Return 扩展方法

repository.GetCustomer(1)
    .Bind(c => c.Address.Return());

为了完成，我也复制在这里：

public static class MaybeExtensions
{
    public static Maybe<T> Return<T>(this T value) where T : class
    {
        return value != null ? new Maybe<T>(value) : Maybe<T>.None();
    }
}

MaybeMonad 中的类型推断

Type inference in MaybeMonad

c#

monads