c# - iSynaptic.Commons 和 Maybe Monad

Question

我一直在努力弄清楚如何在 iSynaptic.Commons 中使用 Maybe monad在我的值检索器可能抛出异常的上下文中:

例如:

dynamic expando = new Expando();
expando.Name = "John Doe";

var maybe = Maybe.Defer(()=>(string)expando.NonExistingProperty);

//In this context I would like the exception which is thrown 
//to result in Maybe<string>.NoValue;
if(maybe.HasValue) {
    //Do something
}

这有可能实现吗？

Answer 1

有几种使用 iSynaptic.Commons 允许异常的方法。我发现的每种方法都需要 .Catch() 扩展方法来让 monad 知道静默捕获异常。另外，访问属性 maybe.Value 时要小心。如果此属性为 Maybe.NoValue，将抛出 InvalidOperationException。

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1) 创建一个“OnExceptionNoValue”扩展方法。这将检查 Maybe 以查看它是否有异常。如果是，则返回 NoValue Maybe。否则原始的 Maybe 将被退回。

public static class MaybeLocalExtensions
{
    public static Maybe<T> OnExceptionNoValue<T>(this Maybe<T> maybe)
    {
        return maybe.Exception != null ? Maybe<T>.NoValue : maybe;
    }
}

// Sample Use Case:
var maybe = Maybe.Defer(() => (string)expando.NonExistingProperty).Catch()
    .OnExceptionNoValue();

---

2) 创建一个“BindCatch”扩展方法。这会在出现异常时更改正常绑定(bind)的行为以返回 Maybe.NoValue 而不是抛出异常。

public static class MaybeLocalExtensions
{
    public static Maybe<TResult> BindCatch<T, TResult>(this Maybe<T> @this, Func<T, Maybe<TResult>> selector)
    {
        var self = @this;

        return new Maybe<TResult>(() => {
            if (self.Exception != null)
                return Maybe<TResult>.NoValue;

            return self.HasValue ? selector(self.Value) : Maybe<TResult>.NoValue;
        });
    }
}

// Sample Use Case:
var maybe = Maybe.Defer(() => (string)expando.NonExistingProperty).Catch()
    .BindCatch(m => m.ToMaybe());

---

3) 这种方式也使用了 Catch() 扩展方法，但是使用了 maybe.HasValue 属性，而不是依赖于扩展方法。如果 Maybe 中存在异常，则 HasValue 属性为 false。当此值为 false 时，Maybe.NoValue 可以替换变量的值 maybe 或在这种情况下需要做的任何事情。

dynamic expando = new ExpandoObject();
expando.Name = "John Doe";

// This example falls to the else block.
var maybe = Maybe.Defer(() => (string)expando.NonExistingProperty).Catch();

//In this context I would like the exception which is thrown 
//to result in Maybe<string>.NoValue;
if (maybe.HasValue) {
    //Do something
    Console.WriteLine(maybe.Value);
} else {
    maybe = Maybe<string>.NoValue; // This line is run
}

// This example uses the if block.
maybe = Maybe.Defer(() => (string)expando.Name).Catch();
//to result in Maybe<string>.NoValue;
if (maybe.HasValue) {
    //Do something
    Console.WriteLine(maybe.Value); //This line is run
} else {
    maybe = Maybe<string>.NoValue;
}

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这些答案都是同一主题的变体，但我希望它们对您有所帮助。