在C＃中使用Monads的令人信服的场景是什么？

Question

Let me state up front that I have an infantile understanding of Monads. I have read the various threads on Monads here and have done a few hours of study on the concept. I hardly feel comfortable with the term, but I think it is safe to say that I generally understand what a Monad is/does.

I'm a C# developer who is looking to improve the way I work. What would help me further in my Monaducation is see a real world application of a Monad in C# (ie via a linq SelectMany() or somesuch) that is clearly an improvement over other ways of solving the same sort of problem in oldskool C#.

Has anyone seen such a beast?

Answer 1

Here is one such scenario: you want to author a parsing library (a nice example of an embedded DSL), and you discover that the best ones are monadic parser combinator libraries. So you write it leveraging LINQ syntax sugars to author C# code that has the same structure as the grammar of the language you're parsing, and you get the benefits of an awesome programming model for on-the-fly semantic analysis and error-recovery. See this blog for a description.

Answer 2

Find Pythagorean triples:

  var r = from a in Enumerable.Range(1, 25)
          from b in Enumerable.Range(a, 25-a)
          from c in Enumerable.Range(b, 25-b)
          where a*a + b*b == c*c
          select new [] { a, b, c };

Answer 3

Here is one such scenario: you want to write code that makes sequential async calls (eg IO) without holding threads, but you don't want to write the hopeless tangle of spaghetti that the async programming model (BeginFoo/EndFoo) forces you into. So you can use a monad and LINQ sugars and write code that looks straight-line but it releases/switches threads throughout. See this blog for a short description.

Answer 4

一个例子是使用Maybe monad简化null检查，如本文所示。

Answer 5

看看http://memoirsofaprogrammer.blogspot.com

Answer 6

Programming with monads is declarative, describing what you want at a high level rather than the low-level details of how to generate it.

See the exercises at the end of Brian Beckman's state-monad talk on Channel 9 .

Answer 7

I recently blogged about refactoring a typical imperative real-world C# code (a function in NuGet ) to functional, monadic style (more concretely, using the Maybe monad ). I did my best to do it in little steps, explaining the rational behind step, so I think it helps in understanding how monads are useful.

Answer 8

LINQ is used in many solutions (and often requested in questions) here on StackOverflow. Review questions with the LINQ tag and you will see real world usage.