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Comment by louthy | Hacker News Reader

C# has native support for monads (LINQ), so it's trivial to implement an Option monad.

Or you can go the whole hog like I have my language-ext project [1][2].

In C# it would look like this:

    var city = from author  in article.author
               from address in author.address
               from city    in address.city
               select city.ToLower();

Which is arguably even more elegant than the Scala 'for' notation.

As an aside, there was a discussion [3] on one of the language-ext issues about my Try<T> implementation, and I was asked to compare it to Scala's Try monad. The crux of it was that it's perfectly possible to capture the majority of the use-cases, but may be of interest in the context of this discussion.

[1] https://github.com/louthy/language-ext

[2] https://github.com/louthy/language-ext/blob/master/LanguageE...

[3] https://github.com/louthy/language-ext/issues/33#issuecommen...

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Implementing a monad is only 30% of the deal.

Being able to abstract over monads (and other type constructors) ... that's the part that really matters.

C# is not expressive enough for that.

It doesn't do higher-kinded polymorphism, no. If you're referring to something else, please elaborate on what it can't express. But for the example given in the blog piece it's exactly as expressive and concise - so it's not giving C# a fair hearing.

It is actually possible to get most of the way to HKT using extension methods. But it involves a hell of a lot of typing. In my project I used a template generator to do it. It's still pretty limited though, and there isn't a single type-class called Monad.

> It doesn't do higher-kinded polymorphism, no.

That's what I referred to.

> ...

The point is that you can't leverage all the existing monad/functor/applicative/free/yoneda... libraries out there–they don't exist, because they can't be written in a way that would allow re-use.

> all the existing monad/functor/applicative/free/yoneda... libraries out there–they don't exist

Agreed. I can only hope one day that the CLR team decide it's a reasonable thing to support so that C# (and F#) can both support it. It seems to be the blockage to the language teams (they won't move without it, even if they could implement it in a weaker/slower way on top of the existing type system). I've jumped through many hoops to get as much of a functional 'base class library' as possible in C# (I needed it for a very large C# project I maintain and want to bring more stability to), but language support would be preferred.

Just out of curiosity what kind of reusable monad libraries are out there for Scala/Haskell? Given the scope of everything between Maybe and IO, I've never understood what one could make that would be useful to "monads" in general (beyond the sugar for do-notation).

https://github.com/scalaz/scalaz

Monad Transformers for one. Traverse/Sequence are another super common (amazing) pattern that requires abstracting over types.

http://eed3si9n.com/learning-scalaz-day12

is a nice tutorial series on it.

Monad transformers in C# [1] (but yes, only works with the monads in the library)

[1] https://github.com/louthy/language-ext/blob/master/LanguageE...

Ah, light bulb moment (perhaps).

So in F# I've been using specialized asyncSeq and asyncMaybe libraries. With monad transformers (and thus HKT's), would those come largely for free?

And you could traverse a list<maybe<list<_>> combo like [Some [Some [1; 2]; None; Some [3]]; None; Some [None; Some [4]]] with no more difficulty than a regular list-of-lists?

Exactly!

You can also write a generic "sequence" over a list, for instance, so that any 'monad' (task/future, maybe/option, state, either/disjunction etc.) can 'swap places'.

So your List<Future<A>> can become Future<List<A>>. But you only need to write it once. Then it doesn't matter what the inner type of the List is, as long as it's a monad (applicative, really, but all monads are applicatives).

Crazy, I'd always considered monad transformers a sore thumb on, well, all that stuff. The fact that monads weren't enough but then you had to go reproduce all that stuff in monad transformers as well. The whole thing just reeked of code duplication and bad smells so I didn't really look into it further.

Now looking at the Scalaz impl, the transformers are actually the core thing, and monads themselves are just transformers over Identity. So it all fits and isn't redundant. Certainly changes a lot of perspective.

That said, I'd still put LINQ as 99% of "the deal" per the original comment. The extra abstractions allowed by HKT's, while "necessary" mathematically, still only save me about 1% of the work required to implement asyncSeq and asyncMaybe, and in 15 years of pro experience those are the only real uses of HKT's I've ever had the need for.

For one example, in Haskell, all Monads are also Functors. This means you can `fmap` a function over a Maybe Foo just as you would over a [Foo] (read list of Foo) or Vector Foo or indeed an IO Foo.

lens, for example (which is not really a "reusable monad library" but very much of that flavour).

Trivial to implement but not compatible if we both implemented. That's the main benefit of Scala Optional, its baked-in -- I don't have to worry about writing adapter to use your library.

Agreed, there is no functional base with C# aside from LINQ. So if you want to do functional in C# you would have to chose a library, and that's why I went so far building the rest of the functional infrastructure around it. It'd be insane to use multiple Option implementations. My first attempt at bringing common monad support in C# [1] suffered from the problems you state.

But that still doesn't change the crux of the blog post, it's disingenuous comparing the Option monad to a null coalescing operator.

[1] https://github.com/louthy/csharp-monad

[Author] I wasn't trying to say that Option monad is better/worse than null conditional. My point was to compare two different approaches to the same problem.

This article is timely for us as we're using Scala for backend work.

Ruby 2.3 &. operator and Swift can do something like this pseudocode:

    if let city = article?.author?.address?.city?.ToLower() {
       // city present
    }

Too long?:

    if let city = article?
      .author?
      .address?
      .city?
      .ToLower() {
        // city present
    }

All are improvements over pyramids of doom.

(Be aware of subtle gotcha in Scala of var and val, really prefer Rust/Go/Swift keywords.)

I have a standalone implementation here as well: https://github.com/jb55/Data.Maybe.cs

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