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Comment by revelation | Hacker News Reader

C# can do this: http://dotnetfiddle.net/dCm475

(Sorry, it's just a not-so-well-known but awesome feature)

Now here's the same example in F# (a functional language); it compiles into IL similar to that produced by your C# code. Which do you find more readable?

    module Program =
        let n = 42
    
        let rec sqrtGuesses x = seq {
            yield x
            let next_x = 0.5 * (x + (float n / x))
            yield! sqrtGuesses next_x }

        sqrtGuesses 1.0
        |> Seq.pairwise
        |> Seq.pick (fun (x, y) ->
            if abs (x - y) < 1E-10 then Some y else None)
        |> System.Console.WriteLine
        
        System.Console.WriteLine (sqrt (float n))

It's a wash. To understand that thing you posted, I'd have to look up what the exclamation point does to "yield", why adding it forces you to repeat the function name, and whether those "|>" sequences are typos or some weird operator. The other version has some random StudlyCaps, I have to look up. Meh.

Well it's only a wash if you are familiar with one and not the other (and that's the case IIUC). You don't intuitively know what the C# version does either.

For future reference "|>" like "$" in haskell is just short hand for a start and end parenthesis and end of expression or next instance of "|>" or "$".

So in Haskell:

    sum $ filter (> 2) [0..10]

is a less noisy way of saying:

    sum(filter (>2) [0..10])

About the exclamation points, I believe it causes it to evaluate and has to do with ensuring the expression is evalauted. At least that is the case in haskell.

Imperative / shell programmers may be more familiar with |> as a pipe.

  echo "Hello World" | wc -c

  let wc x:String =
    x.length
  "Hello World" |> wc

are equivalent

|> would be the flip of $ since the arg comes before the function

So equivalent to Control.Lens.(&)

yield and yield! are both keywords used in sequence expressions. To compute a sequence of type seq<'T>, you can use a sequence expression, in which the yield keyword takes a value of type 'T, and adds that value to the computed sequence, while yield! takes a value of type seq<'T> and appends that value to the computed sequence.

You also have keywords "return" and "return!" for F#'s version of monads. In Haskell, "return" is just a function, while "return!" is not needed (though I wish it was present in Scala).

The function name is repeated because it's recursive!

In F#, yield emits a new element in the sequence (just as in C#), while yield! emits all of the elements from another sequence you provide. This effectively allows you to "splice" sequences together, or create a new sequence by prepending/appending some elements to an existing sequence.

As another commenter mentioned below, yield! has nothing to do with repeating the function name -- the name is repeated because it's a recursive function (i.e., a function which calls itself). So the sqrtGuesses function produces a sequence which emits (yields) the value it's given (x), computes the next value of x, stores it in the variable next_x, then emits all of the values in the sequence produced by calling sqrtGuesses for the next value of x (next_x).

The |> is called the "forward pipeline operator" in F#, and it's very simple -- whereas you normally call a function f with a value x by writing f x, you can swap the order of the arguments with |> to write x |> f. This effectively allows you to write nested function applications without all of the parentheses: f(g(h(x)) can instead be written x |> h |> g |> f or h x |> g |> f or whatever.

You could use `Seq.scan` to get rid of the manual recursion:

    let n = 42.
        
    Seq.initInfinite float 
    |> Seq.scan (fun x _ -> 0.5 * (x + n/x)) 1.0 
    |> Seq.pairwise
    |> Seq.pick (fun (x, y) -> if abs <| x-y < 1E-10 then Some y else None)
    |> printf "%f"

    printf "%f" <| sqrt n

Thank you for posting this.

When I saw the original version in F# I thought "I think I can do better" after a long absence from using F#. When I got back to my laptop and searched for the to-be-improved snippet I saw yours.

Now I'm hooked again.

Note given appropriate `MyLinqExt.Scan` and `MyLinqExt.Pairwise`, you could write this in C# just as concisely.

Hmm.. No. :) I guess that's discussed all over the thread here. I do like C# in general and agree that it can be used in a functional style, but 'just as concisely'? Can't agree with that.

Needs the boilerplate (class definition, Main vs. .fsx for example), most keywords/methods seem to be longer (especially the Linq ones, SelectMany, FirstOrDefault etc) and there's no inherent way to build a pipeline as in "no |>".

Obviously you can write quite similar code in C# and I would agree that C# doesn't make it especially hard, is even a roughly decent language for that sort of stuff. Still, the F# snippet looks a lot nicer to me.

Which leads to .. Just a matter of preference, ymmv etc - there's no claim of superiority here.

For those complaining about the verbosity of c#, here is an alternate c# representation:

    public static void Main()
    {
        const int n = 21;

        Observable.Generate<double,double>(1, x=>true, x=> 0.5 * (x + (n/x)), x=>x )
        .Scan(new {prv = 0d, cur = 0d}, (prev, curr) => new {prv = prev.cur, cur = curr})
        .FirstAsync(_ => Math.Abs(_.cur - _.prv) < 1E-10)
        .Select(_ => _.cur)
        .Subscribe(Console.WriteLine);

        // this is just to compare values, so is not part of the solution
        Console.WriteLine(Math.Sqrt(n)); 
        Console.ReadLine();
    }

That's is, everything you need to run the program (technically one line of code, line breaks are for readability). And its non-destructive, has no side effects and is pretty easy to read.

I would never argue that c# is a 'proper' functional language, but the poster above wrote in the style of idiomatic java, something a lot of c# devs do. I don't think c# has an 'idiomatic'. It is very flexible, and while never truly functional, can be also be used like a scala.

Having someone re-write my 7 line Haskell program into 40+ lines of C# is the best argument I can think of for why it's worth learning a functional programming language ;)

This kind of lying (why count the usage example?) and gloating attitude is what I find repulsive in Haskell community.

Downvoted. I think the ";)" indicates pretty well that this is tongue in cheek. I wouldn't draw any conclusions about the Haskell community, which is very nice, from this.

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