F# RX Projection [closed] - f#

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Closed 11 years ago.
Want to convert some C# code for RX to F# code.
The following C# code works well:
var seqNum = Observable.Range(1, 5);
var seqString = from n in seqNum
select new string('*', (int)n);
seqString.Subscribe(str => { Console.WriteLine(str); });
Console.ReadKey();
The following is my code in F#:
#light
open System
open System.Collections.Generic
open System.Linq
open System.Reactive
open System.Reactive.Linq
open System.Reactive.Subjects
open System.Threading
open System.IO
let seqNum = Observable.Range(1, 5)
let seqString = from n in seqNum
select new string('*', (int)n)
Console.ReadLine() |> ignore
But I got the following compiler error:
Error: Unexpected keyword 'new' in implementation file
If I deleted the new keyword, I got another error:
Error: Successive arguments should be separated by spaces or tupled, and arguments involving function or method applications should be parenthesized
The "new" keyword are totally different in C# and F#.
Please show me how to do the same job in F#.
Thanks,

Answer 1

In C# string is the shortcut to System.String class. However, in F# string is a function which has obj as its input and returns a string which is overriden in obj.ToString():
let s = string('*', 3);; // tuple to string
// val s : string = "(*, 3)"
What you really want is creating a string by repeating '*' three times:
let s = new String('*', 3)
// val a : String = "***"
To be clear, from ... in ... select ... is C# LINQ syntax which is invalid in F#. Therefore, using computation expression instead:
let seqNum = seq {1..5}
let seqString = seq { for n in seqNum -> new String('*', n) }
To get some ideas of creating/using computation expression for Reactive Extension, take a look at the question and its answers at How do I change the Rx Builder implementation to fix the stack overflow exception?

Answer 2

Instead of using the String constructor use the String.replicate method.
String.replicate n "*"
There is no direct equivalent for String(char, int) but String.replicate: int -> string -> string is roughly the equivalent with string instead of char
F# version for that code
[1 .. 5]
|> Seq.map (fun i -> String.replicate i "*")

Answer 3

Here you go:
open System
open System.IO
open System.Reactive
open System.Reactive.Linq
let seqString = Observable.Range(1,5).Select(fun x -> String.replicate x "*")
using (seqString.Subscribe (fun x -> printfn "%s" x))
(fun _ -> Console.ReadLine() ) |> ignore
EDIT: As Paul suggested below two last lines can be replaced by simple
seqString.Subscribe (printfn "%s") |> ignore
However, if we want to gracefully unsubscribe from our subscription, but get rid of using in lieu of newer use syntax we may replace last two lines by the following three
do
use subscription = seqString.Subscribe(printfn "%s")
Console.ReadLine() |> ignore