How to call System.IO.TextWreader.ReadLine to read stdin until EOF?

Question

I'm new to F# and I'm trying to translate this C# code:

  static System.IO.TextWriter outp = System.Console.Out;
  static System.IO.TextReader inp = System.Console.In;
   string line;
      while ( (line = inp.ReadLine()) != null )
        outp.WriteLine("echo data from stdin: "+ line);

I'm not sure how to do the while loop in F# so it reads the line variable and also terminates on EOF.

Can someone translate this for me?

Thanks

Siegfried

---

siegfried heintze

Answer 1

> let lines =
-     Seq.initInfinite (fun _ -> System.Console.In.ReadLine())
-     |> Seq.takeWhile(fun line -> line <> null);;

val lines : seq<string>

> for line in lines do printfn "%s" line;;
test
test
opr
opr
mmt
mmt
^D
♦
^Z
val it : unit = ()
>

Answer 2

This works. However, the statement

for line in lines do printfn "%s" line

only works once! Why cannot I print out the contents of lines twice using the exact same statement? Why is not not necessary to declare line as mutable?

Thanks

Siegfried

---

siegfried heintze

Answer 3

My example is a lazy sequence. It is recalculated each time you try to enumerate over it.

To use input multiple times you must cache it via Seq.cache.