Reading input from console in F# (as a sequence of lines)

Question

Consuming input line-by-line until the end is often useful. In C# I would write the following loop:

while ((line = Console.ReadLine()) != null) {
 // ...

I wanted to do this using "idiomatic" F# (which to me includes avoiding let mutable) and I came up with this:

let rec readlines = seq {
 let line = Console.ReadLine()
 if line <> null then
 yield line
 yield! readlines
}

The last line shows the following warning

Warning FS0040: This and other recursive references to the object(s) being defined will be checked for initialization-soundness at runtime through the use of a delayed reference. This is because you are defining one or more recursive objects, rather than recursive functions. This warning may be suppressed by using '#nowarn "40"' or '--nowarn:40'.

The warning would suggest I am actually not writing idiomatic F# (as warnings usually point out problematic code). Is there a better way to write the function that avoids the warnings?

• octopusgrabbus: How are you invoking this function? The rules for code review don't allow me to post my version of this with the () addition in @MarkSeeman's answer.

Answer 1

What you have there is a value, but you probably want it to be a function:

let rec readlines () = seq {
 let line = Console.ReadLine()
 if line <> null then
 yield line
 yield! readlines ()
}

• Dan: was this meant to be yield line instead of yield v? There is a pending edit on this post by another user making the change to yield line and I want to make sure before accepting it.
• octopusgrabbus: In Visual Studio 2012, I'm getting a type constraint mismatch on the yield! readlines () line. Any thoughts as to why?
• Mark Seemann: @octopusgrabbus What does the error message say?
• octopusgrabbus: Must have been a weird alignment thing. The error is gone. Thanks for inquiring.

Answer 2

Mark's response does answer as to the warning you received.

An alternate approach could be

Seq.initInfinite // (1)
 (fun _ -> Console.ReadLine())
|> Seq.takeWhile (fun line -> line <> null) // (2)

Edit: Could be restructured and more succinct as

 fun _ -> Console.ReadLine()
 |> Seq.initInfinite // (1)
 |> Seq.takeWhile ((<>) null) // (2) (3)

1. Seq.initInfinite<'T> Function (F#)
2. Seq.takeWhile<'T> Function F#
3. Partial Application

• octopusgrabbus: Thanks for a very helpful alternative. I was also searching for a non-recursive function example using the Seq methods.