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?

  • \$\begingroup\$ 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. \$\endgroup\$ Sep 22, 2016 at 20:40

2 Answers 2


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 ()
  • \$\begingroup\$ 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. \$\endgroup\$
    – Dan
    Jul 10, 2015 at 16:59
  • \$\begingroup\$ In Visual Studio 2012, I'm getting a type constraint mismatch on the yield! readlines () line. Any thoughts as to why? \$\endgroup\$ Sep 22, 2016 at 17:34
  • \$\begingroup\$ @octopusgrabbus What does the error message say? \$\endgroup\$ Sep 22, 2016 at 17:41
  • \$\begingroup\$ Must have been a weird alignment thing. The error is gone. Thanks for inquiring. \$\endgroup\$ Sep 22, 2016 at 17:44

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
  • 1
    \$\begingroup\$ Thanks for a very helpful alternative. I was also searching for a non-recursive function example using the Seq methods. \$\endgroup\$ Sep 22, 2016 at 18:26

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