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Flushing F# Interactive output on something other than newline.
f#

I've noticed the fsi at least in Visual Studio 2008 won't flush stdout to the screen until it encounters a newline. Is there some way to force the interactive session to do this before it encounters a newline?

For example, this code snippet prints in an awkward fashion. Instead of printing the integer, ellipsis, immediately and done after one second, it prints the whole line after one second. Since I want the ellipsis to convey a computation in progress, I want it to show up immediately, then after the computation is finished print "done".

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for i in seq { 1..100 } do printf "%d..." i; Thread.Sleep(1000); printfn "done";;

I've tried stdout.Flush(). Any suggestions? Anyone else encounter this problem and find a solution?

If such a function could force the flushing, I could define a printfThenFlush function a la:

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let printfThenFlush format = Printf.kprintf (fun _ -> (*force output to screen*)) format
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I've tried your sample in fsi running in Emacs, and it works fine; this ellipsis is printed immediately. I guess it's a problem with the VS plugin (could you try and run the compiled version?), and you won't find a function that fixes your problem.

By on 11/7/2009 6:44 PM ()

Thanks, yes... also works for me in the console fsi, and as a ".exe". Only a problem within VS.

It also works without the stdout.Flush

(for i in seq { 1..20 } do printf "%d..." i; Thread.Sleep(1000)); printfn "done";;

By on 11/7/2009 6:58 PM ()

Thanks for the help. I'll send a report to the F# team. I'd like the VS plugin to either pickup the behavior seen in fsi.exe or a compiled F# program or they could implement some kind of function to force output to the screen.

By on 11/8/2009 1:46 PM ()

I don't have a solution - just posting to confirm the problem.

i tried:

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(for i in seq { 1..20 } do printf "%d..." i; Thread.Sleep(1000); stdout.Flush()); printfn "done";;

I changed it to include the stdout.Flush, and also bracket the for loop so to exclude the printfn

Yep - nothing is printed until the printfn.

By on 11/7/2009 6:29 PM ()

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