How to put a literal percent sign (%) inside F#'s printf format strings?

Question

I want to print a % sign using F#'s printfn function. After googling the format syntax, this should do the trick: printfn "%%". Apparently not...

F# interactive output:

> printfn "%%";;
%%
val it : unit = ()

Weird...

I'm using F#3.1 and .NET 4.5, the F# interactive session uses .NET 4.0. Same thing.

For reference: printfn "%" doesn't compile (missing format specifier) and printfn "%s" "%" is my current workaround...

Update:

When I change the target F# runtime from 3.1 to 3.0, it works. Is this a bug in the 3.1 runtime?

Seems to be working for me too: http://pastebin.com/iz6yWzMR but I think this might have been a bug in earlier version - what's your exact version of FSI? (Mine prints "12.0.30110.0") — Tomas Petricek, Jun 02 '14 at 21:36
My version of FSI is 12.0.21005.1; http://pastebin.com/HdYtULG6 But compiling it into an exe doesn't work, either... — Nikon the Third, Jun 02 '14 at 21:39
FWIW, It works on dotnetfiddle: https://dotnetfiddle.net/bhg9b5 — Robert Jeppesen, Jun 02 '14 at 21:44
Oh wow. When I change to F# 3.0 in the project settings, it works. This seems to be a F# 3.1-only thing... — Nikon the Third, Jun 02 '14 at 21:53
I noticed I don't have VS2013 Update 2 installed on my machine. I'm going to update now to see if this fixes things in case it's just an old bug like Tomas mentioned... — Nikon the Third, Jun 02 '14 at 22:01

score 9 · Accepted Answer · answered Jun 02 '14 at 23:17

If printfn "%%" outputs two percent signs (%%) instead of one percent sign (%), then you have to update F# 3.1 to at least version 3.1.1.

If you are using Visual Studio 2013, you can do this via Tools → Extensions and Updates → Updates → Visual Studio Gallery → Visual FSharp Tools.

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