If you’re not familiar with what FizzBuzz is, check out this blog post by Jeff Atwood (Coding Horror). FizzBuzz is sort of that bare-minimum-entry-level-bar that when you can clear you know you’ve at least understood the basic grammar of a language. Typically I find in learning languages what takes the most time is really learning the standard library and built in functions.

F# has proven to be no exception to this and even though it has the entire .NET framework library behind it, there were some quirky things there to trip me up. A great example is the printfn function.

Consider the following, which, though it seems like a trivial bit of code is actually wrong:

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let x = “Hello World”
printfn x

Running the above will thrown an exception…

The type ‘string’ is not compatible with the type ‘Printf.TextWriterFormat<’a>’

Really? A string can’t be printed out? There’s a question on StackOverflow which summarizes why this happens, but the short version is that printfn expects input to be of type TextWriterFormat<T>. Right.

So the fix for the code above would be:

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let x = “Hello World”
printfn “%s” x

I’ve been trying to write FizzBuzz in F# now for a week and while I probably have had it correct for a while, the printfn issue has been throwing me off this whole time.

In any case, here is FizzBuzz in F# (where the side effect of printing has been isolated to outside of the function):

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let fizzbuzz n =
    match n with
    | n when n % 15 = 0 -> “FizzBuzz”
    | n when n % 5 = 0  -> “Buzz”
    | n when n % 3 = 0  -> “Fizz”
    | _                 -> n.ToString()

[1 .. 100] |> Seq.iter(fun x -> printfn “%s” (fizzbuzz x))