While F# can easily access the standard .NET formatting functions (e.g. String.Format()), it also provides its own set of functions for outputting formatted text. In fact, F# offers the a printf-based family of functions that should be familiar to C programmers. Consider the following simple example using F#'s interactive environment.

Most of these formatting functions also have an additional "n" version that implicitly adds a new-line character. For example, we could modify the above code to use printfn like so:

Of course, using an invalid argument will result in an error. Notice what happens if we pass 12 instead of 12.8 for the %f format specifier:

What should give .NET developers pause is the fact that the error above does not occur at runtime. This isn't some exception being thrown—it's a compiler error. In other words, the compiler actually parses and type-checks format strings!

This behavior becomes more useful inside of Visual Studio. When a type mismatch occurs within a format string, the F# background compiler marks the problem with a red squiggly underline:

Hovering the mouse over the error will show a tooltip containing the same message that the interactive environment displayed.

This is another example of how F# is extremely statically-typed. The F# compiler works to make even format strings type-safe.