[lkitching]

The Haskell version of printf does not statically analyse the format string and will raise an exception if the provided arguments are invalid e.g.

    Prelude Text.Printf> printf "%d\n" "invalid"
    *** Exception: printf: bad formatting char 'd'

The behaviour of F#'s printfn is hard-coded in the compiler and could not be written in F# iteself.