How to pass a printf-style function to another function in F#

Question

I\'d like to make a function in F# that accepts a printf-style function as an argument, and uses that argument to output data. Usage would be something like the following:

Answer 1

The problem is that when you say (output : Printf.TextWriterFormat<'a> -> 'a), that means "there is some 'a for which output takes a Printf.TextWriterFormat<'a> to an 'a". Instead, what you want to say is "for all 'a output can take a Printf.TextWriterFormat<'a> and return a 'a.

This is a bit ugly to express in F#, but the way to do it is with a type with a generic method:

type IPrinter =
    abstract Print : Printf.TextWriterFormat<'a> -> 'a

let OutputStuff (output : IPrinter) =
    output.Print "Header"
    output.Print "Data: %d" 42

OutputStuff { new IPrinter with member this.Print(s) = printfn s }