Question

Since I do research with F# (in particular, using F# interactive), I’d like to have switchable “print-when-in-debug” function.

I can do

let dprintfn = printfn

F# interactive says

val dprintfn : (Printf.TextWriterFormat<'a> -> 'a)

and I can use

dprintfn "myval1 = %d, other val = %A" a b

whenever I want in my scripts.

Now I’d like to define dprintfn differently, so that it would ignore all its arguments yet being syntax-compatible with printfn. How?

---

The closest (yet non-working) variant I have in mind is:

let dprintfn (arg: (Printf.TextWriterFormat<'a> -> 'a)) = ()

but it the following doesn’t compile then dprintfn "%A" "Hello", resulting in error FS0003: This value is not a function and cannot be applied.

P.S. I currently use an alias for Debug.WriteLine(...) as work-around, but the question is still interesting for understading F#’s type system.

Answer 1

You can use the kprintf function, which formats a string using the standard syntax, but then calls a (lambda) function you specify to print the formatted string.

For example, the following prints the string if debug is set and otherwise does nothing:

let myprintf fmt = Printf.kprintf (fun str -> 
  // Output the formatted string if 'debug', otherwise do nothing
  if debug then printfn "%s" str) fmt