Question

I want to add debug printing to my project with a function having a type signature something like:

bool -> Printf.TextWriterFormat<'a> -> 'a

i.e. it should take a bool indicating whether or not we are in verbose mode, and use that to take the decision about whether to print or not.

For example, lets say dprint : bool -> Printf.TextWriterFormat<'a> -> 'a then I would like this behaviour:

> dprint true "Hello I'm %d" 52;;
Hello I'm 52
val it : unit = ()
> dprint false "Hello I'm %d" 52;;
val it : unit = ()

The idea is that a command line flag can be used to avoid control this output. I also want to avoid a runtime cost in the “not verbose” case. It is possible to define a function that works like this using kprintf:

let dprint (v: bool) (fmt: Printf.StringFormat<'a,unit>) =  
  let printVerbose (s: string) =
    if v then System.Console.WriteLine(s)

  fmt |> Printf.kprintf printVerbose

but printing/ignoring a sequence of numbers with List.iter (dprint b "%A") [1..10000] (b \in {true,false}) takes amount 1.5s for both values of b on my machine.

I came up with another method using reflection that builds an appropriately typed function to discard the formatting arguments:

let dprint (v: bool) (fmt: Printf.TextWriterFormat<'a>) : 'a =
  let rec mkKn (ty: System.Type) =
    if FSharpType.IsFunction(ty) then
      let _, ran = FSharpType.GetFunctionElements(ty)
      FSharpValue.MakeFunction(ty,(fun _ -> mkKn ran))
    else
      box ()
  if v then
    printfn fmt
  else
    unbox<'a> (mkKn typeof<'a>)

but here the reflection seems too expensive (even more so than that done inside the standard libraries complicated definition of printf sometimes).

I don’t want to litter my code with things like:

if !Options.verbose then
    printfn "Debug important value: %A" bigObject5

or closures:

dprint (fun () -> printfn "Debug important value: %A" bigObject5)

so, are there any other solutions?

Answer 1

I like your solution using reflection. How about caching it on the type level so that you pay the price of reflection only once per type? For example:

let rec mkKn (ty: System.Type) =
    if Reflection.FSharpType.IsFunction(ty) then
        let _, ran = Reflection.FSharpType.GetFunctionElements(ty)
        // NOTICE: do not delay `mkKn` invocation until runtime
        let f = mkKn ran
        Reflection.FSharpValue.MakeFunction(ty, fun _ -> f)
    else
        box ()

[<Sealed>]
type Format<'T> private () =
    static let instance : 'T =
        unbox (mkKn typeof<'T>)
    static member Instance = instance

let inline dprint verbose args =
    if verbose then
        printfn args
    else
        Format<_>.Instance

A pragmatist would just use the fast C# formatted printing machinery instead of this. I avoid Printf functions in production code because of the overhead they have, as you point out. But then F# printing definitely feels nicer to use.

My #time results for List.iter (dprint false "%A") [1..10000]:

• Original version : 0.85
• Original version with reflection : 0.27
• The proposed version : 0.03