Excerpts from Brian Hurt's message of Wed Apr 23 22:42:45 +0200 2008:
> So, I'm trying to write code like (simplifying):
> 
> let my_output (_: string) = ();; (* the real code is much more 
> complicated but not relevant *)
> 
> let foo b fmt =
>     if not b then
>        Printf.ifprintf () fmt
>     else
>        Printf.ksprintf my_output fmt
> ;;

ifprintf works well with fprintf

let foo b fmt =
   if not b then
      Printf.ifprintf oc fmt
   else
      Printf.fprintf oc fmt
;;

Otherwise  using  Format.ifprintf  could help due to its generalized notion of
formatter.

> The problem is that the above code doesn't compile- ifprintf wants fmt 
> to be ('b, unit, unit) format = ('b, unit, unit, unit) format4, while 
> ksprintf wants it to be ('b, unit, string, 'a) format4.  Now, I could do 
> the above like:
> 
> let foo b fmt =
>     Printf.ksprintf (fun s -> if b then my_output s) fmt
> 
> but the point and purpose of using ifprintf is to avoid the cost of 
> converting the arguments to strings that are just going to be thrown away.

Yes this defeats the purpose.

> So, my questions are:
> 
> 1: is there a way to make this work without using Obj.magic or 
> rewritting isprintf?

With Printf.ksprintf I would say no.

> 2: is there a reason ifprintf has the type 'a -> ('b, 'a, unit) format 
> -> 'b, instead of ('b, 'a, 'c) format -> 'b, or better yet ('b, 'a, 'c, 
> 'd) format4 -> 'b, or even better yet ('b, 'a, 'c, 'd, 'e, 'f) format6 
> -> 'b (allowing it to unify with more different formats)?

Hum there perhaps room for a more general ifprintf.

> 3: Does ifprintf actually avoid the cost of converting it's arguments to 
> strings?  The code is unclear.  If the answer to this is 'no', the other 
> two questions are moot.

Yes it does avoid the cost of converting it's arguments.

-- 
Nicolas Pouillard aka Ertai