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
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