From: "Nicolas Pouillard" <nicolas.pouillard@gmail.com>
To: Brian Hurt <bhurt@janestcapital.com>
Cc: Caml_mailing list <caml-list@yquem.inria.fr>
Subject: Re: [Caml-list] ifprint and format type question
Date: Thu, 24 Apr 2008 10:39:04 +0200 [thread overview]
Message-ID: <1209026061-sup-6115@ausone.inria.fr> (raw)
In-Reply-To: <480F9F45.4040101@janestcapital.com>
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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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prev parent reply other threads:[~2008-04-24 8:39 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-04-23 20:42 Brian Hurt
2008-04-24 8:39 ` Nicolas Pouillard [this message]
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