Re: [Caml-list] format type

[Cezary Kaliszyk <ck189400@zodiac.mimuw.edu.pl>]

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On Mon, Nov 04, 2002 at 08:54:13AM +0100, Alessandro Baretta wrote:
> >>From: Cezary Kaliszyk <ck189400@zodiac.mimuw.edu.pl>
> >>Date: Sun, 3 Nov 2002 08:08:58 +0100
> >>To: caml-list@inria.fr
> >>Subject: [Caml-list] format type
> >>
> >>I'm trying to write a function that takes a format and a function and
> >>applies some (got from elsewere) arguments to the function.
> >>
> >><snip>
> >>
> >>The only known workaround for me for now is to pass "%t" and
> >>make my function not (unit -> unit) but ('a -> unit).
> 
> As far as I can see from the docs, "%t" is only meaningful 
> with the Printf module. I see no mention of it in Scanf. 
> What exactly are you trying to do anyway?
I am trying to create a protocol for passing messages via network.
Each message may be acompanied by a certain amount of parameters
(of type char, int, float or string) (the parameters are dependant on the
message).

output/input_value aren't apropriate. (For network speed reasons, and no
type extensibility)

I am writing a function which adds a message to the protocol, which takes the
type of arguments accompanying the message and the receiving function.
It should return function used to send this message type. Eg:

let two_char_sender = add_to_protocol "%c%c" two_char_receiver;;

Where two_char_receiver : (char -> char -> unit)
And   tho_char_sender : (char -> char -> unit).

And I'd like the types to be controlled.

And I've written it. But it doesn't work if I want to create a message
that does not take any parameters. And I suppose the builtin type format
doesn't have the functionality necessary to write it. 
> Assuming you are talking about the Scanf module, I'd say you 
> can't. Of course, if your function takes a unit input, it 
> can very well be a perfectly polymorphic function (? la 
> ignore) with type ('a -> unit). In which case you can force 
> a call to such function while scanning the input buffer by 
> passing it a range conversion with an empty range: 
> "%[^\000-\255]". Your function would be called with an empty 
> string as an actual parameter.
I'm not talkin about Printf nor Scanf, but about my module that uses the
format type.
> Or, better yet, you can use the "%N" conversion, which 
> passes the number of characters consumed. Just discard this 
> value.
As above.
> >>Writing code with changed types just for the sake of the language is
> >>very bad. And with "%t" 'a seems to be (unit -> unit).
> 
> It is also very bad to complain without doing any work 
> yourself. You are probably the only one on this list wishing 
> to call a function with no input while parsing a buffer. 
I'm not parsing a buffer...
> Nothing wrong with this, apart from stylistic 
> considerations, but it implies two things: 1) I don't 
> suppose Pierre ever even thought of such a conversion for 
> Scanf, and 2) even if he did, I doubt he'd care to implement 
> it. If you really think it is necessary, you might consider 
> hacking it into the Scanf module and submitting the patch to 
> the maintainers.
It's not scanf relative...
> Alex
The format argument is parsed by me, not by scanf or printf (as I wrote
in the example code) and I don't know where in ocaml code is type
checking of the arguments accompanying the format type.

Cezary Kaliszyk
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