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From: Pierre Weis <pierre.weis@inria.fr>
Message-Id: <200309292110.XAA16929@pauillac.inria.fr>
Subject: Re: [Caml-list] unread_char function
In-Reply-To: <20030929184457.GA31008@bourg.inria.fr> from Basile Starynkevitch at "Sep 29, 103 08:44:57 pm"
To: basile.starynkevitch@inria.fr (Basile Starynkevitch)
Date: Mon, 29 Sep 2003 23:10:53 +0200 (MET DST)
Cc: miles@caddr.com, caml-list@inria.fr
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> On Mon, Sep 29, 2003 at 08:42:13AM -0700, Miles Egan wrote:
> > I need an "unread_char" function to go along with the input_char
> > function for a tool I'm writing in OCaml, but I can't seem to find one
> > in OCaml 3.07beta.  
> 
> Perhaps you might want to use streams? They provide some form of lookahead
> (by factorising common prefixes).
> 
> Ungetting functions might be not always possible in general (i.e.
> considering reading from a raw serial line, or a raw network...).
> 
> -- 
> Basile STARYNKEVITCH -- basile dot starynkevitch at inria dot fr
> Project cristal.inria.fr - phone +33 1 3963 5197 - mobile 6 8501 2359
> http://cristal.inria.fr/~starynke --- all opinions are only mine 

You may also consider scanf and the format %0c that gives you access
to the current character without incorporating it to the current token
(as Basile said, this is some form of lookahead, limited to 1
character as is normally the case for streams).

To give a more concrete example, here is an elementary scanner for
strings, numbers, chars, idents and single character symbols: the
scanneer just decides on the first character (after proper skipping of
spaces) what is the kind of token to read.

let scan_tok sb =
  Scanf.bscanf sb " %0c" (function
  | '"' ->
     Scanf.bscanf sb "%S"
       (fun v -> String v)
  | '0' .. '9' | '-' | '+' ->
     Scanf.bscanf sb "%f"
       (fun v -> Float v)
  | '\'' ->
     Scanf.bscanf sb "%C"
       (fun v -> Char v)
  | 'a' .. 'z' | 'A' .. 'Z' ->
     Scanf.bscanf sb "%[a..zA..Z0..9]"
       (fun v -> Ident v)
  | c ->
     Scanf.bscanf sb "%1c"
       (fun v -> Symbol v));;
val scan_tok : Scanf.Scanning.scanbuf -> tok = <fun>

Cute, isn't it ?

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://pauillac.inria.fr/~weis/


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