[Caml-list] unread_char function

[Caml-list] unread_char function

[Miles Egan]

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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.  Was it excluded for a technical reason?  If not, it
looks like it would be fairly easy to add.  Would there be any
objections to adding it?

-- 
Miles Egan <miles@caddr.com>

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Re: [Caml-list] unread_char function

[Basile Starynkevitch]

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 

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Re: [Caml-list] unread_char function

[Pierre Weis]

> 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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Re: [Caml-list] unread_char function

[Miles Egan]

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On Mon, 2003-09-29 at 14:10, Pierre Weis wrote:
> 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>

I'm trying to write a function that seeks to an arbitrary string
delimiter in an input stream.  It's not immediately obvious to me how
I'd do this with a parser but I'll poke around with it.

-- 
Miles Egan <miles@caddr.com>

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