Block comments and `read-hash-extend'

[ludovic.courtes@laas.fr (Ludovic Courtès)]

Hi,

The patch below (1) fixes `#! ... !#' block comments and (2) allows to
override them with `read-hash-extend'.


(1)  This example makes Guile 1.6 and the current Guile 1.7 hang (for
     some reason which I did not track down):

       guile> #! this is a comment !# (+ 2 2)
       [wait forever]

     Once the patch is applied:

       guile> #! this is a comment !# (+ 2 2)
       4

     I guess block comments have not been widely used over the past
     years.  ;-)


(2)  It makes it possible to discard the block comment syntax with
     `read-hash-extend'.  The example below allows to recognize the
     DSSSL keyword syntax[*], at the expense of losing the block
     comment syntax:

       (read-hash-extend #\! (lambda (chr port)
                               (symbol->keyword (read port))))

     I find this particular example useful since, even if DSSSL is
     apparently not widely used, other Scheme implementations such as
     Bigloo do support this syntax, and allowing such extensions is what
     `read-hash-extend' is for.


Comments?

Thanks,
Ludovic.

[*] http://www.ibiblio.org/pub/sun-info/standards/dsssl/dssslo/do960816.htm


2005-06-16  Ludovic Courtès  <ludovic.courtes@laas.fr>

	* read.c (scm_flush_ws):  Do not handle `#! .. !#' block
	comments here so that the behavior of `#!' can be extended with
	`read-hash-extend'.
	(scm_lreadr):  Handle the `#!' case here instead of in
	`scm_flush_ws ()' after making sure that no hash reader
	extension exists for `!'.  Jump to `tryagain' after call to
	`skip_scsh_block_comment ()', which fixes a bug in block
	comments handling (where Guile would hang).



Index: read.c
===================================================================
RCS file: /cvsroot/guile/guile/guile-core/libguile/read.c,v
retrieving revision 1.117
diff -u -B -b -p -r1.117 read.c
--- read.c	23 May 2005 19:57:21 -0000	1.117
+++ read.c	16 Jun 2005 09:13:28 -0000
@@ -227,9 +227,6 @@ scm_flush_ws (SCM port, const char *eofe
 	  case EOF:
 	    eoferr = "read_sharp";
 	    goto goteof;
-	  case '!':
-	    skip_scsh_block_comment (port);
-	    break;
 	  default:
 	    scm_ungetc (c, port);
 	    return '#';
@@ -478,9 +475,10 @@ scm_lreadr (SCM *tok_buf, SCM port, SCM 
 	  goto num;
 
 	case '!':
-	  /* should never happen, #!...!# block comments are skipped
-	     over in scm_flush_ws. */
-	  abort ();
+	  /* Only handle `#! ... !#' block comments if no user extension was
+	     defined for `!' using `read-hash-extend'.  */
+	  skip_scsh_block_comment (port);
+	  goto tryagain;
 
 	case '*':
 	  j = scm_read_token (c, tok_buf, port, 0);



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