Block comments and `read-hash-extend'

[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  <address@hidden>

        * 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);