Subject: bug in SRFI-30 comment parser

From: Bruno Haible <[email-masked]>
Date: Tue, 4 Oct 2011 23:45:19 +0200

Hi,

Block comments whose contents end with an odd number of vertical bars are not
parsed correctly by guile-2.0.2.

The guile manual [1] says:
  "Guile (starting from version 2.0) supports nested block comments
   as specified by R6RS and SRFI-30"

R6RS [2] says:
  "Block comments may be indicated with properly nested #|and |# pairs."

More formally, SRFI-30 [3] implies that in particular

  #| <comment-text> |#

is a comment, where

  <comment-text> ---> <character sequence not containing #| or |#>

Now look at this guile session: When I use a <comment-text> that ends in
1 or 3 or 5 ... trailing vertical bars, guile misses the end of the
block comment and hangs.

scheme@(guile-user)> '(a #|foo|# b)
$1 = (a b)
scheme@(guile-user)> '(a #|foo|||# b)
$2 = (a b)
scheme@(guile-user)> '(a #|foo|||||# b)
$3 = (a b)
scheme@(guile-user)> '(a #|foo||# b)
[hangs]

scheme@(guile-user)> '(a #|foo||||# b)
[hangs]

The bug is most likely located in libguile/read.c,
function scm_read_r6rs_block_comment, line 1204.

Bruno

[1] http://www.gnu.org/software/guile/manual/html_node/Block-Comments.html
[2] http://www.r6rs.org/final/html/r6rs/r6rs-Z-H-7.html#node_sec_4.2.3
[3] http://srfi.schemers.org/srfi-30/srfi-30.html
--
In memoriam Irena Iłłakowicz <http://en.wikipedia.org/wiki/Irena_Iłłakowicz>