From bc5ab1e03128db600a6909e2150ebfdfb9b3dee6 Mon Sep 17 00:00:00 2001
From: Matthew Flatt <mflatt@racket-lang.org>
Date: Sun, 30 Jan 2011 07:47:10 -0600
Subject: [PATCH] fix problems with `read-language' error reporting

---
 collects/scribblings/reference/read.scrbl | 4 +++-
 collects/tests/racket/read.rktl           | 8 ++++++++
 src/racket/src/read.c                     | 9 ++++++---
 3 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/collects/scribblings/reference/read.scrbl b/collects/scribblings/reference/read.scrbl
index 9355611abb..3e63ed48b3 100644
--- a/collects/scribblings/reference/read.scrbl
+++ b/collects/scribblings/reference/read.scrbl
@@ -158,7 +158,9 @@ is @racket[#f].
 
 If @racket[in] has a @litchar{#lang} or @litchar{#!} specification,
 but parsing and resolving the specification raises an exception, the
-exception is propagated by @racket[read-language].
+exception is propagated by @racket[read-language]. Having at least
+@litchar{#l} or @litchar{#!} (after comments and whitespace) counts as
+starting a @litchar{#lang} or @litchar{#!} specification.
 
 If @racket[in] does not specify a @tech{reader language} with
 @litchar{#lang} or @litchar{#!}, then @racket[fail-thunk] is
diff --git a/collects/tests/racket/read.rktl b/collects/tests/racket/read.rktl
index 55a860963a..38f93b84d3 100644
--- a/collects/tests/racket/read.rktl
+++ b/collects/tests/racket/read.rktl
@@ -1095,6 +1095,14 @@
   (check-nothing ";" exn:fail:read:eof?)
   (check-nothing "#| |#" exn:fail:read:eof?)
   (check-nothing "8 9" exn:fail:read?))
+(err/rt-test (read-language (open-input-string "#l") void) exn:fail:read:eof?)
+(err/rt-test (read-language (open-input-string "#la") void) exn:fail:read:eof?)
+(err/rt-test (read-language (open-input-string ";;\n;\n#la") void) exn:fail:read:eof?)
+(err/rt-test (read-language (open-input-string ";;\n;\n#lx") void) exn:fail:read?)
+(test (void) read-language (open-input-string ";;\n;\n#xa") void)
+;; Check error-message formatting:
+(err/rt-test (read (open-input-string "#l"))
+             (lambda (exn) (regexp-match? #rx"`#l'" (exn-message exn))))
 
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
diff --git a/src/racket/src/read.c b/src/racket/src/read.c
index a1e31523d0..6f0535b408 100644
--- a/src/racket/src/read.c
+++ b/src/racket/src/read.c
@@ -1547,16 +1547,19 @@ read_inner_inner(Scheme_Object *port, Scheme_Object *stxsrc, Scheme_Hash_Table *
                     }
                     return v;
                   } else {
+                    if (ch == EOF) --fl;
                     scheme_read_err(port, stxsrc, line, col, pos, 6, ch, indentation,
-                                    "read: expected a single space after `#lang'",
-                                    found, fl);
+                                    "read%s: expected a single space after `#lang'",
+                                    (get_info ? "-language" : ""));
                     return NULL;
                   }
                 }
               }
             }
+            if (ch == EOF) --fl;
             scheme_read_err(port, stxsrc, line, col, pos, fl, ch, indentation,
-                            "read: bad input: `#%u'",
+                            "read%s: bad input: `#%u'",
+                            (get_info ? "-language" : ""),
                             found, (intptr_t)fl);
             return NULL;
           }