From ed2b579e3c353d92f11cfe8e7f724cad00a16758 Mon Sep 17 00:00:00 2001
From: Eli Barzilay <eli@barzilay.org>
Date: Thu, 3 May 2012 03:29:42 -0400
Subject: [PATCH] Fail early on a non-pair input to `list-ref'.

There's no index that works with non-pairs, and the generic error
message in such cases is confusing.

Closes PR12740.
---
 collects/scribblings/reference/pairs.scrbl  |  2 +-
 collects/scribblings/reference/unsafe.scrbl |  2 +-
 src/racket/src/list.c                       | 20 ++++++++++++--------
 3 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/collects/scribblings/reference/pairs.scrbl b/collects/scribblings/reference/pairs.scrbl
index 60529e57bc..d660d3ae2e 100644
--- a/collects/scribblings/reference/pairs.scrbl
+++ b/collects/scribblings/reference/pairs.scrbl
@@ -206,7 +206,7 @@ Returns the number of elements in @racket[lst].
 ]}
 
 
-@defproc[(list-ref [lst any/c] [pos exact-nonnegative-integer?])
+@defproc[(list-ref [lst pair?] [pos exact-nonnegative-integer?])
          any/c]{
 
 Returns the element of @racket[lst] at position @racket[pos], where
diff --git a/collects/scribblings/reference/unsafe.scrbl b/collects/scribblings/reference/unsafe.scrbl
index 3ea2d5739d..85e0a36d4b 100644
--- a/collects/scribblings/reference/unsafe.scrbl
+++ b/collects/scribblings/reference/unsafe.scrbl
@@ -192,7 +192,7 @@ Unsafe variants of @racket[car], @racket[cdr], @racket[mcar],
 
 
 @deftogether[(
-@defproc[(unsafe-list-ref [lst any/c] [pos (and/c exact-nonnegative-integer? fixnum?)]) any/c]
+@defproc[(unsafe-list-ref [lst pair?] [pos (and/c exact-nonnegative-integer? fixnum?)]) any/c]
 @defproc[(unsafe-list-tail [lst any/c] [pos (and/c exact-nonnegative-integer? fixnum?)]) any/c]
 )]{
 
diff --git a/src/racket/src/list.c b/src/racket/src/list.c
index 73cceb0249..957b340166 100644
--- a/src/racket/src/list.c
+++ b/src/racket/src/list.c
@@ -743,7 +743,7 @@ scheme_init_unsafe_list (Scheme_Env *env)
                                 | SCHEME_PRIM_IS_UNSAFE_FUNCTIONAL);
   scheme_add_global_constant ("unsafe-cdr", p, env);
 
-  p = scheme_make_folding_prim(unsafe_list_ref, "unsafe-list-tail", 2, 2, 1);
+  p = scheme_make_folding_prim(unsafe_list_ref, "unsafe-list-ref", 2, 2, 1);
   SCHEME_PRIM_PROC_FLAGS(p) |= (SCHEME_PRIM_IS_UNARY_INLINED
                                 | SCHEME_PRIM_IS_UNSAFE_FUNCTIONAL);
   scheme_add_global_constant ("unsafe-list-ref", p, env);
@@ -1291,20 +1291,24 @@ do_list_ref(char *name, int takecar, int argc, Scheme_Object *argv[])
   intptr_t i, k;
   Scheme_Object *lst, *index, *bnindex;
 
-  if (SCHEME_BIGNUMP(argv[1])) {
-    bnindex = argv[1];
+  lst = argv[0];
+  index = argv[1];
+
+  if (!SCHEME_PAIRP(lst) && takecar) {
+    scheme_wrong_type(name, "pair", 0, argc, argv);;
+  }
+
+  if (SCHEME_BIGNUMP(index)) {
+    bnindex = index;
     k = 0;
-  } else if (!SCHEME_INTP(argv[1])) {
+  } else if (!SCHEME_INTP(index)) {
     scheme_wrong_type(name, "non-negative exact integer", 1, argc, argv);
     return NULL;
   } else {
     bnindex = NULL;
-    k = SCHEME_INT_VAL(argv[1]);
+    k = SCHEME_INT_VAL(index);
   }
 
-  lst = argv[0];
-  index = argv[1];
-
   if ((bnindex && !SCHEME_BIGPOS(bnindex))
       || (!bnindex && (k < 0))) {
     scheme_wrong_type(name, "non-negative exact integer", 1, argc, argv);