Subject: bug#25154: 25.1; Bindings in cl-letf are in reverse order

From: Philipp Stephani <[email-masked]>
Date: Fri, 23 Dec 2016 12:17:54 +0000

Eli Zaretskii <[email-masked]> schrieb am Sa., 10. Dez. 2016 um 15:09 Uhr:

> > From: Philipp Stephani <[email-masked]>
> > Date: Sat, 10 Dec 2016 13:41:16 +0000
> > Cc: [email-masked], Alex <[email-masked]>
> >
> >  > Isn't it true that the order of evaluation in a 'let' is unspecified?
> >  > If you want a particular order, use 'let*'.
> >  Right, the order of evaluation in a let is up to the implementation. A
> program
> >  should not rely on such details.
> >  The same statement should apply to cl-letf.
> >
> > I think that should be mentioned explicitly in the manuals: given that
> the order of value evaluations is specified,
> > people might expect the same for the bindings themselves.
>
> I agree, patches to that effect are welcome.  (AFAICT, the manual
> tries to say that already, but the wording could be more explicit.)
>

OK, I've attached a patch that hopefully clarifies this a bit.
From 42d7450c41d69a713eb8f9492cc169e8c2bc15ca Mon Sep 17 00:00:00 2001
From: Philipp Stephani <[email-masked]>
Date: Fri, 23 Dec 2016 13:14:55 +0100
Subject: [PATCH] Document that variable binding order is unspecified

* doc/lispref/variables.texi (Local Variables):
* cl.texi (Modify Macros): Document that assignment order in 'let' and
'cl-letf' is unspecified.
---
 doc/lispref/variables.texi | 12 ++++++++++++
 doc/misc/cl.texi           |  5 +++++
 2 files changed, 17 insertions(+)

diff --git a/doc/lispref/variables.texi b/doc/lispref/variables.texi
index a2d64815d9..e2c8c542ab 100644
--- a/doc/lispref/variables.texi
+++ b/doc/lispref/variables.texi
@@ -221,6 +221,18 @@ Local Variables
      @result{} (1 2)
 @end group
 @end example
+
+On the other hand, the order of @emph{assignments} is unspecified: in
+the following example, either 1 or 2 might be printed.
+
+@example
+(let ((x 1)
+      (x 2))
+  (print x))
+@end example
+
+Therefore, avoid binding a variable more than once in a single
+@code{let} form.
 @end defspec
 
 @defspec let* (bindings@dots{}) forms@dots{}
diff --git a/doc/misc/cl.texi b/doc/misc/cl.texi
index c62fa727c1..aa047e2122 100644
--- a/doc/misc/cl.texi
+++ b/doc/misc/cl.texi
@@ -1179,6 +1179,11 @@ Modify Macros
 as @code{setf} places; each will accept either an integer or a
 marker as the stored value.)
 
+Like in the case of @code{let}, the @var{value} forms are evaluated in
+the order they appear, but the order of assignments is unspecified.
+Therefore, avoid assigning to the same @var{place} more than once in a
+single @code{cl-letf} form.
+
 Since generalized variables look like lists, @code{let}'s shorthand
 of using @samp{foo} for @samp{(foo nil)} as a @var{binding} would
 be ambiguous in @code{cl-letf} and is not allowed.
-- 
2.11.0