bug#33199: lambda forms are not self-quoting

bug#33199: lambda forms are not self-quoting

[Noam Postavsky]

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X-Debbugs-Cc: Garreau, Alexandre <galex-713@galex-713.eu> 
Tags: patch
Severity: minor

Alexandre's recent post about pcase-lambda made me notice that lambda's
docstring claims lambda forms are self-quoting.  However, they are not:

    ;; -*- lexical-binding: t -*-
    (setq lexical-binding t) ; for *scratch*

    (lambda (x) (+ 1 x)) ;=> (closure (t) (x) (+ 1 x))


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From f816c64bf2fe59fc3c77a45d828aea06599f346a Mon Sep 17 00:00:00 2001
From: Noam Postavsky <npostavs@gmail.com>
Date: Mon, 29 Oct 2018 19:01:07 -0400
Subject: [PATCH v1] Don't claim lambda forms are self-quoting

* doc/lispref/functions.texi (Anonymous Functions):
* lisp/subr.el (lambda): Don't say that a lambda form yields itself,
under lexical binding it yields a closure object.
---
 doc/lispref/functions.texi | 13 ++-----------
 lisp/subr.el               | 10 ++++------
 2 files changed, 6 insertions(+), 17 deletions(-)

diff --git a/doc/lispref/functions.texi b/doc/lispref/functions.texi
index 9b8057080e..09e43621e4 100644
--- a/doc/lispref/functions.texi
+++ b/doc/lispref/functions.texi
@@ -1082,17 +1082,8 @@ Anonymous Functions
 @var{args}, documentation string @var{doc} (if any), interactive spec
 @var{interactive} (if any), and body forms given by @var{body}.
 
-In effect, this macro makes @code{lambda} forms self-quoting:
-evaluating a form whose @sc{car} is @code{lambda} yields the form
-itself:
-
-@example
-(lambda (x) (* x x))
-     @result{} (lambda (x) (* x x))
-@end example
-
-The @code{lambda} form has one other effect: it tells the Emacs
-evaluator and byte-compiler that its argument is a function, by using
+The @code{lambda} form has one effect: it tells the Emacs evaluator
+and byte-compiler that its argument is a function, by using
 @code{function} as a subroutine (see below).
 @end defmac
 
diff --git a/lisp/subr.el b/lisp/subr.el
index 59f6949b21..1fc58d0ce3 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -93,12 +93,10 @@ def-edebug-spec
   `(put (quote ,symbol) 'edebug-form-spec (quote ,spec)))
 
 (defmacro lambda (&rest cdr)
-  "Return a lambda expression.
-A call of the form (lambda ARGS DOCSTRING INTERACTIVE BODY) is
-self-quoting; the result of evaluating the lambda expression is the
-expression itself.  The lambda expression may then be treated as a
-function, i.e., stored as the function value of a symbol, passed to
-`funcall' or `mapcar', etc.
+  "Return an anonymous function.
+A call of the form (lambda ARGS DOCSTRING INTERACTIVE BODY) evaluates
+to a function, i.e., it may be stored as the function value of a
+symbol, passed to `funcall' or `mapcar', etc.
 
 ARGS should take the same form as an argument list for a `defun'.
 DOCSTRING is an optional documentation string.
-- 
2.11.0

bug#33199: lambda forms are not self-quoting

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@gmail.com>
> Date: Mon, 29 Oct 2018 19:06:25 -0400
> Cc: garreau@debbugs.gnu.org, alexandre <galex-713@galex-713.eu>
> 
> Alexandre's recent post about pcase-lambda made me notice that lambda's
> docstring claims lambda forms are self-quoting.  However, they are not:
> 
>     ;; -*- lexical-binding: t -*-
>     (setq lexical-binding t) ; for *scratch*
> 
>     (lambda (x) (+ 1 x)) ;=> (closure (t) (x) (+ 1 x))

The doc string is still correct if lexical-binding is not in effect,
isn't it?

> From: Noam Postavsky <npostavs@gmail.com>
> Date: Mon, 29 Oct 2018 19:01:07 -0400
> Subject: [PATCH v1] Don't claim lambda forms are self-quoting
> 
> * doc/lispref/functions.texi (Anonymous Functions):
> * lisp/subr.el (lambda): Don't say that a lambda form yields itself,
> under lexical binding it yields a closure object.

This removes useful information, IMO.  I'd be much happier if we added
to the existing docs that lambda is not self-quoting in the
lexical-binding environment, and described (and explained) what does
happen in the lexical-binding environment.

Thanks.

bug#33199: lambda forms are not self-quoting

[Noam Postavsky]

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Eli Zaretskii <eliz@gnu.org> writes:

> This removes useful information, IMO.

What's it really useful for though?  IMO, all anyone needs to know is
that (lambda ...) evaluates to a function.

> I'd be much happier if we added to the existing docs that lambda is
> not self-quoting in the lexical-binding environment, and described
> (and explained) what does happen in the lexical-binding environment.

It seems to me like too much useless detail, but if you insist.


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From bd397a3cec219a453e3118cb59e5955f5a09b870 Mon Sep 17 00:00:00 2001
From: Noam Postavsky <npostavs@gmail.com>
Date: Mon, 29 Oct 2018 19:01:07 -0400
Subject: [PATCH v2] Note that lambda forms are not self-quoting under lex
 binding

* doc/lispref/functions.texi (Anonymous Functions):
* lisp/subr.el (lambda): Note that under lexical binding it yields a
closure object (Bug#33199).
---
 doc/lispref/functions.texi |  9 ++++++---
 lisp/subr.el               | 13 +++++++------
 2 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/doc/lispref/functions.texi b/doc/lispref/functions.texi
index 9b8057080e..7f8a6fc40b 100644
--- a/doc/lispref/functions.texi
+++ b/doc/lispref/functions.texi
@@ -1082,15 +1082,18 @@ Anonymous Functions
 @var{args}, documentation string @var{doc} (if any), interactive spec
 @var{interactive} (if any), and body forms given by @var{body}.
 
-In effect, this macro makes @code{lambda} forms self-quoting:
-evaluating a form whose @sc{car} is @code{lambda} yields the form
-itself:
+Under dynamic binding, this macro effectively makes @code{lambda}
+forms self-quoting: evaluating a form whose @sc{car} is @code{lambda}
+yields the form itself:
 
 @example
 (lambda (x) (* x x))
      @result{} (lambda (x) (* x x))
 @end example
 
+Note that when evaluting under lexical binding the result is a closure
+object (@pxref{Closures}).
+
 The @code{lambda} form has one other effect: it tells the Emacs
 evaluator and byte-compiler that its argument is a function, by using
 @code{function} as a subroutine (see below).
diff --git a/lisp/subr.el b/lisp/subr.el
index 59f6949b21..d09789340f 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -93,12 +93,13 @@ def-edebug-spec
   `(put (quote ,symbol) 'edebug-form-spec (quote ,spec)))
 
 (defmacro lambda (&rest cdr)
-  "Return a lambda expression.
-A call of the form (lambda ARGS DOCSTRING INTERACTIVE BODY) is
-self-quoting; the result of evaluating the lambda expression is the
-expression itself.  The lambda expression may then be treated as a
-function, i.e., stored as the function value of a symbol, passed to
-`funcall' or `mapcar', etc.
+  "Return an anonymous function.
+Under dynamic binding, a call of the form (lambda ARGS DOCSTRING
+INTERACTIVE BODY) is self-quoting; the result of evaluating the
+lambda expression is the expression itself.  Under lexical
+binding, the result is a closure.  Regardless, the result is a
+function, i.e., it may be stored as the function value of a
+symbol, passed to `funcall' or `mapcar', etc.
 
 ARGS should take the same form as an argument list for a `defun'.
 DOCSTRING is an optional documentation string.
-- 
2.11.0

bug#33199: lambda forms are not self-quoting

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@gmail.com>
> Cc: 33199@debbugs.gnu.org,  garreau@debbugs.gnu.org,  galex-713@galex-713.eu
> Date: Tue, 30 Oct 2018 19:07:16 -0400
> 
> > This removes useful information, IMO.
> 
> What's it really useful for though?  IMO, all anyone needs to know is
> that (lambda ...) evaluates to a function.

It sounds non-trivial to me, and someone obviously thought it was
important enough to write that text.

> > I'd be much happier if we added to the existing docs that lambda is
> > not self-quoting in the lexical-binding environment, and described
> > (and explained) what does happen in the lexical-binding environment.
> 
> It seems to me like too much useless detail, but if you insist.

Thanks the updated patch LGTM.

bug#33199: lambda forms are not self-quoting

[Noam Postavsky]

tags 33199 fixed
close 33199 26.2
quit

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Noam Postavsky <npostavs@gmail.com>
>> Cc: 33199@debbugs.gnu.org,  garreau@debbugs.gnu.org,  galex-713@galex-713.eu
>> Date: Tue, 30 Oct 2018 19:07:16 -0400
>> 
>> > This removes useful information, IMO.
>> 
>> What's it really useful for though?  IMO, all anyone needs to know is
>> that (lambda ...) evaluates to a function.
>
> It sounds non-trivial to me, and someone obviously thought it was
> important enough to write that text.

Hmm, it's hard to argue with the anonymous absentee.

> Thanks the updated patch LGTM.

Okay, pushed to emacs-26.

[1: 39e85a0c6c]: 2018-11-08 20:13:56 -0500
  Note that lex bound lambda forms are not self-quoting (Bug#33199)
  https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=39e85a0c6c8de75b446e8e4dc41cdfdca96907e3