GNU bug report logs - #16238
24.3.50; pcase docs (and possibly pcase) wrong

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Tassilo Horn <tsdh <at> gnu.org>
To: bug-gnu-emacs <at> gnu.org
Subject: 24.3.50; pcase docs (and possibly pcase) wrong
Date: Tue, 24 Dec 2013 10:13:50 +0100

I'm just reading through the `pcase' docs in the info manual.  The
simple expression language example has a small bug.  The `env' parameter
is missing at the location marked below:

--8<---------------cut here---------------start------------->8---
(defun evaluate (exp env)
  (pcase exp
    (`(add ,x ,y)         (+ (evaluate x env) (evaluate y env)))
    (`(call ,fun ,arg)    (funcall (evaluate fun) (evaluate arg env)))
                                                `--- HERE!
    (`(fn ,arg ,body)     (lambda (val)
                            (evaluate body (cons (cons arg val) env))))
    ((pred numberp)       exp)
    ((pred symbolp)       (cdr (assq exp env)))
    (_                    (error "Unknown expression %S" exp))))
--8<---------------cut here---------------end--------------->8---

But even then, I can't get `fn' forms working:

ELISP> (evaluate '(fn x (add 1 x)) nil)
(lambda
  (val)
  (evaluate body
            (cons
             (cons arg val)
             env)))

But shouldn't `arg' be substituted with 'x and `body' with '(add 1 x)?
Now when I call the function, I get a void-variable error whereas I
should get 3:

ELISP> (evaluate '(call (fn x (add 1 x)) 2) nil)
*** Eval error ***  Symbol's value as variable is void: body

Strange enough, the `call' pattern which uses the same pattern syntax
seems to work, though:

ELISP> (evaluate '(call foo 2) '((foo . (lambda (x) (+ 1 x)))))
3 (#o3, #x3, ?\C-c)

In any case, it would be good if the docs would contain a sample program
defined in the simple expression language.  Getting the syntax right
just from the patterns is not enough.  For example, it's not obvious
that `fn' forms take only one symbol as `arg' whereas you'd usually
expect an argument list (which might be restricted to length 1).



In GNU Emacs 24.3.50.2 (x86_64-pc-linux-gnu, GTK+ Version 3.8.7)
 of 2013-12-22 on thinkpad
Bzr revision: eliz <at> gnu.org-20131222183138-zlzn9zzpz1hwd110
Windowing system distributor `The X.Org Foundation', version 11.0.11499904
System Description:     NAME=Gentoo

Configured using:
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Important settings:
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  value of $LC_MONETARY: de_DE.utf8
  value of $LC_NUMERIC: de_DE.utf8
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Major mode: Lisp Interaction

Minor modes in effect:
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Message #8 received at 16238 <at> debbugs.gnu.org (full text, mbox):

From: Tassilo Horn <tsdh <at> gnu.org>
To: 16238 <at> debbugs.gnu.org
Subject: Re: bug#16238: 24.3.50; pcase docs (and possibly pcase) wrong
Date: Tue, 24 Dec 2013 10:37:06 +0100

Tassilo Horn <tsdh <at> gnu.org> writes:

> ELISP> (evaluate '(fn x (add 1 x)) nil)
> (lambda
>   (val)
>   (evaluate body
>             (cons
>              (cons arg val)
>              env)))
>
> But shouldn't `arg' be substituted with 'x and `body' with '(add 1 x)?

It seems the `lambda' is the problem that prevents substitution of arg
and body.  By doing some kinda strange quoting I can get it right, but
IMHO that shouldn't be needed:

--8<---------------cut here---------------start------------->8---
(defun evaluate (exp env)
  (pcase exp
    (`(add ,x ,y)         (+ (evaluate x env) (evaluate y env)))
    (`(call ,fun ,arg)    (funcall (evaluate fun env) (evaluate arg env)))
    (`(fn ,arg ,body)     `(lambda (val)
			     (evaluate ',body (cons (cons ',arg val) env))))
    ((pred numberp)       exp)
    ((pred symbolp)       (cdr (assq exp env)))
    (_                    (error "Unknown expression %S" exp))))
--8<---------------cut here---------------end--------------->8---

ELISP> (evaluate '(fn x (add 1 x)) nil)
(lambda
  (val)
  (evaluate
   '(add 1 x)
   (cons
    (cons 'x val)
    env)))

ELISP> (evaluate '(call (fn x (add 1 x)) 3) nil)
4 (#o4, #x4, ?\C-d)

Bye,
Tassilo

Message #11 received at 16238 <at> debbugs.gnu.org (full text, mbox):

From: Michael Heerdegen <michael_heerdegen <at> web.de>
To: Tassilo Horn <tsdh <at> gnu.org>
Cc: 16238 <at> debbugs.gnu.org
Subject: Re: bug#16238: 24.3.50; pcase docs (and possibly pcase) wrong
Date: Tue, 24 Dec 2013 10:55:00 +0100

Tassilo Horn <tsdh <at> gnu.org> writes:

> But shouldn't `arg' be substituted with 'x and `body' with '(add 1 x)?
> Now when I call the function, I get a void-variable error whereas I
> should get 3:
>
> ELISP> (evaluate '(call (fn x (add 1 x)) 2) nil)
> *** Eval error ***  Symbol's value as variable is void: body

I think the example is designed for lexical-binding.  That should be
mentioned.  It is not so nice anyway, since people will likely C-x C-e
the example and then it won't work.

Michael.

Message #14 received at 16238 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> IRO.UMontreal.CA>
To: Tassilo Horn <tsdh <at> gnu.org>
Cc: 16238 <at> debbugs.gnu.org
Subject: Re: bug#16238: 24.3.50; pcase docs (and possibly pcase) wrong
Date: Tue, 24 Dec 2013 09:53:35 -0500

> (defun evaluate (exp env)
>   (pcase exp
>     (`(add ,x ,y)         (+ (evaluate x env) (evaluate y env)))
>     (`(call ,fun ,arg)    (funcall (evaluate fun) (evaluate arg env)))
>                                                 `--- HERE!

Indeed, feel free to fix it.

> But shouldn't `arg' be substituted with 'x and `body' with '(add 1 x)?

No, the code is meant for lexical-binding.


        Stefan

Message #17 received at 16238 <at> debbugs.gnu.org (full text, mbox):

From: Tassilo Horn <tsdh <at> gnu.org>
To: Stefan Monnier <monnier <at> IRO.UMontreal.CA>
Cc: Michael Heerdegen <michael_heerdegen <at> web.de>, 16238 <at> debbugs.gnu.org
Subject: Re: bug#16238: 24.3.50; pcase docs (and possibly pcase) wrong
Date: Tue, 24 Dec 2013 16:34:31 +0100

Stefan Monnier <monnier <at> IRO.UMontreal.CA> writes:

>> (defun evaluate (exp env)
>>   (pcase exp
>>     (`(add ,x ,y)         (+ (evaluate x env) (evaluate y env)))
>>     (`(call ,fun ,arg)    (funcall (evaluate fun) (evaluate arg env)))
>>                                                 `--- HERE!
>
> Indeed, feel free to fix it.

Done.

>> But shouldn't `arg' be substituted with 'x and `body' with '(add 1
>> x)?
>
> No, the code is meant for lexical-binding.

Ok, I see.  I've also mentioned that in the docs, though I guess you can
refine that comment to be more specific.

Bye,
Tassilo

Message #20 received at 16238 <at> debbugs.gnu.org (full text, mbox):

From: Glenn Morris <rgm <at> gnu.org>
To: Tassilo Horn <tsdh <at> gnu.org>
Cc: Michael Heerdegen <michael_heerdegen <at> web.de>, 16238 <at> debbugs.gnu.org,
 Stefan Monnier <monnier <at> IRO.UMontreal.CA>
Subject: Re: bug#16238: 24.3.50; pcase docs (and possibly pcase) wrong
Date: Tue, 24 Dec 2013 15:13:12 -0500

Tassilo Horn wrote:

>> No, the code is meant for lexical-binding.
>
> Ok, I see.  I've also mentioned that in the docs, though I guess you can
> refine that comment to be more specific.

That _example_ assumes lexical binding. _Pcase itself_ does not require
lexical binding. (You've written in the manual that it does.)

Message #23 received at 16238 <at> debbugs.gnu.org (full text, mbox):

From: Tassilo Horn <tsdh <at> gnu.org>
To: Glenn Morris <rgm <at> gnu.org>
Cc: Michael Heerdegen <michael_heerdegen <at> web.de>, 16238 <at> debbugs.gnu.org,
 Stefan Monnier <monnier <at> IRO.UMontreal.CA>
Subject: Re: bug#16238: 24.3.50; pcase docs (and possibly pcase) wrong
Date: Wed, 25 Dec 2013 10:15:10 +0100

Glenn Morris <rgm <at> gnu.org> writes:

>>> No, the code is meant for lexical-binding.
>>
>> Ok, I see.  I've also mentioned that in the docs, though I guess you
>> can refine that comment to be more specific.
>
> That _example_ assumes lexical binding. _Pcase itself_ does not
> require lexical binding. (You've written in the manual that it does.)

Ah, yes.  I'm so used to lexical binding that I can't see a closure when
it's immediately in front of me.  I've corrected the sentence and moved
it directly after the `evaluate' code.

Bye,
Tassilo

Message #28 received at 16238-done <at> debbugs.gnu.org (full text, mbox):

From: Tassilo Horn <tsdh <at> gnu.org>
To: 16238-done <at> debbugs.gnu.org
Subject: Re: bug#16238: 24.3.50; pcase docs (and possibly pcase) wrong
Date: Wed, 25 Dec 2013 21:04:41 +0100

Closing.

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