lexical closure problem

[Jinsong Zhao]

Hi there,

Recently, I read Lisp 3rd Edition by Winston and Horn. In Chapter 15,
there is a example code:

(setf generator-with-dispatch-procedure
(let ((previous-power-of-two 1)
(reset-procedure
#'(lambda ()
(setf previous-power-of-two 1)))
(value-procedure
#'(lambda ()
(setf previous-power-of-two
(* previous-power-of-two 2)))))
;;The dispatch procedure:
#'(lambda (accessor)
(cond ((eq 'reset accessor) reset-procedure)
((eq 'value accessor) value-procedure)))))

I tried to invoke it like:
(funcall (funcall generator-with-dispatch-procedure 'value))

SBCL gives the error message:
The variable PREVIOUS-POWER-OF-TWO is unbound.

If I run
(funcall (funcall generator-with-dispatch-procedure 'reset))
before
(funcall (funcall generator-with-dispatch-procedure 'value))
I could get what I expect.

I could not understand why the error appears. Any hint?

Best,
Jinsong

[Sam Steingold]

> * Jinsong Zhao <wfm...@lrnu.arg> [2022-09-01 22:06:39 +0800]:

>
> (setf generator-with-dispatch-procedure
> (let ((previous-power-of-two 1)

replace `let' with `let*'

> (reset-procedure
> #'(lambda ()
> (setf previous-power-of-two 1)))
> (value-procedure
> #'(lambda ()
> (setf previous-power-of-two
> (* previous-power-of-two 2)))))
> ;;The dispatch procedure:
> #'(lambda (accessor)
> (cond ((eq 'reset accessor) reset-procedure)
> ((eq 'value accessor) value-procedure)))))
>
> I tried to invoke it like:
> (funcall (funcall generator-with-dispatch-procedure 'value))
>
> SBCL gives the error message:
> The variable PREVIOUS-POWER-OF-TWO is unbound.

Actually, SBCL tells you that this will happen as soon as you evaluate
the above `setf' form:

--8<---------------cut here---------------start------------->8---
; in: SETF GENERATOR-WITH-DISPATCH-PROCEDURE
; (PREVIOUS-POWER-OF-TWO 1)
;
; caught STYLE-WARNING:
; The variable PREVIOUS-POWER-OF-TWO is defined but never used.
; in: SETF GENERATOR-WITH-DISPATCH-PROCEDURE
; (* PREVIOUS-POWER-OF-TWO 2)
;
; caught WARNING:
; undefined variable: COMMON-LISP-USER::PREVIOUS-POWER-OF-TWO
;
; compilation unit finished
; Undefined variable:
; PREVIOUS-POWER-OF-TWO
; caught 1 WARNING condition
--8<---------------cut here---------------end--------------->8---

this tells you that `previous-power-of-two' is not known to
`reset-procedure' and `value-procedure' and it is not used.

--
Sam Steingold (https://aphar.dreamwidth.org/) on darwin Ns 10.3.2113
https://lastingimpactpsychology.com https://steingoldpsychology.com
https://jihadwatch.org https://fairforall.org https://ij.org/ https://ffii.org
Don't hit a man when he's down -- kick him; it's easier.

[Ben Bacarisse]

Jinsong Zhao <jsz...@yeah.net> writes:

> Recently, I read Lisp 3rd Edition by Winston and Horn. In Chapter 15,
> there is a example code:
>
> (setf generator-with-dispatch-procedure
> (let ((previous-power-of-two 1)
> (reset-procedure
> #'(lambda ()
> (setf previous-power-of-two 1)))
> (value-procedure
> #'(lambda ()
> (setf previous-power-of-two
> (* previous-power-of-two 2)))))
> ;;The dispatch procedure:
> #'(lambda (accessor)
> (cond ((eq 'reset accessor) reset-procedure)
> ((eq 'value accessor) value-procedure)))))
>
> I tried to invoke it like:
> (funcall (funcall generator-with-dispatch-procedure 'value))
>
> SBCL gives the error message:
> The variable PREVIOUS-POWER-OF-TWO is unbound.

Indeed. I find the code odd. What is it intended to illustrate? To
work as it apparently should you need let* there.

> If I run
> (funcall (funcall generator-with-dispatch-procedure 'reset))
> before
> (funcall (funcall generator-with-dispatch-procedure 'value))
> I could get what I expect.

Well, except for the fact that a global variable called
previous-power-of-two comes into existence! That is not intended, I
think.

> I could not understand why the error appears. Any hint?

The two anonymous functions in the let can't see the binding for
previous-power-of-two.

--
Ben.

[Spiros Bousbouras]

Because reset-procedure and value-procedure are not within the scope of
previous-power-of-two ; or , to use the terminology of the book , within the
"gatekeeper's list" created for previous-power-of-two .Compare with an
earlier example :

(setf g1
(let ((previous-power-of-two 1))
#'(lambda () ..... )))

This works because the lambda is within the scope for previous-power-of-two
created by LET .I suspect that the authors made a mistake : they intended to
use LET* for the generator-with-dispatch-procedure example but they forgot
and in their testing they always happened to do first
(funcall (funcall generator-with-dispatch-procedure 'reset))
hence they never noticed the mistake. The way they have written the code ,
the first time you do
(funcall (funcall generator-with-dispatch-procedure 'reset))

a special variable previous-power-of-two gets created. Note that then
you can just type on the REPL
previous-power-of-two

and it shows you its current value which violates what the book says on page
217 : "However some programmers dislike having the state variable exposed. In
principle other procedures could alter it inadvertently." and the examples
which follow {including the one your question is about} are meant to show you
how you can avoid this.

With the definition in the book , I have the following interaction with SBCL
{omitting warnings}

* (funcall (funcall generator-with-dispatch-procedure 'reset))
1

* previous-power-of-two
1

* (funcall (funcall generator-with-dispatch-procedure 'value))
2

* (setf previous-power-of-two 7)
7

* (funcall (funcall generator-with-dispatch-procedure 'value))
14

Now if I terminate and restart SBCL and replace the let in the code of
the book by let* , I get the following :

* (funcall (funcall generator-with-dispatch-procedure 'value))
2

{ Note that there is no error now. }

* previous-power-of-two

debugger invoked on a UNBOUND-VARIABLE in thread #<THREAD "initial thread" RUNNING {A8346F9}>:

The variable PREVIOUS-POWER-OF-TWO is unbound.

* (setf previous-power-of-two 7)
7

* (funcall (funcall generator-with-dispatch-procedure 'value))
4

* (funcall (funcall generator-with-dispatch-procedure 'reset))
1

* (funcall (funcall generator-with-dispatch-procedure 'value))
2

* previous-power-of-two
7

When you do (funcall generator-with-dispatch-procedure ...) , the 2
functions access the lexical previous-power-of-two which is private to
the 2 functions and has a separate value from the special
previous-power-of-two which was created when I did
(setf previous-power-of-two 7)

.I believe this is what the book was aiming to exhibit.

--
vlaho.ninja/prog

[steve g]

It seems to work for me I think..?

CL-USER> (funcall (funcall generator-with-dispatch-procedure 'value))
2
CL-USER> (funcall (funcall generator-with-dispatch-procedure 'reset))
1
CL-USER>


Try using a let to bind the value outside the `setf'...

(let ((previous-power-of-two 1)
(setq generator-with-dispatch-procedure

(reset-procedure
#'(lambda ()
(setf previous-power-of-two 1)))
(value-procedure
#'(lambda ()
(setf previous-power-of-two
(* previous-power-of-two 2)))))
;;The dispatch procedure:
#'(lambda (accessor)
(cond ((eq 'reset accessor) reset-procedure)
((eq 'value accessor) value-procedure)))))

The use of setq is stylistic; this might help...

[Kaz Kylheku]

On 2022-10-24, steve g <sgoned...@gmail.com> wrote:

> Jinsong Zhao wrote:
>> I could not understand why the error appears. Any hint?
>
> It seems to work for me I think..?

Three others had to change let to let*, so that is amazing.

Do you have a customized environment in which *let* is sequential
by default, or do you just filter such problems in your
fingers without being consciously aware of it any more?

--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal

[steve]

Kaz Kylheku <864-11...@kylheku.com> writes:

> On 2022-10-24, steve g <sgoned...@gmail.com> wrote:
>> Jinsong Zhao wrote:
>>> I could not understand why the error appears. Any hint?
>>
>> It seems to work for me I think..?
>
> Three others had to change let to let*, so that is amazing.

an old thread.

> Do you have a customized environment in which *let* is sequential
> by default, or do you just filter such problems in your
> fingers without being consciously aware of it any more?

I dunno what you mean. could you explain filtering?

[steve]

Kaz Kylheku <864-11...@kylheku.com> writes:

> On 2022-10-24, steve g <sgoned...@gmail.com> wrote:
>> Jinsong Zhao wrote:
>>> I could not understand why the error appears. Any hint?
>>
>> It seems to work for me I think..?
>
> Three others had to change let to let*, so that is amazing.
>
> Do you have a customized environment in which *let* is sequential

I just ment to bring the binding outside of the setf; that is why i use
setq.

i just typed that in; it looks like it worked...

(setq generator-with-dispatch-procedure

(let ((previous-power-of-two 1)
(reset-procedure
#'(lambda ()
(setf previous-power-of-two 1)))
(value-procedure
#'(lambda ()
(setf previous-power-of-two
(* previous-power-of-two 2)))))
;;The dispatch procedure:
#'(lambda (accessor)
(cond ((eq 'reset accessor) reset-procedure)
((eq 'value accessor) value-procedure)))))

> by default, or do you just filter such problems in your
> fingers without being consciously aware of it any more?

i usually use more humility.