Precedence of for clauses in cl-loop

Precedence of for clauses in cl-loop

[Plamen Tanovski]

Hi,

the CL info states:

    If you include several ‘for’ clauses in a row, they are treated
    sequentially (as if by ‘let*’ and ‘setq’).

but in this simple example

(cl-loop for y = x
         for x in '(1 2)
	 for z in '(1 2))


the "for in" clauses are executed before the "for =" clause. Emacs is:

GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.16.7) of
2018-06-08

best regards

-- 
Plamen Tanovski

Re: Precedence of for clauses in cl-loop

[Noam Postavsky]

On 6 September 2018 at 14:33, Plamen Tanovski <pgt@tanovski.de> wrote:

> in this simple example
>
> (cl-loop for y = x
>          for x in '(1 2)
>          for z in '(1 2))
>
>
> the "for in" clauses are executed before the "for =" clause.

I disagree. I invite you to prove me wrong.

Re: Precedence of for clauses in cl-loop

[Plamen Tanovski]

Noam Postavsky <npostavs@gmail.com> writes:

> On 6 September 2018 at 14:33, Plamen Tanovski <pgt@tanovski.de> wrote:
>
>> in this simple example
>>
>> (cl-loop for y = x
>>          for x in '(1 2)
>>          for z in '(1 2))
>>
>>
>> the "for in" clauses are executed before the "for =" clause.
>
> I disagree. I invite you to prove me wrong.

simply edebug the loop shows the precedence, but below as example a loop
which gives error, beacuse the z part is executed before the y line:

(cl-loop
 for x in (number-sequence 1 10)
 for y = (1+ x) 
 for z in (elt (number-sequence 1 10) y))


-- 
Plamen Tanovski

Re: Precedence of for clauses in cl-loop

[Noam Postavsky]

On 7 September 2018 at 03:08, Plamen Tanovski <pgt@tanovski.de> wrote:
> gives error, beacuse the z part is executed before the y line:
>
> (cl-loop
>  for x in (number-sequence 1 10)
>  for y = (1+ x)
>  for z in (elt (number-sequence 1 10) y))

This example also gives an error:

(cl-loop
 for x in (number-sequence 1 10)
 for z in (elt (number-sequence 1 10) (1+ x)))

because a `for VAR in LIST' clause evaluates LIST just once before the
loop starts.

The "treated sequentially" in the manual refers to the assignment of
VAR. For example, this gives an error:

(cl-loop
 for y = (1+ x)
 for x in (number-sequence 1 10))

And this does not:

(cl-loop
 for x in (number-sequence 1 10)
 for y = (1+ x))

Re: Precedence of for clauses in cl-loop

[Plamen Tanovski]

Noam Postavsky <npostavs@gmail.com> writes:

> On 7 September 2018 at 03:08, Plamen Tanovski <pgt@tanovski.de> wrote:
>> gives error, beacuse the z part is executed before the y line:
>>
>> (cl-loop
>>  for x in (number-sequence 1 10)
>>  for y = (1+ x)
>>  for z in (elt (number-sequence 1 10) y))
>
> This example also gives an error:
>
> (cl-loop
>  for x in (number-sequence 1 10)
>  for z in (elt (number-sequence 1 10) (1+ x)))
>
> because a `for VAR in LIST' clause evaluates LIST just once before the
> loop starts.

But

(cl-loop
 for x on (number-sequence 1 10)
 for z in (elt (number-sequence 1 10) (car x)))

works fine, which means, x was assigned a value before the for z clause.

> The "treated sequentially" in the manual refers to the assignment of
> VAR.

That's what I don't understand. After a "for VAR in" clause VAR is not
assigned and available for the rest of the for clauses. Obviously "for in
(across, etc.)" behaves different than "for on" and other for clauses.
I've tested with SBCL and there seems to be the same issue.  

best regards

-- 
Plamen Tanovski

Re: Precedence of for clauses in cl-loop

[Noam Postavsky]

On 7 September 2018 at 08:35, Plamen Tanovski <pgt@tanovski.de> wrote:

> (cl-loop
>  for x on (number-sequence 1 10)
>  for z in (elt (number-sequence 1 10) (car x)))
>
> works fine, which means, x was assigned a value before the for z clause.

Hmm, I wasn't aware of this, but it seems to be on purpose,
https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node239.html says:

    All variables are initialized first, regardless of where the
establishing clauses appear in the source. The order of initialization
follows the order of these clauses.

Re: Precedence of for clauses in cl-loop

[Udyant Wig]

On 9/8/18 7:07 AM, Noam Postavsky wrote:
> Hmm, I wasn't aware of this, but it seems to be on purpose,
> https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node239.html says:
>
>     All variables are initialized first, regardless of where the
> establishing clauses appear in the source. The order of initialization
> follows the order of these clauses.
>

Indeed.  The Common Lisp Standard concurs:

<URL:http://www.lispworks.com/documentation/HyperSpec/Body/06_aaf.htm>

The phrasing is identical there.

Udyant Wig
-- 
We make our discoveries through our mistakes: we watch one another's
success: and where there is freedom to experiment there is hope to
improve.
                                -- Arthur Quiller-Couch