Order of the evaluation of elements in a quasiquote list.

Order of the evaluation of elements in a quasiquote list.

[Kjetil Svalastog Matheussen]

Hi, I have recently stumbled upon various bugs in my code
because the order of the evaluation of the elements in a quasiquote
list changes.

I know that the order of arguments aren't specified in r5rs, but I 
personally think it would have been very practicaly if they were...

Please, I don't want to start a flame war, I read comp.lang.scheme,
I just want to know if there is a quick way in Guile to get
left-to-right evaluation order for quasiquoted lists?



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Re: Order of the evaluation of elements in a quasiquote list.

[Neil Jerram]

Kjetil Svalastog Matheussen wrote:
> Hi, I have recently stumbled upon various bugs in my code
> because the order of the evaluation of the elements in a quasiquote
> list changes.

Changes how?  Can you give us an example, to make things concrete?

	Neil


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Re: Order of the evaluation of elements in a quasiquote list.

[Kjetil Svalastog Matheussen]

On Sun, 3 Apr 2005, Neil Jerram wrote:

> Kjetil Svalastog Matheussen wrote:
> > Hi, I have recently stumbled upon various bugs in my code
> > because the order of the evaluation of the elements in a quasiquote
> > list changes.
> 
> Changes how?  Can you give us an example, to make things concrete?
> 

Sorry, this is the stripped down version of the cause of the bug I'm 
finding various places:

(let ((a 1))
  `( ,(begin (set! a 2) a)
     ,a))
-> (2 1)


Wouldn't it be better if this one returned (2 2)?

Also, I'm seeing different behaviour in the order of evaluation between
1.6.4 and 1.7 (doesn't have the code available now). I think that is
really bad actually...

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Re: Order of the evaluation of elements in a quasiquote list.

[rm]

On Sun, Apr 03, 2005 at 01:38:04PM +0200, Kjetil Svalastog Matheussen wrote:
> On Sun, 3 Apr 2005, Neil Jerram wrote:
> 
> > Kjetil Svalastog Matheussen wrote:
> > > Hi, I have recently stumbled upon various bugs in my code
> > > because the order of the evaluation of the elements in a quasiquote
> > > list changes.
> > 
> > Changes how?  Can you give us an example, to make things concrete?
> > 
> 
> Sorry, this is the stripped down version of the cause of the bug I'm 
> finding various places:
> 
> (let ((a 1))
>   `( ,(begin (set! a 2) a)
>      ,a))
> -> (2 1)
> 
> 
> Wouldn't it be better if this one returned (2 2)?

Why do you think that "(2 2)" would be _better_? (How do you define
"better" ?)

> Also, I'm seeing different behaviour in the order of evaluation between
> 1.6.4 and 1.7 (doesn't have the code available now). I think that is
> really bad actually...

You get what you ask for: did you do a "manual" expand of your code?
This result in the following form:

     (list (begin (set! a 2) a) a)
                 
 =>  (list 2  1) | (list 2 2)

depending on the order of evaluation of function arguments, and, please,
since you read c.l.scheme: there's not to be argued about here :-/
So guile changed the order of evaluation for function arguments - that's not
good or bad. There is no reason why the implementors shouldn't do this, after
all, that's the whole point of leaving evaluation order unspecified (so implementors
can safely do this).

hth Ralf Mattes

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> 
> 
> 
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Re: Order of the evaluation of elements in a quasiquote list.

[Kjetil Svalastog Matheussen]

On Sun, 3 Apr 2005 rm@fabula.de wrote:

> On Sun, Apr 03, 2005 at 01:38:04PM +0200, Kjetil Svalastog Matheussen wrote:
> > On Sun, 3 Apr 2005, Neil Jerram wrote:
> > 
> > > Kjetil Svalastog Matheussen wrote:
> > > > Hi, I have recently stumbled upon various bugs in my code
> > > > because the order of the evaluation of the elements in a quasiquote
> > > > list changes.
> > > 
> > > Changes how?  Can you give us an example, to make things concrete?
> > > 
> > 
> > Sorry, this is the stripped down version of the cause of the bug I'm 
> > finding various places:
> > 
> > (let ((a 1))
> >   `( ,(begin (set! a 2) a)
> >      ,a))
> > -> (2 1)
> > 
> > 
> > Wouldn't it be better if this one returned (2 2)?
> 
> Why do you think that "(2 2)" would be _better_? (How do you define
> "better" ?)


It just feels more logical, left to right, top to bottom.

Thanks for the information, I had a small hope
that this was a bug in guile, or that the behaviour could be
changed with a switch. I'll fix up my code.

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Re: Order of the evaluation of elements in a quasiquote list.

[Geoffrey Knauth]

By comparison:
   PLT 299.100 and scsh 0.6.6 return (2 2).
   JScheme 7.1 and sisc 1.9.7 return (2 1).

Geoffrey
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Geoffrey S. Knauth | http://knauth.org/gsk

On Apr 3, 2005, at 07:38, Kjetil Svalastog Matheussen wrote:

> On Sun, 3 Apr 2005, Neil Jerram wrote:
>
>> Kjetil Svalastog Matheussen wrote:
>>> Hi, I have recently stumbled upon various bugs in my code
>>> because the order of the evaluation of the elements in a quasiquote
>>> list changes.
>>
>> Changes how?  Can you give us an example, to make things concrete?
>
> Sorry, this is the stripped down version of the cause of the bug I'm
> finding various places:
>
> (let ((a 1))
>   `( ,(begin (set! a 2) a)
>      ,a))
> -> (2 1)
>
> Wouldn't it be better if this one returned (2 2)?



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Re: Order of the evaluation of elements in a quasiquote list.

[Kjetil Svalastog Matheussen]

Geoffrey Knauth:

>By comparison:
>   PLT 299.100 and scsh 0.6.6 return (2 2).
>   JScheme 7.1 and sisc 1.9.7 return (2 1).

Just for the record...:
Actually, after taking a quick peek in the guile source,
the reason for the inconsistency for the result of the
mentioned code, is because the order of arguments
isn't specified in C either. So its because of different
behaviour between C compilators I sometime have got
(2 1) and sometimes (2 2) with Guile.


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