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Re: Bug with Sequence and Assignment by Part...

  • To: mathgroup at smc.vnet.net
  • Subject: [mg121713] Re: Bug with Sequence and Assignment by Part...
  • From: Richard Fateman <fateman at cs.berkeley.edu>
  • Date: Mon, 26 Sep 2011 20:06:45 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <j5f6ai$q8a$1@smc.vnet.net> <201109250942.FAA29324@smc.vnet.net> <j5pcqu$8jc$1@smc.vnet.net>

The pronouncement that a bug is a feature is one way to cure it.

   It can be fixed another way.  Though I have not pondered on all the 
examples, the simplest one by DanL seems to illustrate the problem 
adequately.

Here is the example.

s={1,2,3}
s[[1]]=Sequence[4,5]    ... sets s to apparently  {4,5,2,3} (*)
s[[2]]= aha             ... sets s to apparently  {4,5,aha,2,3}
                              NOT
                  {4,aha,2,3}
which illustrates that (*) is apparently not the value of s.

whereas

s={1,2,3}
s[[1]]=Sequence[4,5]    ... sets s to apparently  {4,5,2,3} (*)
s = s                   ... only change from above, insert this
s[[2]]= aha             ... sets s to apparently  {4,aha,2,3}
  note change, in last line. s is now correct.


..........



Note that z=Sequence[4,5]  is really the same as Set[z,4,5].

Here's a fix. Require that Set take exactly 2 arguments.   In 
particular, that
means that s[[1]]=Sequence[4,5]  is ILLEGAL.
Perhaps it can be converted to mean  s[[1]] = List[4,5]    or {4,5}

............

Will this break stuff?  Only Set of items to Sequences,
and the  implicit set of a pattern x__.  e.g.

f[x__]:=x
y=f[1,2,3]   ... sets y to Sequence[1,2,3]

Ramifications:

I think that the old use of Sequence

stuff= Sequence [a,b,c]

is used only in this way...

f[1,2,stuff,3]   to simulate   f[1,2,a,b,c,3].


Now consider re-writing it this way...

new={a,b,c}
f@@Join[{1,2},new,{3}]

and if you have a pattern Sequence (e.g. stuff) you write

f@@Join[{1,2},{stuff},{3}].

I suppose one could write another program, say conjoin, which would
allow the following...

f@@conjoin[1,2,stuff,3].

Another, more wholesale change would have f[x__]:=x  return a List 
rather than a Sequence.

in the example above, s=s  would then be required -- actually, 
s=conjoin@@s, or something like that

Presumably a change of this magnitude would result in disruption of the 
existing code base.  On the other hand, it seems to be unacceptable to 
have the insertion of the command s=s change the meaning of a program.

.....
Another run through this...

Or I suppose a hack could be put in place forcing some kind of 
re-evaluation after Set[<NonAtomic>, Sequence[...]].  which was 
previously mentioned but thought to be too expensive...

RJF















On 9/26/2011 1:23 AM, DrMajorBob wrote:
> Amen.
>
> Bobby
>
> On Sun, 25 Sep 2011 04:42:00 -0500, BernieTheJet<berniethejet at gmail.com>
> wrote:
>
>> Thank you Leonid for the link, and thank you Oleksandr for pointing me
>> to OwnValues, I hadn't ever used that before.
>>
>> So everyone's opinion seems to be that this is not a bug and that this
>> construct is something of a manipulation of proper Mathematica syntax -
>> something approaching a hack - because Sequence is a low-level
>> function that one should understand Mathematica's evaluation sequence
>> before
>> using.  Ok, that is fine.  Certainly that is one interpretation ('when
>> looked at from a certain perspective').  Certainly one can't claim
>> that the use of Sequence is hidden, or in anyway indicated to be a
>> 'low-level' operator by its presentation in the Help, except insofar
>> as it is given a perfunctory presentation.
>>
>> But I think that it is just as valid to say that this 'bug' it is
>> counter to one of my favourite Mathematica design philosophy which is,
>> somewhat similar to Apple in this regard, to remove all the tedious
>> and computer-ish work from the user's care, to take care of that
>> behind the scenes, while still delivering as much of the power of the
>> language as possible.  So for users to have to understand Mathematica's
>> evaluation sequence, SequenceHold (which I have never once used, or
>> perhaps even needed), or OwnValues (ditto) in order to get logical
>> behaviour from a function seems to clearly go against this
>> philosophy.  Of course one can counter that there is another design
>> philosophy in Mathematica of mixing the best of all worlds, and never
>> forcing
>> oneself to strictly subscribe to any one design structure, which I
>> also support.
>>
>> For me, I just guess that Sequence is a 'hack' that WRI implemented to
>> get around the limitation that everything be a List.  A 'hack' in that
>> it defied their initial plan that everything be representable as
>> Lists, and then a 'hack' in that they couldn't get it to work in a
>> logical fashion in all subsequent interactions with other functions,
>> as seen here.
>>
>> B
>>
>
>





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