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MathGroup Archive 2007

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Re: Re: Sequence as a universal UpValue

  • To: mathgroup at smc.vnet.net
  • Subject: [mg74654] Re: [mg74632] Re: Sequence as a universal UpValue
  • From: "Chris Chiasson" <chris at chiasson.name>
  • Date: Thu, 29 Mar 2007 05:33:56 -0500 (EST)
  • References: <euan47$dni$1@smc.vnet.net> <200703290729.CAA07586@smc.vnet.net>

Also, I think you have sufficiently answered my question. Thank you.

On 3/29/07, Szabolcs <szhorvat at gmail.com> wrote:
> On Mar 27, 11:11 am, "Chris Chiasson" <chris at chiasson.name> wrote:
> > In his presentation on working with held expressions at
> >
> > http://library.wolfram.com/conferences/devconf99/villegas/Unevaluated...
> >
> > Villegas says:
> >
> > "In fact, Sequence itself could almost be implemented as a universal
> > UpValue (maybe Dave Withoff or Roman Maeder remembers if that's not
> > quite true)."
> >
> > So, I am wondering, does the following input disprove that Sequence
> > can be implemented as a universal UpValue? How should I think of
> > Sequence? Importantly, why doesn't blocking Sequence work like
> > blocking the arbitrary symbol?
>
> I'm not sure I understand completely how these things work, but the
> behaviour of Block does seem to make sense if you read its help page.
> It says:
>
> " When you execute a block, values assigned to x, y, ... are cleared.
> When the execution of the block is finished, the original values of
> these symbols are restored. "
>
> When you put blahblah in a Block, the definitions associated with it
> are cleared, and
> its arguments are not spliced into Map. You get the expected result.
>
> But the definitions associated with Sequence are built-in, so they can
> not be cleared.
>
> ...
>
> Hmm ... Now that I experimented some more, Sequence does seem to be
> special in this respect:
>
> In[1]:=
> Block[{},Print[f/@{a,b}]]
> Block[{Map},Print[f/@{a,b}]]
>
> >From In[1]:=
> {f[a],f[b]}
>
> >From In[1]:=
> f/@{a,b}
>
> So built-in definitions can be cleared after all. But the Mathematica
> book does mention that Sequence is treated in a special way (unlike
> other built-ins). Check Section A.4.1 (Mathematica Reference Guide ->
> Evaluation -> The Standard Evaluation Sequence).
>
>
> >
> > In[1]:=
> > blahblah/:h_[l___,blahblah[blahblahArgs___],r___]=h[l,blahblahArgs,r]
> >
> > UpValues@blahblah
> >
> > a[1,blahblah[2,3]]
> >
> > Block[{Sequence},f/@Sequence[1,2,3]]
> >
> > Block[{blahblah},f/@blahblah[1,2,3]]
> >
> > Out[1]=
> > h[l,blahblahArgs,r]
> >
> > Out[2]=
> > {HoldPattern[h_[l___,blahblah[blahblahArgs___],r___]]\[RuleDelayed]
> >     h[l,blahblahArgs,r]}
> >
> > Out[3]=
> > a[1,2,3]
> >
> > Map::nonopt: Options expected (instead of 3) beyond position 3 in
> > Map[f,1,2,3]. An option must be a rule or a list of rules.
> >
> > Out[4]=
> >
> > Map[f,1,2,3]
> >
> > Out[5]=
> > blahblah[f[1],f[2],f[3]]
> >
> > Thanks for your input,
> >
> > --http://chris.chiasson.name/
>
>
>
>


-- 
http://chris.chiasson.name/


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