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/

**References**:**Re: Sequence as a universal UpValue***From:*"Szabolcs" <szhorvat@gmail.com>