Re: Sequence as a universal UpValue

*To*: mathgroup at smc.vnet.net*Subject*: [mg74632] Re: Sequence as a universal UpValue*From*: "Szabolcs" <szhorvat at gmail.com>*Date*: Thu, 29 Mar 2007 02:29:32 -0500 (EST)*References*: <euan47$dni$1@smc.vnet.net>

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/

**Follow-Ups**:**Re: Re: Sequence as a universal UpValue***From:*"Chris Chiasson" <chris@chiasson.name>

**Re: Re: Sequence as a universal UpValue***From:*"Chris Chiasson" <chris@chiasson.name>