Sequence as a universal UpValue

*To*: mathgroup at smc.vnet.net*Subject*: [mg74586] Sequence as a universal UpValue*From*: "Chris Chiasson" <chris at chiasson.name>*Date*: Tue, 27 Mar 2007 04:02:30 -0500 (EST)

In his presentation on working with held expressions at http://library.wolfram.com/conferences/devconf99/villegas/UnevaluatedExpressions/Links/index_lnk_22.html 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? 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/