Re: Curiosity concerning transformation rules for List

• To: mathgroup at smc.vnet.net
• Subject: [mg70919] Re: Curiosity concerning transformation rules for List
• From: "Ray Koopman" <koopman at sfu.ca>
• Date: Wed, 1 Nov 2006 03:56:19 -0500 (EST)
• References: <ei4ktg\$dbf\$1@smc.vnet.net>

Andrew Moylan wrote:
> Since the List symbol is locked, I am curious about the possibility (or
> otherwise) of giving definitions for which the left-hand-side of the
> transformation rule contains only the List symbol. Here's an arbitrary,
> explicit example:
>
> Is it possible to make a definition such that: any list of two
> identical elements evaluates to the empty list? E.g. {x_, x_} -> {}.
>
> I can't see any way this transformation rule can be added. It's not
> possible to modify the DownValues for List; and there are no
> first-level symbols to which an UpValue can be added. Does anyone have
> any ideas?

In[1]:= a = Table[Random[Integer],{10},{2}]
Out[1]= {{0,1},{1,1},{0,0},{1,0},{0,0},{0,0},{1,0},{1,1},{0,0},{0,1}}

In[2]:= a /. {x_,x_}->{}
Out[2]= {{0,1},{},{},{1,0},{},{},{1,0},{},{},{0,1}}

In[3]:= a /. {x_,x_}->Sequence[]
Out[3]= {{0,1},{1,0},{1,0},{0,1}}

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