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Re: Scoping of pattern names

• To: mathgroup at smc.vnet.net
• Subject: [mg122744] Re: Scoping of pattern names
• From: Roger Wilson <rogerhw999 at gmail.com>
• Date: Tue, 8 Nov 2011 07:17:32 -0500 (EST)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <j98djh$3r0$1@smc.vnet.net>

On Nov 7, 10:57 am, Rui <rui.r... at gmail.com> wrote:
> My goal:
> store a pattern in a variable for future use
>
> My problem:
> if the pattern I stored has a pattern name in it, (like i_/;i=8),
> and I use it later to build a new pattern in which I use the stored pattern twice, it thinks its the same name (i)
>
> My investigation:
> So far, couldn't find any way to scope pattern's names in a useful way.
> Module and With, refuse to scope a pattern name that's somewhere on the left hand side of a condition contained in them.
>
> Sure, I probably haven't tried much yet to build what I want with hard expression manipulation, but... I found my objective neat simple and useful...
> I would like to know if there's a proper way to do this, or to achieve my objective somehow else. And, I'd also enjoy knowing why such a resistance to scope pattern names...

I would suggest something along the lines of this (if I understand
your problem correctly)...

11/7/11 13:09:43 In[56]:= Clear[p]
11/7/11 13:09:44 In[57]:= p[]:= Module[{i,z},z=Condition[Apply[Pattern,
{i,Blank[]}],Or[Equal[i,8],Equal[i,9]]]; z];
11/7/11 13:09:44 In[58]:= p[]
11/7/11 13:09:44 Out[58]= i$682_/;i$682==8||i$682==9
11/7/11 13:09:45 In[59]:= MatchQ[1,p[]]
11/7/11 13:09:45 Out[59]= False
11/7/11 13:09:46 In[60]:= MatchQ[8,p[]]
11/7/11 13:09:46 Out[60]= True
11/7/11 13:09:46 In[61]:= MatchQ[{8,9},{p[],p[]}]
11/7/11 13:09:46 Out[61]= True

The Apply prevents the Pattern being evaluated when the Module is
defined so that when p[] is called a new local temporary i is created
each time so two successive calls (as in the last example) create two
different patterns and so can match to 8,9 rather than only 8,8 and
9,9.