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Re: Why does _+_==2_ (or, why is HoldPattern required for
*To*: mathgroup at smc.vnet.net
*Subject*: [mg132231] Re: Why does _+_==2_ (or, why is HoldPattern required for
*From*: Sseziwa Mukasa <mukasa at gmail.com>
*Date*: Tue, 21 Jan 2014 03:01:48 -0500 (EST)
*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com
*Delivered-to*: l-mathgroup@wolfram.com
*Delivered-to*: mathgroup-outx@smc.vnet.net
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*References*: <20140120085945.0A76D69CD@smc.vnet.net>
The fundamental feature of Mathematica's programming language is
"everything is an expression". Blank[] has the value Blank[] and only is
replaced by another value in pattern matching expressions like Replace,
Cases etc. and evaluation of replacement rules such as when a substitution
rule has been defined: f[_]:=<expression>.
Since that is not happening in this case the rule x_+x_->2 x applies, hence
the result.
On Monday, January 20, 2014, Alan <alan.isaac at gmail.com> wrote:
> I'm a relatively new user of Mathematica, and the following behavior seems
> odd to me:
>
> In[1]:= _ + _
> Out[1]= 2 _
>
> Use of `Plus` here is just to illustrate a more general "problem with
> `Blank`. I had expected pattern objects to resist such evaluation. This is
> possibly related to the fact that I was surprised by the following:
>
> In[2]:= Blank[] == Blank[]
> Out[2]= True
>
> Since each Blank[] can match anything, I find this conceptually to be the
> wrong behavior.
As an example of a counter-intuitive result:
>
> In[3]:= MatchQ[a + b, _ + _]
> Out[3]= False
>
> I understand that I can deal with this problem by using HoldPattern.
>
> In[4]:= MatchQ[a + b, HoldPattern[_ + _]]
> Out[4]= True
>
> So my question is not how to deal with the problem, but rather is a
> request for an explanation of why the current pattern evaluation behavior
> is desirable.
>
> Thanks,
> Alan Isaac
>
>
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