Why does _+_==2_ (or, why is HoldPattern required for sensible matching)?
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- Subject: [mg132222] Why does _+_==2_ (or, why is HoldPattern required for sensible matching)?
- From: Alan <alan.isaac at gmail.com>
- Date: Mon, 20 Jan 2014 03:59:45 -0500 (EST)
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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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