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Alan
Posts:
95
Registered:
3/29/11


Why does _+_==2_ (or, why is HoldPattern required for sensible matching)?
Posted:
Jan 20, 2014 3:53 AM


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 counterintuitive 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



