Why does _+_==2_ (or, why is HoldPattern required for sensible matching)?

*To*: mathgroup at smc.vnet.net*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)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-outx@smc.vnet.net*Delivered-to*: mathgroup-newsendx@smc.vnet.net

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

**Follow-Ups**:**Re: Why does _+_==2_ (or, why is HoldPattern required***From:*Itai Seggev <itais@wolfram.com>

**Re: Why does _+_==2_ (or, why is HoldPattern required for***From:*Bob Hanlon <hanlonr357@gmail.com>

**Re: Why does _+_==2_ (or, why is HoldPattern required for***From:*Sseziwa Mukasa <mukasa@gmail.com>