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:= _ + _
Out= 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:= Blank == Blank
Since each Blank can match anything, I find this conceptually to be the wrong behavior. As an example of a counter-intuitive result:
In:= MatchQ[a + b, _ + _]
I understand that I can deal with this problem by using HoldPattern.
In:= MatchQ[a + b, HoldPattern[_ + _]]
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.
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