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How to get non-printing heads?

• To: mathgroup at smc.vnet.net
• Subject: [mg46946] How to get non-printing heads?
• From: kj <socyl at 987jk.com>
• Date: Tue, 16 Mar 2004 19:55:13 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

How can I give a Mathematica object a different head?

I tried

  In[1]:= Head[a] ^= Constant;

But, as shown below, this doesn't work as desired:

  In[2]:= MatchQ[a, _Constant]

  Out[2]= False

  In[3]:= MatchQ[a, _Symbol]

  Out[3]= True

I even tried

  In[4]:= a/: MatchQ[a, _Constant] = True;

  In[5]:= ??a
  Global`a

  Head[a] ^= Constant
   
  a /: MatchQ[a, _Constant] = True

...but

  In[6]:= MatchQ[a, _Constant]

  Out[6]= False

Is there any way to give "a" a user-defined head?

My immediate goal is to be able to define functions that perform
algebraic manipulations on Mathematica expressions according to
the types of the objects they contain.  I want to leave these
objects pretty much undefined, other than specifying their types.
E.g.:

  myDerivative[x_Constant] := 0;

  myDerivative[a]

  ==> 0

  myDerivative[b]

  ==> myDerivative[b]    (* Derivative doesn't know how to handle Symbol *)

TIA,

kj
-- 
NOTE: In my address everything before the period is backwards.

• Follow-Ups:
  • Re: How to get non-printing heads?
    • From: Tomas Garza <tgarza01@prodigy.net.mx>