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Single-step evaluation in Mathematica

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  • Subject: [mg81235] Single-step evaluation in Mathematica
  • From: Szabolcs Horvát <szhorvat at>
  • Date: Mon, 17 Sep 2007 03:32:27 -0400 (EDT)

I would like to have a function, hasOwnValue, that takes the name of a 
symbol as a string, and tells whether the symbol has an own-value.  It 
should do this without evaluating the value of the symbol.


abc := Print["side effect"]
xyz =.

In := hasOwnValue["abc"] (* "side effect" must not be printed *)
Out = True

In := hasOwnValue["xyz"]
Out = False

How can I achieve this?

This is a practical problem that I ran into, but the general question 
is:  What do you do when you feel the need for *single-step evaluation* 
in Mathematica?  E.g. how can I manipulate/transform held expressions 
made up of symbols that have values?  An artificial example:

a = 1

How do I reverse the list inside Hold?  I could map Hold to each 
element, reverse the list, hold the list again, and remove Hold from 
individual elements to get Hold[{c,b,a}], but this is very inconvenient.


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