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I would like to know how can I completely copy one Symbol to another. When I say copy, a refer to UpValues, DownValues, FormatValues.. and so on.

I created this function to do that, but I don't know if it's a good practice.

SetAttributes[copy,HoldFirst];
copy[new_Symbol,org_Symbol]:=Module[{},
    ClearAll@new;
    UpValues@new=UpValues@org/.org:> new;
    DownValues@new=DownValues@org/.org:> new;
    FormatValues@new=FormatValues@org/.org:> new;
    SetAttributes[new,Attributes[org]];
]

So I can copy some symbol b into a using a~copy~b.

There is a simpler way to do that? Or this approach is ok?


Update

Thanks for all comments. This is the evolution of the function above:

SetAttributes[copy,HoldFirst];
new_~copy~org_:=With[{prop={Attributes,UpValues, OwnValues, DownValues, SubValues, NValues, FormatValues, Messages,Options}},
    ClearAll@new;
    Set[#@new,#@org/.HoldPattern@org:>new]&~Scan~prop;
]
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Do you have a good use case for this? Or is it just theoretical? –  Szabolcs Feb 21 '13 at 3:01
2  
There's some clone function of Leonid around, that does basically this. Don't forget Options, NValues, OwnValues, SubValues, Defaults. And make it HoldAll –  Rojo Feb 21 '13 at 3:01
1  
Also, you can't trust this for built-ins, some of which seem to have "hidden" values, or for ReadProtected and Locked symbols, or for symbols that are recognized as special by other functions –  Rojo Feb 21 '13 at 3:02
    
There's a subtlety: if org refers to org in its definition, will new keep referring to org or to new? –  Szabolcs Feb 21 '13 at 3:03
    
In other words, I wouldn't trust this too much unless you know the function you are cloning. And I don't know a better way to copy a symbol –  Rojo Feb 21 '13 at 3:04
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1 Answer

up vote 23 down vote accepted

This is a simplification of your solution:

Language`ExtendedFullDefinition[new] = 
 Language`ExtendedFullDefinition[old] /. HoldPattern[old] :> new

I believe Language`ExtendedFullDefinition is used in transferring definitions between the main kernel and subkernels. Also note the HoldPattern on the LHS of the rule which ensures that OwnValues will work.


One thing to think about is: what should happen if old refers back to itself? Should new keep referring to old, or to itself? This solution, like yours, replaces the old in the definition as well.

share|improve this answer
    
Nice! I didn't know you could assign to that +1 –  Rojo Feb 21 '13 at 3:09
    
@Rojo I learned about it from Oleksandr. I'm not sure if anything can go wrong with this ... check the Options. As I said, it's meant to be used for transferring definitions to subkernels, and for some contexts this should be prevented. –  Szabolcs Feb 21 '13 at 3:12
    
Wow, much more simpler!! Tks! I have to think about the "old to itsef" question. I Don't know yet the best to my case. +1 –  Murta Feb 21 '13 at 3:14
    
Damn... I just came here to write this! Of course, I learned this from you, so it's right that you got to post it :) –  rm -rf Feb 21 '13 at 3:15
    
Does this method properly preserve Unevaluated, which is lost in a regular DownValues call? I am talking about issue described in Leonid's comment and the linked Q&A. (I cannot test it because this function is not in version 7.) –  Mr.Wizard Mar 15 '13 at 1:34
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