What is Undefined ?

[rocky]

Is it treated like value like True or False or ComplexInfinity?

RIght now on input we treat it as a Global

mathics --full-form

Mathics 5.0.3dev0
.. 

In[1]:= True
System`True
Out[1]= True

In[2]:= Undefined
Global`Undefined
Out[2]= Undefined

In[3]:= ComplexInfinity
System`ComplexInfinity
Out[3]= ComplexInfinity

Is this the correct behavior?

If this is not right and it should be in the System context should Undefined be declared as a PredefinedSymbol (that is a value that can't be reassigned like True, and ComplexInfinity.

[mmatera]

https://reference.wolfram.com/language/ref/Undefined.html

In WMA

n[1]:= ?? Undefined                                                            

Out[1]= Undefined is a symbol that represents a quantity with no defined value.


        Attributes[Undefined]={Protected}

[mmatera]

https://reference.wolfram.com/language/ref/Undefined.html

In WMA

n[1]:= ?? Undefined                                                            

Out[1]= Undefined is a symbol that represents a quantity with no defined value.


        Attributes[Undefined]={Protected}

So, it is different than System`True in the sense that you can unprotect it and assign a different value.

[rocky]

But is this supposed to be to have Global or System context?

[rocky]

Should it be treated as a constant like ComplexInfinity?

[mmatera]

The context should System and just has the attribute Protected so it is not a numeric constant.