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front end - Difference between ParentList and Inherited


In this answer Carl Woll uses the ParentList symbol to inherit old input aliases when adding a new one:


CurrentValue[EvaluationNotebook[], 

   InputAliases] = {"0x" -> 
    RowBox[{InterpretationBox[
       StyleBox["\"0\"", ShowStringCharacters -> False], 
       Function[Null, 
        FromDigits[StringDrop[ToString@Unevaluated@#, 1], 16], 
        HoldAll]], "\[InvisibleApplication]", "\[InvisibleSpace]", 
      StyleBox["x", ShowAutoStyles -> False]}], ParentList};
CurrentValue[EvaluationNotebook[], InputAliases] /. Rule[a_, _] :> a

{"0x", ParentList}


And we can still use the all the basic built-in aliases


On the other-hand, we can get the same result using Inherited:


CurrentValue[EvaluationNotebook[], InputAliases] = Inherited;
CurrentValue[EvaluationNotebook[], 
   InputAliases] = {"0x" -> 
    RowBox[{InterpretationBox[
       StyleBox["\"0\"", ShowStringCharacters -> False], 
       Function[Null, 
        FromDigits[StringDrop[ToString@Unevaluated@#, 1], 16], 

        HoldAll]], "\[InvisibleApplication]", "\[InvisibleSpace]", 
      StyleBox["x", ShowAutoStyles -> False]}], Inherited};
CurrentValue[EvaluationNotebook[], InputAliases] /. Rule[a_, _] :> a

{"0x", Inherited}

And we can, again, use the all of the built-in aliases, but if we look at the AbsoluteCurrentValues for the first case:


CurrentValue[EvaluationNotebook[], 
   InputAliases] = {"0x" -> 
    RowBox[{InterpretationBox[

       StyleBox["\"0\"", ShowStringCharacters -> False], 
       Function[Null, 
        FromDigits[StringDrop[ToString@Unevaluated@#, 1], 16], 
        HoldAll]], "\[InvisibleApplication]", "\[InvisibleSpace]", 
      StyleBox["x", ShowAutoStyles -> False]}], ParentList};
AbsoluteCurrentValue[EvaluationNotebook[], InputAliases] /. 
 Rule[a_, _] :> a

{"0x", "intt", "dintt", "sumt", "prodt", "dt", "ia", "cbrt", "surd", \
"ket", "bra", "braket", "delay", "grad", "del.", "delx", "del2", \

"notation", "notation>", "notation<", "symb", "infixnotation", \
"addia", "pattwraper", "madeboxeswraper"}

And the second:


CurrentValue[EvaluationNotebook[], InputAliases] = Inherited;
CurrentValue[EvaluationNotebook[], 
   InputAliases] = {"0x" -> 
    RowBox[{InterpretationBox[
       StyleBox["\"0\"", ShowStringCharacters -> False], 
       Function[Null, 

        FromDigits[StringDrop[ToString@Unevaluated@#, 1], 16], 
        HoldAll]], "\[InvisibleApplication]", "\[InvisibleSpace]", 
      StyleBox["x", ShowAutoStyles -> False]}], Inherited};
AbsoluteCurrentValue[EvaluationNotebook[], InputAliases] /. 
 Rule[a_, _] :> a

{"0x", Inherited}

We see that even though they are operationally similar, the AbsoluteCurrentValue result is different.


So I guess my question is, how do ParentList and Inherited differ outside of this small difference?



ParentList is obviously more scoped to list-type constructs, but why does it need to exist at all?



Extra Info:


Per Albert Retey's comment I checked whether either responded differently to upstream changes:


StyleSheetEdit["MyStyle1" -> "Input",
  {
   InputAliases -> {"0x" -> "\"\[SadSmiley]\"", ParentList}
   },
  "MakeCell" -> True
  ];

CellPrint[Cell[BoxData[""], "MyStyle1"]];
StyleSheetEdit["MyStyle2" -> "Input",
  {
   InputAliases -> {"0x" -> "\"\[SadSmiley]\"", Inherited}
   },
  "MakeCell" -> True
  ];
CellPrint[Cell[BoxData[""], "MyStyle2"]];
CurrentValue[EvaluationNotebook[], InputAliases] = {"a" -> "\"b\""};


Both cells behaved entirely the same



Answer



Its name is misleading, unfortunately, because it can be very useful.


A better name would be something around ParentSequence/ParentArguments. Here is one example:


CurrentValue[EvaluationNotebook[], TaggingRules] = {"old" -> "value"};
cell = EvaluationCell[];

CurrentValue[cell, TaggingRules] = {ParentList, "new" -> "value"};
AbsoluteCurrentValue[cell, TaggingRules]



{"old" -> "value", "new" -> "value"}

CurrentValue[cell, TaggingRules] = {Inherited, "new" -> "value"};
AbsoluteCurrentValue[cell, TaggingRules]


{{"old" -> "value"}, "new" -> "value"}

So in this case you really want to use it.



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