Once[…, "location"] adds $CellContext` to all global symbols

Question

For certain persistence locations, such as "Notebook" or "FrontEndSession", Once adds $CellContext` to all Global` symbols, e.g.

Once[x, "Notebook"]

after the second evaluation (when the persistent object has been created) will only give

$CellContext`x

and not just x.

I'm sure there's a good reason for this (avoiding collisions), but I see two problems at the moment:

1. The obvious problem that all names of symbols are now changed.
2. In my case, in place of x I have an expression that returns a fairly long result, and the addition of $CellContext` to each symbol name blows up the stored result unnecessarily.

How can I get Once not to add $CellContext` to every symbol name?

I have already tried some tricks such as Block[{$Context = "System`"}, …] (as in Exporting notebooks inserts unwanted $CellContext. Workaround?), but Once appears to be treatment-resistant.

(I'm aware that one can access and change the content of the corresponding PersistentObject and in this way manually remove the added context, but this seems like a risky brute force solution to me.)

Answer 1

I don't have a solution, only a workaround:

undoCellContext[expr_] := Module[{ccSymbols, rules},
  ccSymbols = 
   Cases[expr, _Symbol, \[Infinity]] // DeleteDuplicates // 
    Select[StringStartsQ["$CellContext`"]@*ToString];
  rules = (# -> Symbol["Global`"<>SymbolName[#]]) & /@ ccSymbols;
  expr /. rules
  ]

Once[...,"Notebook"] // undoCellContext

As an optimization, when LeafCount[expr] is suitably large, undoCellContext could switch from building a list and deleting duplicates to populating a HashSet within the delayed rule given to Cases.

It would also perhaps help if undoCellContext held the expression, and the replacement symbols, unevaluated until it was done with it.

We could "Fix" Once on versions where it's misbehavinbg:

If[$VersionNumber<=14.,
  Unprotect[Once];
  Once`Private`$overridden = True;
  Once[expr__,"Notebook"] := Block[{Once`Private`$overridden = False},
    Once[expr, "Notebook"] // undoCellContext
  ] /; Once`Private`$overridden
  Protect[Once];
]

For reference, Once is not guilty. The underlying problem is that PersistentObject[...,"Notebook"]["Value"] converts Global</code> symbols to <code>$CellContext on readout.