ValuePreprocessingFunction

is an option for functions such as PersistentSymbol and InitializationValue that specifies a function to apply to a new value that is being assigned.

Details

When a value is to be assigned with PersistentSymbol["name",…]=value or InitializationValue["name",…]=value, the setting of the ValuePreprocessingFunction option is applied to value; the result is then assigned. If the result is $Failed, no assignment is done.
The default setting is ValuePreprocessingFunction->Identity, which does not modify the value to be assigned.
Settings for ValuePreprocessingFunction are typically done for the "Installation" persistence location.

Examples

Define a value preprocessing function that accepts only integers:

Wolfram Language code: validateInteger[x_] := If[IntegerQ[x], x, $Failed]

Set up a default value and the value preprocessing function for a persistent value named "int":

Wolfram Language code: PersistentSymbol["int", "Installation"] = 0;

Wolfram Language code: io = PersistentObject["int", "Installation"]

Wolfram Language code: io["ValuePreprocessingFunction"] = validateInteger;

Assign an integer at any persistence location:

Wolfram Language code: PersistentSymbol["int", "KernelSession"] = 42;

Assigning anything else generates an error, and the assignment is not performed:

Wolfram Language code: PersistentSymbol["int", "KernelSession"] = 0.5;

Wolfram Language code: PersistentSymbol["int"]

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