How to adapt MSPToExpression function in $PrePrint?

Question

I am introducing the mathematica Security (MSP module) to our system. The Document suggests use MSPToExpression as followed:

Needs["MSP`"]

SetSecurity[];

$$var = "5+7";

MSPToExpression[ $$var] (*the output is 12*)

---

But when I tried to adapt this function to $PrePrint with scripts:

$PrePrint = With[{$$var = ToString[#, InputForm]}, MSPToExpression[$$var]] &;
(*or*)

$PrePrint = With[{expr = #}, MSPToExpression[expr]] &;
(*or*)

$PrePrint = With[{$$var = #}, MSPToExpression[$$var]] &;

Input: 5+7

there's same error says:

It can't recognize $$var as symbol, can anyone help me on this? Many thanks! :)

Answer 1

I suppose this is what you need.

So the first thing is to use Module since With injects values into held expressions.

Moreover PrePrint is probably to late to catch "5+7". So I'd go with $Pre:

$Pre = Function[expr, 
  Module[
        {expr$ = ToString[Unevaluated[expr], InputForm]},
         MSPToExpression[expr$]
  ],
  HoldAllComplete
]