programming - A pre-parsed HoldForm

HoldForm holds an expression's parsed form not its actual "inputted form" e.g.

 SetAttributes[AttemptedRespect, HoldAll];

 AttemptedRespect[x_] := ExpressionCell[HoldForm@x, "Input"]

(assoc = <|f@x -> (x // g)|>) // AttemptedRespect

assoc = Association[f[x] -> g[x]]

where the held short forms/notation of Association/f/g are no longer respected.

To instigate respect:

 FleetingRespect[expr_] :=

 (Unprotect@Association;
 Format[Association[x__]] := <|x|>;
 Format[f[x_]] := Prefix[f@x];
 Format[g[x_]] := Postfix[x // g];
 ExpressionCell[HoldForm@expr, "Input"])

and:

(assoc = <|f@x -> (x // g)|>) // FleetingRespect

assoc = <|f@x -> (x // g)|>

which is, of course, indeed fleeting:

(assoc = Association[(x // f) -> (g@x)]) // FleetingRespect

assoc = <|f@x -> (x // g)|>

Is there a way to cajole HoldForm into more faithfully respecting what she wraps?

In other words - a function PermanentRespect such that:

(assoc = <|f@x -> (x // g)|>) // PermanentRespect

assoc = <|f@x -> (x // g)|>

and

(assoc = Association[(x // f) -> (g@x)]) // PermanentRespect

assoc = Association[(x // f) -> (g@x)]

Yoh - Respect.

Answer: The final effort in Simon Rochester's answer almost creates a pre-parsed HoldForm (without touching $PreRead) and it certainly meets the use-case requirements that originally motivated the question. Essentially I wanted to programmatically generate the "natural input shortforms" with normal output form (without using any frontend manipulations).

MakeExpression[
   RowBox@{"hf", "[", expr_, "]"} | 
   RowBox@{"hf", "@", expr_} | 
   RowBox@{expr_, "//", "hf"}, StandardForm] := HoldComplete@RawBoxes@expr;

SetAttributes[IOCells, HoldAll];


IOCells[expr_] := Grid[{
    {ExpressionCell[expr, "Input", ShowStringCharacters -> True]},         
    {ExpressionCell[ToExpression@First@expr, "Output"]}}, 
          Frame -> True,
          Alignment -> Left,
          Background -> {None, {1 -> GrayLevel[.9], 2 -> White}}];

and now both examples

Grid[{
   {hf[assoc = <|f@x -> (x // g)|>],

    hf[assoc = Association[(x // f) -> (g@x)]]}
      }] // (Map[IOCells, #, {3}] &)

preserve input forms, respectively.

Answer

I think $PreRead may be your only hope (but see below). You can set it up with

$PreRead = (# /. 
  RowBox@{"PermanentRespect", "[", expr_, "]"} | 
  RowBox@{"PermanentRespect", "@", expr_} | 

  RowBox@{expr_, "//", "PermanentRespect"} :> 
    RowBox@{"RawBoxes", "[", MakeBoxes@expr, "]"}
) &;

LoseRespect[expr_] := expr /. RawBoxes -> ToExpression

Now you can do

(assoc = <|f@x -> (x // g)|>) // PermanentRespect

(assoc = <|f@x -> (x // g)|>)

or

expr = PermanentRespect[(assoc = <|f@x -> (x // g)|>)]

(assoc = <|f@x -> (x // g)|>)

The expression hasn't been evaluated:

assoc

assoc

Evaluate it with LoseRespect:

LoseRespect[expr]

<|f[x] -> g[x]|>

---

Despite what I said above, I think $PreRead may not be your only hope. Here's another solution that uses NotebookRead to get the box representation of the currently evaluating cell. It then looks through those boxes to find the box representation of the call to itself, and returns that wrapped in RawBoxes.

(First clear $PreRead with $PreRead =. if the definition above is still active.)

Define PermanentRespect:

SetAttributes[PermanentRespect, HoldAll]

PermanentRespect[expr_] := 
  First@Cases[
    NotebookRead[EvaluationCell[]], 
    RowBox@{"PermanentRespect", "[", boxes_, "]"} | 

      RowBox@{"PermanentRespect", "@", boxes_} | 
      RowBox@{boxes_, "//", "PermanentRespect"} /; 
      MakeExpression[boxes, StandardForm] == HoldComplete[expr] :> RawBoxes[boxes], 
    Infinity
  ]

Now PermanentRespect gives the same results as obtained above.

---

I'm getting more wrong by the minute about $PreRead -- here's a third solution that employs a custom definition for MakeExpression, which is used whenever boxes are converted to expressions by the front end:

ClearAll[PermanentRespect]


MakeExpression[
  RowBox@{"PermanentRespect", "[", expr_, "]"} | 
    RowBox@{"PermanentRespect", "@", expr_} | 
    RowBox@{expr_, "//", "PermanentRespect"}, 
  StandardForm
] := HoldComplete@RawBoxes@expr

This also gives the same results as above.

---

Also, any of the above methods can be used with .m package files, if the package file is evaluated as a notebook instead of using Get. You can define

myGet[file_] := Module[{obj},
  obj = NotebookOpen[file, Visible -> False];
  NotebookEvaluate[obj];
  NotebookClose[obj]
]

Then if the package file is loaded with myGet["package.m"], the PermanentRespect function calls inside the package will behave as above.