Conversion of expressions by the FrontEnd

Question

As I learned recently there are some types of expressions in Mathematica which are automatically parsed by the FrontEnd.

For example if we evaluate HoldComplete[Rotate[Style[expr, Red], 0.5]] we see that the FrontEnd does not display the original expression:

Is it possible to control such behavior of the FrontEnd?

And is it possible to get complete list of expressions those are parsed by the FrontEnd automatically?

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EDIT

We can see calls to MakeBoxes when using Print:

On[MakeBoxes]; Print[HoldComplete@Rotate["text", Pi/2]]

But copy-pasting the printed output gives changed expression: HoldComplete[Rotate["text", 1.5707963267948966]]. It shows that Print does not respect HoldComplete.

When creating output Cell there should be calls for MakeBoxes too. Is there a way to see them?

Answer 1

I have found a post by John Fultz with pretty clear explanation of how graphics functionality works:

In version 6, the kernel has absolutely no involvement whatsoever in generating the rendered image. The steps taken in displaying a graphic in version 6 are very much like those used in displaying non-graphical output. It works as follows:

1) The expression is evaluated, and ultimately produces something with head Graphics[] or Graphics3D[].

2) The resulting expression is passed through MakeBoxes. MakeBoxes has a set of rules which turns the graphics expression into the box language which the front end uses to represent graphics. E.g.,

In[9]:= MakeBoxes[Graphics[{Point[{0, 0}]}], StandardForm]

Out[9]= GraphicsBox[{PointBox[{0, 0}]}]

Internally, we call this the "typeset" expression. It may be a little weird thinking of graphics as being "typeset", but it's fundamentally the same operation which happens for typesetting (which has worked this way for 11 years), so I'll use the term.

3) The resulting typeset expression is sent via MathLink to the front end.

4) The front end parses the typeset expression and creates internal objects which generally have a one-to-one correspondence to the typeset expression.

5) The front end renders the internal objects.

This means that the conversion is performed in the Kernel by a call to MakeBoxes.

This call can be intercepted through high-level code:

list = {};
MakeBoxes[expr_, form_] /; (AppendTo[list, HoldComplete[expr]]; 
    True) := Null;
HoldComplete[Rotate[Style[expr, Red], 0.5]]
ClearAll[MakeBoxes];
list

Here is what we get as output:

One can see that MakeBoxes does not respect HoldAllComplete attribute.

The list of symbols which are auto-converted before sending to the FrontEnd one can get from FormatValues:

In[1]:= list = 
  Select[Names["*"], 
   ToExpression[#, InputForm, 
     Function[symbol, Length[FormatValues@symbol] > 0, HoldAll]] &];
list // Length

During evaluation of In[1]:= General::readp: Symbol I is read-protected. >>

Out[2]= 162

Answer 2

Not quite an answer, but in Preferences > Evaluation there are options to "Only use textual boxes when converting (input|output) to typeset forms."

If you check these, then using Cell > Convert To... > StandardForm etc... will show the Rotate[..] instead of the visually rotated result.

Answer 3

John Fultz has recently answered my question on converting TableForm to "typeset" expressions and it is worth to cite it here since it amplifies (while partially contradicts) the general explanation cited in my previous answer:

ToBoxes is returning precisely what the kernel sends to the front end without variation (except, in the general case, for the evaluation semantics and side effects possibly being different, but that's not an issue in your example).

The issue is that the front end has two different specifications for specifying GridBox options... one of which dates back to version 3, and the other, more expansive set dates to version 6. The front end understands both sets of options, but canonicalizes anything it receives to the version 6 options.

GridBox is the only box which has had such a wholesale change of options, and it was necessary to support new functionality we added in v6. But the front end will continue to understand the old options for a seriously long time (probably forever), as the old options show up not only in certain kernel typesetting constructs, but in legacy notebook files.

ToBoxes[] of TableForm is creating the legacy options, as there's been no need to update the typesetting of TableForm in a while (ToBoxes[] of Grid, on teh other hand, uses modern options). The conversion is done by the front end. You could rely on the front end to do the conversion for you, or you could figure out how the options map yourself.

So in this case the final stage of the conversion of the expression is done by the FrontEnd.

Answer 4

There are two aspects to what you witness. First, transcription of the expression you entered into boxes and rendering those boxes by Front-End. By default the output is typeset using StandardForm, which has a typesetting rule to render graphics and geometric transforms. If you use InputForm, there are no such rule. You can control which form is used via Preferences->Evaluation.

You can convince yourself that HoldComplete correctly did its job by using InputForm or FullForm on the input, or using InputForm display on the output cell.

EDIT Using the OutputForm:

In[13]:= OutputForm[%]

Out[13]//OutputForm= HoldComplete[Rotate[expr, 0.5]]

In regard to your question about complete list of symbols, it includes Graphics, geometric operations, and possibly others, but I do not know of the complete list.