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 Hol

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