When a HoldComplete expression reaches the frontend, there will still be evaluation happening within it, namely for all expressions that have FormatValues.
For example
Format[myx, StandardForm] = "somethingelse";
HoldComplete@myx
displays as
HoldComplete[somethingelse]
If I choose another Form for which I have not defined FormatValues, the formatting is not applied:
OutputForm@HoldComplete@myx
HoldComplete[myx]
Interestingly, SparseArray[{}, {2, 2}] displays differently from HoldComplete@SparseArray[{}, {2, 2}] (which stays unchanged) and I have not been able to find the ...Form that would format the SparseArray while inside HoldComplete. I have tried
Through[(Symbol /@ Names["System`*Form"])@
HoldComplete@SparseArray[{}, {2, 2}]]
They must be using an internal codepath for that.
I'm also a bit surprised that many Forms besides StandardForm display HoldComplete@myx as HoldComplete[somethingelse], try
Through[(Symbol /@ Names["System`*Form"])@HoldComplete[myx]]
What's bitten me is also that Manipulate inside HoldComplete are evaluated:
HoldComplete[Manipulate[0, {x, Abort[]}]]
gives
$Aborted[]
(interestingly, StandardForm@HoldComplete[Manipulate[0, {x, Abort[]}]] gives $Aborted without the extra []).
The workarounds seem to be to apply any of ToString, FullForm, OutputForm, InputForm...
On a final note: FormatValues of Manipulate shows a rule for MakeBoxes -- I thought you are supposed to define output formats by modifying Format[x, form]?!
Where can I learn more about output formatting and format time evaluations?
I found ref/Format, tutorial/DefiningOutputFormats and maybe tutorial/FormattedOutput. Anything else?
Aside I just noticed that all FormatValues report rules for MakeBoxes...
