Why is SparseArray and InterpolationFunction always grayed out?

Question

In the front end of Mathematica version 10, a new feature was implemented which dims output cells that no longer correspond to its input cells. So when you type

1 + 1
(* 2 *)

and go into the input and add another +1, the 2 is slightly gray to indicate that 1+1+1 is not 2. SparseArray seem to be always grey, as if they are never up-to-date

The same is true for InterpolatingFunction but not for CompiledFunction.

Any clues why this is?

Answer 1

This is not related to the output cell greying to indicate mismatch with the input cell. You can easily verify this by editing the input cell — it gets further greyed out.

The new summarized display that is used for things like SparseArray, Interpolation, etc. has a setting "Interpretable" -> False. If this is the case, the output is showed in a gray font. On the other hand, the summarized outputs that have "Interpretable" -> True are shown in black.

You can check the above conclusion with the following example:

foo /: MakeBoxes[foo, StandardForm] := 
 BoxForm`ArrangeSummaryBox[foo, Null, 
  Null, {BoxForm`MakeSummaryItem[{"Message: ", "Hello"}, 
    StandardForm]}, {"World"}, StandardForm, "Interpretable" -> False];
foo