Avoiding eval when overloading a function

I used eval in this answer where I was trying to overload display and it is making me feel sick. Is it possible to avoid the eval? A simple example is overloading display to prepend a string to object of class char ...

If you add the following function to a folder called @char

function display(X)
  X = ['The string: ', X];
  eval([inputname(1), ' = X;']);
  eval(['builtin(''display'', ', inputname(1), ');']);
end

and then do

>> z = 'Hello world'
z =
  The string: Hello world

I need the eval to get the variable name correct in the output. The documentation for display shows a work around where you recreate the functionality of the built-in display, but that is more distasteful to me than eval. Is it possible to overload display to do some preprocessing and then hand off the result to the built-in display without using eval

EDIT

My simplification has apparently confused people. In this question Jonathan essentially noted that

>> str = '<test TEST>'

returns

str =
  TEST

but he wanted it to return

str = 
  <test TEST>

I suggested an answer which overloaded display in a manner analogous to my simplification above.

[Jan]

+1: My favorite question of the week, because it uses trivial commands to dig in the underground of the Matlab interpreter.

Answer 1

[Jan]

I do not get the problem. Would this be useful:

function display(X)
  Name = inputname(1);
  if ~isempty(Name)
    localAssignName(Name, X);
    builtin('display', Name);
  end
end

function localAssignName(Name, Data)
assignin('caller', Name, ['The string: ', Data]);
end

I cannot test this currently. How does the output differ from your expectations?

[Daniel Shub]

It doesn't work.

>> z = 'Hello world'
Name =
  z

I think your approach can get around the first eval, but not the second. You need to pass the built-in display function a variable with the name returned by inputname(1) and the contents stored in X.

[Jan]

Then let me apply a subterfuge: Why on earth do you need such a strange function?!

I'll take a look into the source of DISPLAY in the evening. Obviously it uses inputname itself, such that my approach must fail. What happens with your method, when the variable is called "inputname" or "builtin"?

[Daniel Shub]

The original question that I was answering, which addressed a problem that I have had in the past, is how to display a variable which contains an HTML string. The DISPLAY function is built-in so you cannot see the code. Using poorly named variables causes problems.

Answer 2

[Sean de Wolski]

Perhaps (probably) I'm missing something, but could you just use fprintf?

[Daniel Shub]

No, see this answer and the related question.