EVALIN cannot execute a return?

I am trying to create a function that aborts whatever mfile is currently executing after issuing a warning. I see no way of avoiding EVALIN for this and so implemented the following,

function abort_and_warn

  warning 'Aborting'
  
  evalin('caller', 'return');

end

However, a simple test of this code in R2020a,

function test

    disp 'do this'
    
    abort_and_warn
    
    disp 'do that'

end

fails to block the execution of the final line:

>> test

do this
Warning: Aborting 
> In test>abort_and_warn (line 12)
  In test (line 5) 
do that

Is this an undocumented limitation of EVALIN? I see nothing here to imply that EVALIN should be unable to issue the return command in the caller wokspace.

[Bruno Luong]

To me it is clear that return cannot be carried out. In order return to carry out, all the pending caliing should be terminated, but the calling function (the one that has EVALIN) is till executing. So no, it's not possible logically, unless the (implicit) RETURN of the CALLING function can somehow go back in time and occurs BEFORE that of the caller. Yes, there seems to be some theoretical about wormhole pretending that can happen and it is possible to go back in time, but no one has yet observed that. ;-)

Caller
   |
   +-----> Calling
              |
              +------> Evalin('caller','return') => stuck because Calling is being executing
      

[Fangjun Jiang]

"return forces MATLAB® to return control to the invoking program before it reaches the end of the script or function"

Who is in control when "return" is executed in this case? I think it is "abort_and_warn". evalin() just gives a different set of workspace. It does not give up control.

[Bruno Luong]

A simple EVAL('RETURN') is not execute either

function foo
eval('return');
fprintf('this will be printed out\n');
end

>> foo
this will be printed out
>> 

[Matt J]

Thanks, folks. I guess I'll just stick with

warning 'Aborting...'; return

Answer 1

[Bruno Luong]

Yes it is written in the doc.

From EVALIN/EVAL document page

"evalin(workspace,expression)

...

expression — Expression to evaluate

character vector | string scalar

Expression to evaluate, specified as a character vector or string scalar. expression must be a valid MATLAB expression and must not include any MATLAB keywords."

>> iskeyword('return')

ans =

  logical

   1

[Bruno Luong]

However

eval('global foo')

works and global is a keyword.Go figure....

That exposes few of the hidden paradoxal gems of MATLAB.

Answer 2

[Fangjun Jiang]

It seems that "return" was not executed in the "caller" workspace as it is supposed to by evalin().

But would a "regular" approach work by passing some kind of flag or value?

In function abort_and_warn, return with a flag or a special value after issuing the warning.

In function test, check the return value of function abort_and_warn, then execute return.