How to make assert() debug break?

I attached my solution. Not sure why it's not the default behavior.

function assert1( b )
    if nargin < 1
        b = 0;
    end
    
    if ~b
        disp( 'Assertion failed.' );
        dbstack
        
        if 0
            dbstop in assert.m at 11;
            dbclear in assert.m at 11;
        else
            ST = dbstack;
            if length( ST ) > 1
                ST = ST(2);
            end
            fl = ST.file;
            ln = ST.line + 1;
           
            cmd = [ 'dbstop in ' fl ' at ' int2str(ln) ];
            eval( cmd )
        end
    end
end

[Jan]

What is the behavior of your code? It creates a breakpoint in the next line and Matlab will stop there.

What happens in nested functions, functions attached to a script and in anonymous functions?

Answer 1

[Steven Lord]

Based on Jan's description (I have not read the code) you'd also want to think about what this does if you call assert1 at the MATLAB prompt, on the last line of a file, on the last line of a function, inside an eval call, in a P-coded file (I think this one might work), via mexCallMATLAB in a MEX-file, in a deployed application, ...

IMO there are certain functions that inherently require interactivity and so users won't be surprised if those functions behave interactively. input, uigetfile, and keyboard are examples of these. But if you call a function that requires interactivity in a function that doesn't, or if you make a function require interactivity that users' mental model says shouldn't require interactivity, that can be annoying to the point of unusability.

Imagine if (for example) the plot function prompted users for the X and Y data every time it was called. You couldn't use it in a program you intended to run unattended. Is users' mental model for assert more like input (where you expect it to require interactivity) or more like plot (start it running and let it do its thing)? To me having assert drop into debug mode would be quite annoying.

Besides, there is a way to get assert to stop and drop you into debug mode without requiring any changes to the functionality of assert or changes to code that calls assert. Set an error breakpoint. If the assert triggers it will cause an error which will be caught by the error breakpoint.

[Jan]

Exactly: Typing this in the command window let each failing assert enter the debug mode:

dbstop if error

Or even inside TRY/CATCH:

dbstop if caught error

Then you can stay at the standard assert and perform your debugging dynamically.

[Zohar]

Much better, I'm happy with that.

Up until now, everytime something asserted, I needed to relace "assert(b)" with "if b, assert".

I was looking for a behavior like in c++: in debug mode, assert breaks, in release it's skipped. These options let me attune matlab in the same spirit.

Thanks