OpenEuphoria

Re: Crash or return forbidden value?

• Posted by Jeremy Cowgar <jeremy at cowgar?com> May 08, 2008
• 658 views

Shawn Pringle wrote:
> 
> What you are talking about here is building an Exception system into the
> interpreter.  It would be analogous to try/throw/catch and Exceptions like
> in Java or C++.  Perhaps exceptions could be less invasive than adding C++ 
> syntax if say some new builtin routines such as a register_exception_routine
> were added.  

The problem with this is the exception routine knows nothing about what function
function caused the problem. You could pass a function name, error, etc... but
that still does not help. Nor does it give the ability to recover from anything.

For instance:

function exception_handler(sequence name, sequence msg, integer error)
   ...
end function
regisert_handler(...)

fname = "abc.txt"
f = open(fname, "r") -- abc.txt does not exist, lets say

Now, what is exception_handler suppose to do? It does not know if the app can
recover or how it should recover. However, take this example:

try
    f = open(fname, "r")
catch ex
    if ex[CODE] = ERR_FILE_NOT_EXIST then
        f = open(fname, "w")
        puts(f, "# File created today\n")
    else
        crash("Could not handle exception: %i %s", {ex[CODE], ex[MSG]})
    end if
end try

So... that takes more coding, but it's useful and can do something about the
exception. Sure, the example was not that good as we can handle that now, but
that's the idea.

Ok, an exception system will take a lot of planning and a lot of work. What we
are trying to solve right now what error condition should return an error code
and what error condition should cause a crash?

Many times in Euphoria code, I see ? 1/0 which the programmer feels is an error
that cannot be recovered from, therefore causes a crash to terminate Euphoria
with a ex.err file.

We now have a better way of doing that in 4.0. You can:

crash("Fatal error I cannot recover from. Error in hard drive: %s",
    {hard_drive_name})

So, this keeps a true stack trace, the user is given a realistic error message
(not a divide by zero message), etc... But, the question is, when to use crash()
vs. return an error code. For instance, when opening a file if it does not exist
an error code is returned vs. s="Hello" ? s[500] ... an index out of bounds
causes a crash.

--
Jeremy Cowgar
http://jeremy.cowgar.com