OpenEuphoria

Re: Error Handling

• Posted by Derek Parnell <ddparnell at bigpond.com> Mar 19, 2001
• 656 views

Jeff wrote:
> I would rather use simple error return codes, like for open. I think, for
> example, that this:
>
> integer fn
> fn = open("thefile","w")
> if fn = -1 then
>     -- do something
> end if
> -- a bunch of statements here
>
> Looks a lot better than:
>
> integer fn
> fn = open("thefile","w")
> -- A bunch of statements here
> on error do
>     -- do something
>

I've written some programs using a flavour of Pick Basic. Many of its
statements had the syntax of ...
   <KEYWORD> <PARAMS>
    ON ERROR <statements>
    ELSE <statements>
   END <KEYWORD>
such as

   OPEN "thefile.e" FOR OUTPUT
     ON ERROR
          PRINT "File Failed to open"
   END OPEN

I hated using it but after a while it was occasionally useful. But I
digress.

There are problems with error return codes.
Firstly, it only applies to routines that return stuff - namely functions.
Secondly, functions are usually designed to return something useful, which
means that if they can return either an error code or a non-error value, the
error code(s) must not be from the same set of values that non-error values
belong to. This can often be designed, but not for everything. A function
that is meant to return a string finds it hard to return an error code as
well. Look at how awkward Euphoria's value() function is. A normal return
value and an error code are from the same set so it must return a sequence.
To do that it means you have to do an assignment first then a test for
errors then another assignment! Not a good way to do things.
Thirdly, some errors occur in expressions (eg Divide by zero), and there is
nothing to return an error code when that happens.

Imaging if we could do something like this ....

----------------
function value(sequence text)
   atom thenumber

   for i = 1 to length(text) do
    . . .
    if text[i] = badchar then
        raise #80001 with {i}
    end if
    if text[i] = anotherdot then
        raise #80002 with {i}
    end if
   end for
   return thenumber
end function

 atom v
 sequence text

 v = value(text)
 if exception then
     if errno = #80002 then
        printf (1, "Extra dot in \"%s\" at position %d", {text, errdata[i]})
     else
        printf (1, "Bad character at position %d in \"%s\"", {errdata[i],
text})
     end if
 end if
-------------------

or even ...

   score = sqrt((sideA * sideB)) / Questions
   if exception then
       if errno = ZeroDivide then
           -- Ignore this result
           score = 0
       else
           printf(1, "Fatal error #%d", {errno})
           abort(1)
       end if
   end if

and of course ...
  -- turn off exception handling
  without exceptions

  -- turn on exception handling
  with exceptions

------
Derek Parnell
Melbourne, Australia
"To finish a job quickly, go slower."