Re: Why no exception hierarchy ?

[kilgallen@eisner.decus.org (Larry Kilgallen, LJK Software)]

In article <D5z3sF.1ou@nntpa.cb.att.com>, ka@socrates.hr.att.com (Kenneth Almquist) writes:

> An example of something that the Ada 83 exception mechanism did not handle
> well is binding UNIX system calls to Ada.  POSIX.5 maps all system call
> errors into a single exception named POSIX_Error, and provides a per-task
> error code variable.  This is dangerous.

Dangerous? Sure.  Smells of "errno" !!!

> As far as I can tell, Ada 95 provides two ways to resolve this problem,
> neither of which I entirely like.  One is to encode the error code in the
> exception message.  Then we could have:
> 
> 	function get_error_code(x: exception_occurence) return error_code;
> 
> which would translate the exception_message into an error code.  This
> seems to be an abuse of the intended purpose of the exception_message,
> which is to provide human-readable information on the exception.  In

> The other approach is to map each error code to a separate exception
> and use query functions on exception identities to implement classes.
> In this case, one would write:
> 
> 	exception
> 	    when occurence : others =>
> 		if is_system_call_error(exception_identity(occurence)) then

> Have I missed overlooked any possibilities?

On VMS, DEC Ada (83) provides the pragma IMPORT_EXCEPTION, allowing you
to assign an arbitrary Ada name to your choice of an OS-generated (or
layered-product-generated) OS-style exception.

Perhaps DEC Ada does the same thing on Unix -- I don't know.

I do not know enough about Ada 95 to understand why this would not work,
but I would be interested in comments from those who do know about Ada 95.

Larry Kilgallen