[Haskell-cafe] Re: New Hackage category: Error Handling
lemming at henning-thielemann.de
Mon Dec 7 19:00:54 EST 2009
On Tue, 8 Dec 2009, Ben Franksen wrote:
> Michael, Henning
> There are two meanings to the word 'exception' in this context; both of you
> tend to conflate these different meanings. One meaning is about a
> *mechanism* for non-local control flow; the other is about certain classes
> of un-desired program conditions.
> Michael, you are above arguing that it is ok to use the same /mechanism/ to
> signal errors and exceptions. That is ok with me.
I'm actually arguing that errors and exceptions (or failures) should not
be treated the same way. This is my opinion because
1. You cannot catch all kinds of errors (infinite loops, memory
corruption, and so on) at run-time
2. The program cannot help itself if it is buggy. The next higher level
in the software hierarchy certainly can, but this requires obviously a
The correct treatment of those things is in my opion:
1. Errors must be fixed by the programmer
2. Exceptions must be appreciated by the programmer and handled by the program at runtime
> So, maybe it would help if we could agree to use different terms for those
> two meanings of the word 'exception'. I think 'exception' is too strongly
> associated with the non-local control flow mechanism to introduce a new
> word for it.
I used the term "exception" because this is the term used for the concept
of handling "file not found", "could not allocate resource" in the
introduction to modern languages that I have read. Thus my impression was
that "exception" is the term for the concept, whereas there are different
implementations and ways of handling them. For instance in Modula-3 the
EXIT statement is called an exception that escapes the LOOP structure, but
it cannot be catched with TRY, that is used for user defined exceptions.
In Modula-3 a detected error (range violation, NIL dereference, division
by zero) terminates the program. An undetected error (which is restricted
to UNSAFE modules) can cause memory corruption and all those bad things,
and cannot be handled in any way.
Before exceptions got a special handling in programming language, they
still existed. In C for instance they were expressed by return codes.
The Wikipedia author also seem to consider "exception" to be the term for
the concept, not for the implementation:
"Exception handling is a programming language construct or computer
hardware mechanism designed to handle the occurrence of exceptions,
special conditions that change the normal flow of program execution."
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