Re: Proposal: Extensible exceptions

On Sun, 6 Jul 2008, David Menendez wrote:

On Sun, Jul 6, 2008 at 4:31 PM, Henning Thielemann wrote:

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On Sun, 6 Jul 2008, David Menendez wrote:

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data DivideByZero = DivideByZero deriving (Show, Typeable)

Maybe I annoy you with my distinction of errors and exceptions, but I consider DivideByZero a bad example for an exception, because it is more an error (I see it is used in the extensible exception paper anyway).

It's also in the current Control.Exception library.

There are many things wrong about Errors and Exceptions in the current Haskell library: http://www.haskell.org/haskellwiki/Exception http://www.haskell.org/haskellwiki/Error

While dividing by zero or accessing an array out of bounds isn't the same as a file not existing, I'm not sure we need different mechanisms for dealing with them.

Yes! Because there is no need to recover from an error. Instead an error must be fixed by the programmer. The program cannot do this by itself. I consider recovering from an error like in a web-server a hack, like catching and recovering from an 'error' in IO is a hack, just like unsafePerformIO. I accept that we need a hack in order to tell the user "please send a bug-report to XYZ", but a hack should be called a hack, not "proper exception handling".

If your code divides by zero, you still want any "finally" or "bracket" clauses to get called before the program terminates.

A program which divides by zero is broken and must be fixed. A program which divides by zero but cleans up a bit, is still broken and must be fixed. Cleaning up may make things better, but may also make things worse! Handling errors is the task of Debugging, not that of Exception Handling. I suggest special variants of 'finally' and 'bracket' for bracketing bugs should be located below "Debug" in the module hierarchy.