Re: [Caml-list] Static exception analysis or alternative to using exceptions

[Alain Frisch <alain@frisch.fr>]

On 05/26/2010 06:15 PM, Hans Ole Rafaelsen wrote:
> What experience does people have to using alternatives to exceptions,
> such as option types or exception monads? Does use of third part
> libraries that still throws exceptions make such approaches hard to use?
> Performance wise it seems to be comparable to catching exceptions or
> matching for options, so I guess the difference be might a question of
> programming style?

Indeed, this is mostly a matter of taste and style.

My personal "Rule #1" to decide to use exceptions or not is that one:

  Raising an exception is ok if you know where it will be caught
  or which global effect it will have on the application.


This includes the following cases:

- (Short jumps) Using exceptions locally as shortcuts in loops or more 
complex  algorithms. In that case, the runtime exception is encapsulated 
in a function or module, and should not cause any harm.

- (Long jumps) Using exceptions to report unexpected errors, to which 
the application has little chance to react in a clever way except 
displaying some error messages to the screen or to a log file. The catch 
point is a global exception handler, even though the code raising the 
exception (maybe in a general purpose library) might not know exactly 
how the application will react to the exception.  It is much better if 
the exception describes the problems in terms that can be understood by 
a human (Not_found, even with a stack trace, is not very helpful as an 
error message).

Library function that perform some kind of lookup should return an 
option type and let the caller raise a proper exception to report error 
in a way that makes sense for the application. Similarly for other kind 
of functions that can "fail": use a sum type (or polymorphic variants) 
instead to describe the possible "errors". A variant of the function 
that raises an exception is acceptable for cases where the caller really 
expect the function to succeed (it can fail only because of bugs in the 
application, not because of user or system errors). In that case, 
Assert_failure, or a custom exception, is probably better than 
Not_found. More generally, if exceptions are to be used, custom ones are 
usually better than generic ones (Failure, Invalid_argument).

Another interesting use of exceptions comes from the fact that exn is an 
extensible sum type. (Unfortunately, it is the only one in OCaml.) This 
enables some kinds of loosely typed scenarios, like notification 
messages to be dispatched across global objects in the application: the 
list of possible messages is not fixed in advance, and each object can 
simply use pattern matching to react on messages it is interested in. 
But this is another story...



Alain