Re: [Caml-list] Uncaught exceptions in function type.

["Daniel Bünzli" <daniel.buenzli@erratique.ch>]

Le lundi, 26 mai 2014 à 18:02, Philippe Veber a écrit :
> Thanks! BTW core still uses exceptions. Is there an explicit rule as to how to decide between Result type or exceptions. For instance, why not write the Array.create function like this:
>  
> val create : int -> 'a -> 'a array Or_error.t
>  
> where create fails for a negative integer?
Because that would be utterly annoying. You need to make the following distinctions:

* Programming errors, for contracts with the programmer that cannot be enforced through types. For that raises Invalid_argument if the contract is violated. Invalid_argument is not supposed to be handled, it denotes an API misuse, like calling Array.create with a negative integer.

* Exceptional errors, for errors that the programmer is unlikely to handle at all (e.g. out of memory). For that raise a custom exception. This should occur very rarely, you are unlikely to ever define one such exception.

* Non-exceptional errors, errors that the programmer will have to handle (e.g. failing to connect a socket), for that do not use a custom exception but use variants or options types.  

In general if you write libraries it’s better to err on the side of exceptionless design: never use exceptions beyond Invalid_argument (and especially never use Not_found or Failure). Leave exception definition/usage at the discretion of the user (if he wishes to shoot himself in the foot).

Best,

Daniel