This isn't just a question about Haskell. It applies to any language with an exception mechanism, including C++ and Java. Even C (segv is an exception mechanism...) The question is really how to communicate failure to the caller, in a way the caller can not ignore, without unduely inconvienencing the caller. So there is a two pronged test. Could the caller have anticipated the failure, and should the caller anticipate failure.
head of an empty list is a good example of the first prong. It is easy to check for. In many cases it can be shown to be impossible. So burdoning all callers with dealing with failure explicitly is not good. An exception is good here. Contrast that with lookup in a table. The caller has no reasonable way of telling before hand that the call will be succesful. The caller should be prepared to handle lookup failure, so a Maybe is good here. In C++ a null value of some kind is the norm. One of the questions that C++ and Java answered was whether or not a caller should be forced to deal with an exception explicitly by the called function. The answer has turned out to be no, for somewhat different reasons in both languages. But it is something that Haskell should consider. Particularly Java library integrator's discovery that strongly typed exceptions do not scale. Frameworks like Tomcat seem to have found that there isn't much middle ground between exceptions that are handled close to the library, and just barely escape the library interface, and the generic 'something went wrong somewhere', 'abort,retry,fail?' kind of error. Of course, for library reuse, this is secondary to the incipient package nameing disaster. Not tertiary, since it is at the heart of lib composability.... On 3/12/07, Dougal Stanton <[EMAIL PROTECTED]> wrote:
The Maybe construction is very useful for explicitly handling circumstances where the function cannot produce a sensible answer. But how far should this notion be taken? When you're writing a function which you realise may not produce what you want, in what circumstances would you use a Maybe, and when would you just throw an error? I'm wondering both in terms of good engineering practise and also for correctness. Should elementary partial functions like > 5 `div` 0 or > head [] return Nothing? I guess it's a bit of a silly suggestion, but it helps to highlight why we use Maybe in the first place. So --- where's the cutoff point in your code? Cheers, D. -- Dougal Stanton _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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