On Tue, May 4, 2010 at 9:09 AM, Limestraël
Are there other methods than Maybe or exceptions to handle the errors in Haskell? Is the monad Error(T) useful?
I believe the usual Error monad is just (Either e), with Left indicating failure. It's the same idea as Maybe, only returning information about the error instead of simply Nothing. At any rate, earlier you said:
The Maybe method is the simplest but it also comes with a little overhead, since you always have to unpack the Maybe a value return
...which sounds odd to me. If you have a complex computation, in which many subcomputations can fail, it makes perfect sense to lift the whole thing into an error-handling monad. Particularly at points where nothing sensible can be done with a Left or Nothing, unpacking the result is unnecessary; instead, leave it as is and continue the computation inside the monad. Then, unpack the value at whatever point that you actually need the result, or can handle errors gracefully. I'd actually argue that error handling with Maybe/Either is the single best, most persuasive use for monadic structure in code--it's certainly the thing I miss most in other languages. For something so simple (the entire implementation of Maybe with instances is shorter than this message!) it's amazingly useful, letting you simplify code while simultaneously having static assurance that you won't get runtime errors because of not checking for failure somewhere. Using "fromJust" or "maybe (error foo) ..." seems bizarre, as if trying to recreate in Haskell the mind-numbing experience of dealing with unexpectedly null pointers being dereferenced. For that matter, null references tend to be frustrating to debug for exactly the same reason that Haskell errors can be: Crashing only when and if the value is actually needed, not when the null pointer or _|_ is first introduced. - C.