> Furthermore, this isn't using exceptions, like a similar piece of C++ code might try. There's no stack unwinding and nothing can escape to contaminate the rest of your program.
It always escapes to contaminate the rest of your program. In the C code, it escapes as a returned NULL. In C++/Java, it would be a thrown exception. In Haskell, it's the Maybe.
The Maybe is pretty close to isomorphic to a checked exception. The caller either has to handle it, or to return a Maybe/declare that it throws an exception itself. At some layer, you have to handle the Nothing/catch the exception. The Nothing short-circuits part of the computation; so does the exception. From where I sit, they're playing almost exactly the same roles. (So is the NULL in C, but it's clumsier than either an exception or a Maybe.)
You're right that the Maybe and Either types are very similar to checked exceptions. And, from that perspective, you're absolutely right that it contaminates the rest of the code around above it, similar to a checked exception. However, I would subjectively argue that Maybe is the less clumsy alternative. For the lazy programmer, the easy way to handle exceptions is to ignore them and let upstream deal with it. Even for checked exceptions, ignoring the problem is just a throws statement away. My subjective argument, though, is that monads are just slightly awkward enough to handle that you're encouraged to deal with the problem, instead of just ignoring it. I'd much rather handle whatever condition occurred than constantly deal with liftM and >>=, so I make sure to handle my "execeptions" as soon as I have enough information, just to make my own life easier. Your mileage may vary.
It always escapes to contaminate the rest of your program. In the C code, it escapes as a returned NULL. In C++/Java, it would be a thrown exception. In Haskell, it's the Maybe.
The Maybe is pretty close to isomorphic to a checked exception. The caller either has to handle it, or to return a Maybe/declare that it throws an exception itself. At some layer, you have to handle the Nothing/catch the exception. The Nothing short-circuits part of the computation; so does the exception. From where I sit, they're playing almost exactly the same roles. (So is the NULL in C, but it's clumsier than either an exception or a Maybe.)