[wyager]

Monad transformers offer a more disciplined and pleasant alternative to your #1. You should handle errors at the value level, not with some magic error passing system provided by your language. You can catch the errors at whatever level you like, and you are forced to deal with them properly by construction.

[jstimpfle]

Are you talking about things like EitherT? IMO here isn't so much of a difference to exception handling. Both approaches make it hard to see at a glance where most code could fail, and you can (but are not encouraged to) transform errors explicitly.

Add Java's checked exceptions, now the practical differences are quite subtle. Of course it's nice to be able to be able to do transformations with higher level functions.

[fnl]

Standard "canned" reply: Java doesn't force you to annotate exceptions or even to handle them all. Typed returns do. And as to the "where", well, in the originating function.

But that gets us to some real criticism here: You run the danger of getting an Either Monad out of about every function in your code-base... So maybe throwing exceptions isn't the worst thing after all. (/me ducks all the stuff being thrown my way from hardcore [EDIT:] ~Haskell~ functional programming fans :-))

EDIT: s/Haskell/functional programming/ (because its unfair - Haskell can throw stuff: http://www.randomhacks.net/2007/03/10/haskell-8-ways-to-repo...)

[jstimpfle]

Source? I was under the impression that it does (apart from things like NullPointerException, which of course has Haskell equivalents). And I've had to do a significant project in Java.

[BoorishBears]

You mean Unchecked Exceptions (that extend from RuntimeException) vs Checked Exceptions?

[wyager]

You can manipulate value-level things like EitherT in ways that are not convenient or possible in Java.

EitherT and friends also force you to handle exceptions explicitly before getting a value out. In Java, you can usually just ignore it and "let it bubble up" as someone suggested earlier.

[jstimpfle]

"Manipulate", that's what I meant by "transformations by higher order functions". The point is acknowledged, but I feel in practice it's not always beneficial. There are typically not many more points of use than different transformations. Doing the stuff inline (with catch blocks) is often better since there's less conceptual overhead.

"Bubbling up", that's exactly what the monad instance gives you. How EitherT is supposed to be used. That's why I said there's not much difference from a practical standpoint. So no, EitherT does not force any better style than checked exceptions (but it makes it really inconvenient to traverse regions of code with differing sets of exceptions).

I still feel that the C-style way of handling error codes is superior in most situations, from a writeability and readability perspective. The big problem is it doesn't enforce error handling. Another problem is it's totally unsuited for quick and dirty scripts like I can write with Python "unchecked" exceptions: Just do it, and tell me if there were errors (I might or might know which ones are possible) only at runtime.