I don't really see the value in `Except` either, but it's there to satisfy Ross who wants an `Identity`-specialized version of the monad transformer. On Tue, Aug 13, 2013 at 1:50 PM, Dag Odenhall wrote:

I don't see why we need type Except e r = EitherT e Identity r if it's exactly the same as Either anyway. We don't have this for MaybeT?

On Tue, Aug 13, 2013 at 8:49 PM, Gabriel Gonzalez wrote:

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My preference is to call the new transformer ExceptT, with a basic monad called Except, in line with most of the other transformers, and to deprecate ErrorT. (The rationale for the name is that Either isn't just for exceptions, and exceptions aren't just for errors.)

Specializing to an identity base monad is usually a misfeature in real code and only useful for pedagogical purposes. Experts leave it polymorphic like this:

expertCode :: (Monad m) => EitherT E m R

By designing the API entirely around the identity specialization you're optimizing for a narrow skill range of intermediate Haskell programmers who are:

a) Smart enough to figure out that `ExceptT` is the official generalization of `EitherT`

b) Not smart enough to figure out how to keep the base monad polymorphic

So I propose that you leave the monad transformer name as `EitherT`, but use `Except` for the `Identity` specialization:

type Except e r = EitherT e Identity r

This has the nice properties that:

* The identity specialization doesn't conflict with `Either`

* Beginners find `transformers` when they search for `EitherT` as they inevitably will

* Existing packages that use `EitherT` won't break, thus preserving compatibility with the code bases that Edwards already mentioned

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Am 13.08.2013 23:09, schrieb David Luposchainsky:

On 2013-08-13 22:57, Gabriel Gonzalez wrote:

...

I don't really see the value in `Except` either, but it's there to satisfy Ross who wants an `Identity`-specialized version of the monad transformer.

I think at this rate, we're running out of terms for "crashing" pretty soon. Exception vs. error is bad enough, and now some want to add Except to that list.

On the other hand, EitherT already exists in a current and used library, it is very clear what that transformer does, and it goes hand in hand with MaybeT.

Furthermore I think Hennings objections are not very convincing:

In all cases I listed you can define an instance that is mathematically sound, that is, it fulfills some nice laws. But from the view of a programmer, I want additionally safety. I want that programs are rejected that are "obviously" wrong. Returning to "Either": You can define a monad instance on it that fulfills all monad laws, i.e. it is mathematically sound. This instance already exist. But for me Either is just a plain set sum, a union type. I may use it for lists that may contain two types of elements. I may use it for constructing larger sum types without the need to define a new 'data'. This is for example useful in GHCi. But then - why should this type also be a monad, where the Left and Right are handled very differently? When I want to have the exception semantics I prefer to make that explicit using the Except type. (Currently I use my own Exceptional type for that purpose.) My general concern is that the discussion is focussed on whether an instance fulfills mathematical laws, but not on software engineering aspects. Although some types are structurally equivalent (Either and Except, pair and Writer, function and Reader monad), they have very different uses. If I want to combine two types in one, I want to express this by Either. If I want exception handling, I want to express this by Except. Either should not have a monad instance, and Except should not have a function like partitionEithers. A strength of Haskell's type system is safety. Safety means that certain things are forbidden. In contrast to that, a discussion focussed on fulfilling mathematical laws tends to allow as much as possible. On the one hand we encourage people to artificially make types distinct by newtype wrapping. E.g. we define newtype Id = Id Int, in order to forbid arithmetic operations that are useless for Id's. On the other hand we throw together many applications to a single type by defining more and more instances for base types and base classes. In the long run we end up with MATLAB semantics: They put so much applications into one type (complex valued tensors used for bools, reals, complex numbers, polynomials, matrices, graphs etc.) that you better not touch a working program, if you want to keep it running. That said, instance Monad Either and instance Monad (->) already exist. We cannot remove them easily, because may people rely on them. However, we can discourage their use and propose the clean way via types from the transformers package. Then the exception handling monad in 'transformers' should not have a name that resembles "Either", because its use is very different from a plain union type. For me "Exept" is a good choice, because the intended application is exception handling.

- Monad for pairs duplicates Writer, which is existing functionality but without the newtype wrapper safety. There is no equivalent for this with Either.

In transformer the Writer monad could also have been defined as data Writer w a = Writer a w and vice versa the Except monad could be defined by newtype Except e a = Except (Either e a) I don't think there is a substantial difference. I order to be consistent with the current style of type definitions in 'transformers', I think the definition will be: newtype ExceptT e m a = ExceptT (m (Either e a)) type Except = ExceptT e Identity a

- Num for functions - I have yet to see an example where that is beneficial beyond "funny how that works".

I guess that people liked to write sin+cos or 2*exp. If not, there must be enough other reasons to publish http://hackage.haskell.org/package/NumInstances