Add `MonadPlus (Either e)` and `Alternative (Either e)` instances

The following 2 instances are currently Orphans in transformers but could be defined in base without instead:

instance Error e => MonadPlus (Either e)
instance Error e => Alterantive (Either e)

This would, however, require us to move Error from transformers into base, which may be a controversial move.

changed milestone to %7.10.1

changed weight to 5

added Ttask Trac import core libraries labels

I'm hitting a problem while trying to implement this.

While the IO instances are rather straightforward (and btw, there's also an Alternative IO instance defined in transformers):

instance MonadPlus IO where
    mzero       = ioError (userError "mzero")
    m `mplus` n = m `catchIOError` \ _ -> n

instance Alternative IO where
    empty = mzero
    (<|>) = mplus

The instances for Either, however, depend on an Error class in transformers to produce an empty/mzero value:

instance (Error e) => Alternative (Either e) where
    empty        = Left noMsg
    Left _ <|> n = n
    m      <|> _ = m

instance (Error e) => MonadPlus (Either e) where
    mzero            = Left noMsg
    Left _ `mplus` n = n
    m      `mplus` _ = m

how shall this issue be resolved?

The IO case is easy and should definitely go in. That one is a no-brainer.

However, looking at it, I'm rather uncomfortable about the notion of moving the rather ad hoc Error class into base. Nobody likes it.

Perhaps the fate of the Either instance should go back out to the libraries mailing list to try to round up ideas.

We could

a.) Leave it where it is b.) Bring Error into base. c.) Switch it to Monoid, which is sadly too big and may break some usecases where noMsg /= mempty holds now. d.) Bring Default into base, which IIRC was already rejected by the libraries@ list, and use that instead. e.) Figure out something else.

Moving over to new owning component 'Core Libraries'.

Trac metadata

Trac field	Value
Component	libraries/base → Core Libraries

removed milestone

Moving to 7.12.1 milestone; if you feel this is an error and should be addressed sooner, please move it back to the 7.10.1 milestone.

Trac metadata

Trac field	Value
CC	ekmett, hvr, simonpj → core-libraries-committee@haskell.org, ekmett, hvr, simonpj

We should at least split this up into the easy and the hard cases and bite off the IO instances for now, and we can explicitly punt on the Either e case until someone comes up with a better idea.

mentioned in issue #10755 (closed)

changed title from Add MonadPlus {-{IO,Either e}-} and Alternative (Either e) instances to Add MonadPlus {+(Either e)+} and Alternative (Either e) instances

changed the description

changed milestone to %⊥

I've split off the easy doable part into #10755 (closed)

Trac metadata

Trac field	Value
Related	- → #10755 (closed)

changed the description

I've updated the description to note that the CLC approved the IO instances unconditionally, but expressed reservations about the Either instances.

mentioned in issue #12160 (closed)

Trac metadata

Trac field	Value
Related	#10755 (closed) → #10755 (closed), #12160 (closed)

intended?

$ ghci-8.0.1
GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /apps/strats/.ghci
> import Data.Foldable
> import Control.Applicative
> import Control.Monad.Trans.Error

<interactive>:1:1: warning: [-Wdeprecations]
    Module ‘Control.Monad.Trans.Error’ is deprecated:
      Use Control.Monad.Trans.Except instead
> Left "A" <|> Left "B"
Left "B"
> asum [Left "A", Left "B"]
Left ""

Sadly.

The current Error based instance in transformers is somewhat lacking in this and other regards.

Replying to [ticket:9588#comment:138576 ekmett]:

Sadly.

The current Error based instance in transformers is somewhat lacking in this and other regards.

I mindlessly applied that and it broke a parser in our code :)

added report impact label

added Pnormal label

These instances have been removed as of transformers-0.6, since the module where they were defined have been long deprecated and have now been removed.