Support for GHC 9.6

[Vekhir]

Hi @sebastiaanvisser,
The module Control.Monad.Trans.Error was deprecated a long time ago and finally removed in transformers 0.6 which is shipped with GHC 9.6.
This leads to this error:

[1 of 1] Compiling Main             ( test/TestSuite.hs, dist/build/suite/suite-tmp/Main.dyn_o )

test/TestSuite.hs:26:1: error:
    Could not find module ‘Control.Monad.Trans.Error’
    Perhaps you meant
      Control.Monad.Trans.Writer (from transformers-0.6.1.0)
      Control.Monad.Trans.RWS (from transformers-0.6.1.0)
      Control.Monad.Trans.Cont (from transformers-0.6.1.0)
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
   |
26 | import Control.Monad.Trans.Error (Error (noMsg))
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Removing the offending line, another error occurs:

[1 of 1] Compiling Main             ( test/TestSuite.hs, dist/build/suite/suite-tmp/Main.dyn_o )

test/TestSuite.hs:187:8: error: [GHC-39999]
    • No instance for ‘MonadPlus (Either String)’
        arising from a use of ‘mA’
    • In the expression: mA
      In an equation for ‘mA_f’: mA_f = mA
    |
187 | mA_f = mA
    |        ^^

In the relevant file, test/TestSuite.hs, there is the following remark:

-- These instance are needed for the `Failing.Lens String` instance,
-- since that needs a `MonadZero` constraint on `Kleisli (Either String)`,
-- which in turn needs a `MonadPlus (Either String)` constraint.
-- These instances used to exist in transformers but were removed in
-- 0.5.0.0 accidentally, and added in 0.5.2.0. We can probably remove
-- this ifdef after GHC 8 rc3 is released, which will include
-- transformers-0.5.2.0.

#if MIN_VERSION_transformers(0,5,0) && !MIN_VERSION_transformers(0,5,2) && MIN_VERSION_base(4,9,0)
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
#endif

Not quite sure what to make of it. Do you have an idea how to fix this issue?

-- Vekhir