Incorrect scoping rules for standalone deriving via

[AlexandreTunstall]

While trying to build mtl, the following error occurred:

mhs: error: "./Control/Monad/Accum.hs": line 173, col 24: undefined type: m

The location in question is the following rather unusual deriving declaration:

deriving via
  (LiftingAccum MaybeT m)
  instance
    (MonadAccum w m) =>
    MonadAccum w (MaybeT m)

It seems that the via clause is not inheriting the scope of the instance declaration.

[augustss]

I'm not quite done with standalone deriving via yet.

[AlexandreTunstall]

For now, I'm working around it with the following patch.

diff --git a/Control/Monad/Accum.hs b/Control/Monad/Accum.hs
index 7be9140..cbe3a5a 100644
--- a/Control/Monad/Accum.hs
+++ b/Control/Monad/Accum.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DerivingVia #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE FunctionalDependencies #-}
@@ -108,6 +109,9 @@ import qualified Control.Monad.Trans.State.Strict as StrictState
 import qualified Control.Monad.Trans.Writer.CPS as CPSWriter
 import qualified Control.Monad.Trans.Writer.Lazy as LazyWriter
 import qualified Control.Monad.Trans.Writer.Strict as StrictWriter
+#if defined(__MHS__)
+import Data.Coerce (coerce)
+#endif
 import Data.Functor (($>))
 import Data.Functor.Identity (Identity)
 import Data.Kind (Type)
@@ -165,6 +169,7 @@ instance (Monoid w) => MonadAccum w (AccumT w Identity) where
   add = Accum.add
   accum = Accum.accum
 
+#if !defined(__MHS__)
 -- | The accumulated value \'survives\' an error: even if the
 -- computation fails to deliver a result, we still have an accumulated value.
 --
@@ -273,6 +278,79 @@ deriving via
   instance
     (MonadAccum w' m, Monoid w) =>
     MonadAccum w' (StrictWriter.WriterT w m)
+#else
+-- MicroHs has incorrect scoping rules that break standalone MPTC derivingvia
+-- See https://github.com/augustss/MicroHs/issues/139
+instance MonadAccum w m => MonadAccum w (MaybeT m) where
+  look = lift look
+  add = lift . add
+  accum = lift . accum
+
+instance MonadAccum w m => MonadAccum w (ContT r m) where
+  look = lift look
+  add = lift . add
+  accum = lift . accum
+
+instance MonadAccum w m => MonadAccum w (ExceptT e m) where
+  look = lift look
+  add = lift . add
+  accum = lift . accum
+
+instance MonadAccum w m => MonadAccum w (IdentityT m) where
+  look = lift look
+  add = lift . add
+  accum = lift . accum
+
+instance MonadAccum w m => MonadAccum w (CPSRWS.RWST r w' s m) where
+  look = lift look
+  add = lift . add
+  accum = lift . accum
+
+instance (MonadAccum w m, Monoid w') => MonadAccum w (LazyRWS.RWST r w' s m) where
+  look = lift look
+  add = lift . add
+  accum = lift . accum
+
+instance (MonadAccum w m, Monoid w') => MonadAccum w (StrictRWS.RWST r w' s m) where
+  look = lift look
+  add = lift . add
+  accum = lift . accum
+
+instance MonadAccum w m => MonadAccum w (ReaderT r m) where
+  look = lift look
+  add = lift . add
+  accum = lift . accum
+
+instance MonadAccum w m => MonadAccum w (SelectT r m) where
+  look = lift look
+  add = lift . add
+  accum = lift . accum
+
+instance MonadAccum w m => MonadAccum w (LazyState.StateT s m) where
+  look = lift look
+  add = lift . add
+  accum = lift . accum
+
+instance MonadAccum w m => MonadAccum w (StrictState.StateT s m) where
+  look = lift look
+  add = lift . add
+  accum = lift . accum
+
+instance MonadAccum w m => MonadAccum w (CPSWriter.WriterT w' m) where
+  look = lift look
+  add = lift . add
+  accum = lift . accum
+
+instance (MonadAccum w m, Monoid w') => MonadAccum w (LazyWriter.WriterT w' m) where
+  look = lift look
+  add = lift . add
+  accum = lift . accum
+
+instance (MonadAccum w m, Monoid w') => MonadAccum w (StrictWriter.WriterT w' m) where
+  look = lift look
+  add = lift . add
+  accum = lift . accum
+#endif
 
 -- | A helper type to decrease boilerplate when defining new transformer
 -- instances of 'MonadAccum'.
diff --git a/Control/Monad/Error/Class.hs b/Control/Monad/Error/Class.hs
index b2465e7..ced4df6 100644
--- a/Control/Monad/Error/Class.hs
+++ b/Control/Monad/Error/Class.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
@@ -75,6 +76,11 @@ import Control.Exception (IOException, catch, ioError)
 import Control.Monad (Monad)
 import Data.Monoid (Monoid)
 import Prelude (Either (Left, Right), Maybe (Nothing), either, flip, (.), IO, pure, (<$>), (>>=))
+#if defined(__MHS__)
+-- A MicroHs bug requires this symbol to be in scope.
+-- See https://github.com/augustss/MicroHs/issues/140
+import Prelude (Char)
+#endif
 
 {- |
 The strategy of combining computations that can throw exceptions
diff --git a/Control/Monad/Select.hs b/Control/Monad/Select.hs
index 4896042..b877633 100644
--- a/Control/Monad/Select.hs
+++ b/Control/Monad/Select.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DerivingVia #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE FunctionalDependencies #-}
@@ -125,6 +126,7 @@ class (Monad m) => MonadSelect r m | m -> r where
 instance MonadSelect r (SelectT r Identity) where
   select = Select.select
 
+#if !defined(__MHS__)
 -- | \'Extends\' the possibilities considered by @m@ to include 'Nothing'; this
 -- means that 'Nothing' gains a \'rank\' (namely, a value of @r@), and the
 -- potential result could also be 'Nothing'.
@@ -288,6 +290,51 @@ deriving via
   instance
     (MonadSelect r m, Monoid w) =>
     MonadSelect r (AccumT w m)
+#else
+-- MicroHs has incorrect scoping rules that break standalone MPTC derivingvia
+-- See https://github.com/augustss/MicroHs/issues/139
+instance MonadSelect r m => MonadSelect r (MaybeT m) where
+  select = lift . select
+
+instance MonadSelect r m => MonadSelect r (ContT r' m) where
+  select = lift . select
+
+instance MonadSelect r m => MonadSelect r (ExceptT e m) where
+  select = lift . select
+
+instance MonadSelect r m => MonadSelect r (IdentityT m) where
+  select = lift . select
+
+instance MonadSelect r m => MonadSelect r (ReaderT r' m) where
+  select = lift . select
+
+instance MonadSelect r m => MonadSelect r (LazyState.StateT s m) where
+  select = lift . select
+
+instance MonadSelect r m => MonadSelect r (StrictState.StateT s m) where
+  select = lift . select
+
+instance MonadSelect r m => MonadSelect r (CPSWriter.WriterT w m) where
+  select = lift . select
+
+instance (MonadSelect r m, Monoid w) => MonadSelect r (LazyWriter.WriterT w m) where
+  select = lift . select
+
+instance (MonadSelect r m, Monoid w) => MonadSelect r (StrictWriter.WriterT w m) where
+  select = lift . select
+
+instance MonadSelect r m => MonadSelect r (CPSRWS.RWST r' w s m) where
+  select = lift . select
+
+instance (MonadSelect r m, Monoid w) => MonadSelect r (LazyRWS.RWST r' w s m) where
+  select = lift . select
+
+instance (MonadSelect r m, Monoid w) => MonadSelect r (StrictRWS.RWST r' w s m) where
+  select = lift . select
+
+instance (MonadSelect r m, Monoid w) => MonadSelect r (AccumT w m) where
+  select = lift . select
+#endif
 
 -- | A helper type to decrease boilerplate when defining new transformer
 -- instances of 'MonadSelect'.