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{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE DeriveAnyClass #-}
 
import Data.Monoid
import Data.Kind
import Data.Functor.Identity
 
data Logger = LFile | LStdout deriving Show
 
data Database = DPG | DMem deriving Show
 
data App (m :: Type -> Type) = App
  { _logger :: m Logger
  , _db     :: m Database
  }
 
deriving instance forall m a. Monoid (m a) => Monoid (App m)
 

[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:19:19: error:
    • Variable ‘a’ occurs more often
        in the constraint ‘Monoid (m a)’ than in the instance head
      (Use UndecidableInstances to permit this)
    • In the stand-alone deriving instance for
        ‘forall m a. Monoid (m a) => Monoid (App m)’
   |
19 | deriving instance forall m a. Monoid (m a) => Monoid (App m)
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Standard output is empty