The following code compiles with older compilers but does not compile with
GHC 9.6.1:
{-# LANGUAGE KindSignatures #-}
module A () where
import Control.Monad.IO.Class
import Control.Monad.Trans.Class
data T (m :: * -> *) a = T
instance Functor (T m) where
fmap f T = undefined
instance Applicative (T m) where
pure = undefined
(<*>) = undefined
instance MonadIO m => Monad (T m) where
return = pure
(>>=) = undefined
instance MonadTrans T where
lift = undefined
It fails with the following error:
xx.hs:20:10: error: [GHC-39999]
• Could not deduce ‘MonadIO m’
arising from the head of a quantified constraint
arising from the superclasses of an instance declaration
from the context: Monad m
bound by a quantified context at xx.hs:20:10-21
Possible fix:
add (MonadIO m) to the context of a quantified context
• In the instance declaration for ‘MonadTrans T’
|
20 | instance MonadTrans T where
| ^^^^^^^^^^^^
What is the correct resolution for this?
-harendra
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