haskell - Haskeline: Implementing `MonadException` instance for `ReaderT r (StateT s m)` stack -

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• monadexception instance not deduced 1 answer

let's have following monad transformer stack (r , s left () simplicity),

newtype mymonad m = mm (readert () (statet () m a) 

if want use base monad haskeline's inputt, need system.console.haskeline.monadexception instance. given apparent complexity of these instances, i'd prefer let compiler derive me generalizednewtypederiving. specifically, expect following typecheck,

{-# language generalizednewtypederiving, standalonederiving, flexiblecontexts #-}  import control.monad.state import control.monad.reader import control.monad.io.class import control.applicative import system.console.haskeline.monadexception  newtype mymonad m = mm (readert () (statet () m) a)                     deriving (functor, applicative, monad, monadio) deriving instance (monadexception m) => monadexception (mymonad m)         

yet sadly, gives me,

/home/bgamari/hi.hs:11:1:     not deduce (monadexception (statet () m))       arising superclasses of instance declaration     context (monadio (mymonad m), monadexception m)       bound instance declaration @ /home/bgamari/hi.hs:11:1-66     possible fix:       add instance declaration (monadexception (statet () m))     in instance declaration `monadexception (mymonad m)' 

looking @ provided instances statet , readert,

instance monadexception m => monadexception (readert r m) instance monadexception m => monadexception (statet s m) 

it seems reasonable expect compiler deduce statet instance. expecting of cunning generalizednewtypederiving? how 1 implement instance short of open-coding it?

there 2 versions of state monad: strict , lazy. import control.monad.state brings in lazy version, instance of monadexception seems strict version.

try import control.monad.state.strict instead of import control.monad.state.