[Haskell-cafe] Selecting a transformer in a monad transformer stack

Paul Brauner polux2001 at gmail.com
Tue May 29 16:06:16 UTC 2018


I have a newtype for a monad transformer stack:

newtype MyMonad a = MyMonad { runMyMonad :: ((ReaderT Env (ExceptT String
(StateT Store (ListT Identity)))) a) }

and I'd like to it derive MonadPlus. The default instance for MonadPlus I
get via GeneralizedNewtypeDeriving is the ExceptT one because it is the
outermost transformer implementing MonadPlus. I would like to use the ListT
instance instead. The best solution I have come up with is to define:

instance MonadPlus MyMonad where
  mzero = MyMonad $ ReaderT $ \e -> ExceptT $ StateT $ \s -> ListT $
Identity []
  mplus a b =
    MyMonad $ ReaderT $ \e -> ExceptT $ StateT $ \s -> ListT $ Identity $
        run x = runIdentity (runListT (runStateT (runExceptT (runReaderT
(runAbstract x) e)) s))
        as = run a
        bs = run b
        as ++ bs

My questions are:
  - Is there a shorter way to define mplus?
  - Is there maybe some variation of monad transformer stacks where the
layers are named and can be accessed by name? (I know of effects handlers
but I'd like to stick to monad transformers for now.)

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