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

[Paul Brauner]

There is a typo in the definition of mplus in my previous message:
"runAbstract" should be "runMyMonad".

On Tue, May 29, 2018 at 6:06 PM Paul Brauner <polux2001 at gmail.com> wrote:

> Hello,
>
> 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 $
>       let
>         run x = runIdentity (runListT (runStateT (runExceptT (runReaderT
> (runAbstract x) e)) s))
>         as = run a
>         bs = run b
>       in
>         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.)
>
> Paul
>
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