[Haskell-cafe] Help with triple stack monad

[William Yager]

Just manually unrolling the definitions for ReaterT and StateT, I think
this is correct.

instance RandomGen g => MonadRandom (ReaderStateRandom r s g) where
    getRandom = RSR (ReaderT (\r -> StateT (\s -> getRandom >>= (\random ->
return (random,s)))))

Or, using TupleSelections,

    RSR (ReaderT (\r -> StateT (\s -> (,s) <$> getRandom)))

You could also write this out in terms of execReaderT, execStateT, etc. but
I couldn't be arsed.

You also have to add Applicative and Functor instances to RSR.

Cheers,
Will

On Mon, Jun 20, 2016 at 11:12 PM, Christopher Howard <ch.howard at zoho.com>
wrote:

> Hi, I was expanding on my earlier learning, to try a triple monad stack:
>
> {-# LANGUAGE GeneralizedNewtypeDeriving #-}
>
> <...snip...>
>
> import System.Random
> import Control.Monad.Random
> import Control.Monad.State.Lazy
> import Control.Monad.Reader
>
> newtype ReaderStateRandom r s g a = RSR {
>   rSR :: ReaderT r (StateT s (Rand g)) a
>   } deriving (Monad, MonadReader r, MonadState s)
>
> However, it seems that I must implement MonadRandom myself, as there is
> no instance for this sort of arrangement already. Probably this is
> trivial, but I'm having trouble wrapping my mind around how to do it.
> Would anybody perhaps assist me in implementing one function, to help
> guide me in the correct direction?
>
> instance MonadRandom (ReaderStateRandom r s g) where
>
>   getRandom = ...?
>
>
> --
> http://justonemoremathproblem.com
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