[Haskell-cafe] Help with triple stack monad

William Yager will.yager at gmail.com
Tue Jun 21 07:00:05 UTC 2016

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.


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

> 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 = ...?
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