[Haskell-cafe] Help with triple stack monad

[Christopher Howard]

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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[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

[Chris Wong]

Since the monad type `m` only appears to the right of the arrow, we
can write the instance using `lift`:

instance RandomGen g => MonadRandom (ReaderStateRandom r s g) where
getRandom = RSR . lift . lift $ getRandom

--
Chris Wong (https://lambda.xyz)

"I had not the vaguest idea what this meant and when I could not
remember the words, my tutor threw the book at my head, which did not
stimulate my intellect in any way." -- Bertrand Russell

[Daniel Bergey]

On 2016-06-21 at 02:12, Christopher Howard <ch.h...@zoho.com> wrote:
> However, it seems that I must implement MonadRandom myself, as there is
> no instance for this sort of arrangement already.

Deriving the class also works, at least with GHC-7.10.3:

newtype ReaderStateRandom r s g a = RSR {
rSR :: ReaderT r (StateT s (Rand g)) a

} deriving (Functor, Applicative, Monad, MonadReader r, MonadState s,
MonadRandom)

bergey

[Christopher Howard]

This doesn't seem to work for me, but I'm using 7.6.3 from Debian
Jessie. I'm curious what the difference is. What extensions and modules
do you import? I get error

ReaderStateRandom.hs:19:73:
No instance for (MonadRandom (ReaderT r (StateT s (Rand g))))
arising from the 'deriving' clause of a data type declaration
Possible fix:
add an instance declaration for
(MonadRandom (ReaderT r (StateT s (Rand g))))
or use a standalone 'deriving instance' declaration,
so you can specify the instance context yourself
When deriving the instance for (MonadRandom
(ReaderStateRandom r s g))

On 06/21/2016 09:33 AM, Daniel Bergey wrote:
> On 2016-06-21 at 02:12, Christopher Howard <ch.h...@zoho.com> wrote:
>> However, it seems that I must implement MonadRandom myself, as there is
>> no instance for this sort of arrangement already.
>
> Deriving the class also works, at least with GHC-7.10.3:
>
> newtype ReaderStateRandom r s g a = RSR {
> rSR :: ReaderT r (StateT s (Rand g)) a
> } deriving (Functor, Applicative, Monad, MonadReader r, MonadState s,
> MonadRandom)
>
> bergey
>

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_______________________________________________

[Daniel Bergey]

On 2016-06-21 at 21:12, Christopher Howard <ch.h...@zoho.com> wrote:
> This doesn't seem to work for me, but I'm using 7.6.3 from Debian
> Jessie. I'm curious what the difference is. What extensions and modules
> do you import? I get error

That's very interesting. Here's my full code. I needed to add the
`Control.Applicative` import for GHC-7.6. In GHC-7.10, Applicative is
in Prelude.

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module Test where

import Control.Applicative
import Control.Monad.Random
import Control.Monad.Reader
import Control.Monad.State.Lazy

newtype ReaderStateRandom r s g a = RSR {
rSR :: ReaderT r (StateT s (Rand g)) a
} deriving (Functor, Applicative, Monad, MonadReader r, MonadState s, MonadRandom)